Airbus_Fuel_Economy_Material

航空

Airbus_Fuel_Economy_Material

游客，如果您要查看本帖隐藏内容请回复

航空

getting to grips with
fuel economy
Issue 3 - July 2004
Flight Operations Support & Line Assistance
getting to grips with
fuel economy
Issue 3 - July 2004
Flight Operations Support & Line Assistance
Customer Services
1, rond-point Maurice Bellonte, BP 33
31707 BLAGNAC Cedex FRANCE
Telephone (+33) 5 61 93 33 33
Telefax (+33) 5 61 93 29 68
Telex AIRBU 530526F
SITA TLSBI7X
Getting to grips with Fuel Economy Table of contents
- 1 -
TABLE OF CONTENTS
TABLE OF CONTENTS 1
1. SUMMARY 4
2. PREAMBLE 5
3. INTRODUCTION 6
4. PRE-FLIGHT PROCEDURES 7
4.1 CENTER OF GRAVITY POSITION 8
4.1.1 INTRODUCTION 8
4.1.2 AUTOMATIC CENTER OF GRAVITY MANAGEMENT 8
4.1.3 INFLUENCE ON FUEL CONSUMPTION 9
4.2 TAKEOFF WEIGHT 11
4.2.1 INTRODUCTION 11
4.2.2 OVERLOAD EFFECT 11
4.2.3 AIRCRAFT OPERATING WEIGHT 12
4.2.4 PAYLOAD 13
4.2.5 EMBARKED FUEL 13
4.3 FLIGHT PLANNING 14
4.4 TAXIING 17
4.5 FUEL FOR TRANSPORTATION 19
4.6 AUXILIARY POWER UNIT 21
4.7 AERODYNAMIC DETERIORATION 23
5. IN FLIGHT PROCEDURES 24
5.1 Take-off and Initial Climb 25
5.1.1 INTRODUCTION 25
5.1.2 BLEEDS 25
5.1.3 CONFIGURATION 25
5.1.4 SPEEDS 26
5.1.5 FLEX THRUST 26
5.1.6 NOISE FLIGHT PATHS 26
5.1.7 COURSE REVERSAL 26
1 - Summary Getting to Grips with Fuel economy
- 2 -
5.2 CLIMB 27
5.2.1 INTRODUCTION 27
5.2.2 THE EFFECT OF CLIMB TECHNIQUE ON FUEL BURN 28
5.2.3 CORRELATION OF FUEL BURN & TIME WITH CLIMB TECHNIQUE 31
5.2.4 CLIMB TECHNIQUE COMPARISON TABLES 34
5.2.5 DERATED CLIMB 35
5.3 CRUISE 36
5.3.1 INTRODUCTION 36
5.3.2 CRUISE ALTITUDE OPTIMISATION 37
5.3.3 CRUISE SPEED OPTIMISATION 47
5.3.4 WIND INFLUENCE 49
5.3.5 MANAGED MODE 52
5.3.6 EFFECT OF SPEED INCREASE ON MANAGED MODE 56
5.4 DESCENT 57
5.4.1 INTRODUCTION 57
5.4.2 THE EFFECT OF DESCENT TECHNIQUES ON FUEL BURN 58
5.4.3 MANAGED MODE DESCENT 60
5.4.4 EARLY DESCENT 60
5.5 HOLDING 62
5.5.1 INTRODUCTION 62
5.5.2 VARIOUS CONFIGURATION / SPEED COMBINATIONS 63
5.5.3 LINEAR HOLDING 68
5.6 APPROACH 70
5.6.1 FLIGHT PATH PRIOR TO GLIDE SLOPE INTERCEPTION 70
5.6.2 LANDING GEAR EXTENSION 70
6. DETAILED SUMMARY 71
6.1 INTRODUCTION 71
6.2 General guidelines 71
6.2.1 PRE-FLIGHT PROCEDURES 71
6.2.2 TAKE-FF AND INITIAL CLIMB 72
6.2.3 CLIMB 72
6.2.4 CRUISE 72
6.2.5 DESCENT 72
6.2.6 HOLDING 73
6.2.7 APPROACH 73
6.3 FUEL SAVINGS 73
6.4 ECONOMIC BENEFITS 75
7. CONCLUSIONS 76
Getting to grips with Fuel Economy Table of contents
- 3 -
8. APPENDICES 77
APPENDIX A (Climb Charts) 78
APPENDIX B (Descent Charts) 80
1 - Summary Getting to Grips with Fuel economy
- 4 -
1. SUMMARY
Fuel Consumption is a major cost to any airline, and airlines need to focus
their attention on this in order to maintain their profitability. This brochure looks
at all the significant operating variables that affect fuel economy for the current
Airbus range of aircraft.
This brochure shows that there are many factors that affect fuel consumption
and that the potential gains and losses are huge. Most of these factors are directly
controlled by the airlines own employees (flight crew, operations/dispatch,
maintenance, etc.).
It can be also seen that what is good for one type of aircraft is not
necessarily good for another, and that certain conceptions regarding best
techniques for fuel economy are wrong.
Finally for a fuel and cost economic airline, the following are the main
features:
• Good flight planning based on good data.
• Correct aircraft loading (fuel weight and CG).
• An aerodynamically clean aircraft.
• Optimal use of systems (APU, Bleed, Flaps/Slats, Gear, etc).
• Flight Procedures using speeds and altitudes appropriate to the
companies economic priorities.
• Use of the FMGS in the managed mode.
• Use of performance factors in flight planning and in the FMGS derived
from an ongoing aircraft performance monitoring program.
Getting to grips with Fuel Economy 2 - Preamble
- 5 -
2. PREAMBLE
The very competitive and deregulated aviation market as well as the fear of
a fuel price rise have made airlines understand how important it is to work on the
fuel consumption of their fleet. Indeed airlines try to reduce their operational
costs in every facet of their business, and fuel conservation has become one of
the major preoccupations for all airlines, as well as aircraft manufacturers. That’s
why all ways and means to reduce fuel costs have to be envisaged, safety being
of course the number one priority in any airline operation.
The purpose of this document is to examine the influence of flight operations
on fuel conservation with a view towards providing recommendations to enhance
fuel economy.
It is very rare that the reduction of fuel used is the sole priority of an airline.
Such instances are to maximize range for a given payload, or to decrease fuel
uplift from a high fuel cost airport. Generally fuel is considered one of the direct
operating costs and an airline tries to minimize total direct operating costs. This
introduces the concept of Cost Index and is the scope of another brochure
(Getting to Grips with the Cost Index). However it is sometimes necessary to
consider the cost implication of a fuel economy, and this is done where necessary
in this brochure.
This brochure systematically reviews fuel conservation aspects relative to
ground and flight performance. Whilst the former considers center of gravity
position, excess weight, flight planning, auxiliary power unit (A.P.U.) operations
and taxiing, the latter details climb, step climb, cruise, descent, holding and
approach.
None of the information contained herein is intended to replace procedures
or recommendations contained in the Flight Crew Operating Manuals (FCOM), but
rather to highlight the areas where maintenance, operations and flight crews can
contribute significantly to fuel savings.
3 - INTRODUCTION Getting to grips with Fuel Economy
- 6 -
3. INTRODUCTION
This brochure considers the two flight management modes: “managed”
mode and “selected” mode.
The managed mode corresponds to flight management by means of a
dedicated tool, the flight management system (FMS). Crews interface through the
multipurpose control and display unit (MCDU) introducing basic flight variables
such as weight, temperature, altitude, winds, and the cost index. From these
data, the FMS computes the various flight control parameters such as the climb
law, step climbs, economic Mach number, optimum altitude, descent law. Hence,
when activated, this mode enables almost automatic flight management.
When in managed mode, aircraft performance data is extracted from the
FMS database. This database is simplified to alleviate computation density and
calculation operations in the FMS, but individual aircraft performance factors can
produce good correlation with actual aircraft fuel burns.
When in selected mode, crews conduct the flight and flight parameters
such as speed, altitude, and heading have to be manually introduced on the flight
control unit (FCU).
The cost index (CI) used in the managed mode provides a flexible tool to
control fuel burn and trip time to get the best overall economics. A technique that
reduces fuel burn often requires more trip time. Hence fuel savings are offset by
time related costs (hourly maintenance costs, flight and cabin crew costs and
marginal depreciation or leasing costs). The cost index is the cost of time ($/min)
compared with the cost of fuel ($/kg) and is used to obtain the best economics.
If fuel costs were the overriding priority, because fuel costs were much
more significant than the cost of time, then the cost index would be low. With
zero cost of time it would be zero and the FMS would fly the aircraft at Mach for
max range (MMR).
However if the cost of fuel was very cheap compared to the cost of time,
then speed would be important and the CI would be high. For zero cost of fuel,
the Cost Index would be 999 and the FMS would fly the aircraft just below MMO.
Best economics would be between these two speeds and would depend on
the operator’s cost structure and operating priorities. For more information on
Cost Index see “Getting to Grips with the Cost Index”
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 7 -
4. PRE-FLIGHT PROCEDURES
Operation of the aircraft starts with the aircraft on the ground by aircraft
maintenance, preparation and loading.
This part intends to highlight the impact of some ground operations on fuel
consumption. Even if these operations enable only little savings in comparison
with savings made during the cruise phase, ground staff has to be sensitive to
them and should get into good habits.
This part is divided into seven different sections:
• Center of gravity position
• Excess Takeoff weight
• Flight Planning
• Ways of taxiing to save fuel
• Auxiliary Power Unit
• Fuel Tankering
• Aerodynamic Deterioration
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 8 -
4.1 CENTER OF GRAVITY POSITION
4.1.1 INTRODUCTION
The gross weight is the sum of the dry operating weight, payload and fuel
and acts as one force through the center of gravity (CG) of the aircraft. The
balance chart allows the determination of the overall center of gravity of the
airplane taking into account the center of gravity of the empty aircraft, the fuel
distribution and the payload. It must be ensured that the center of gravity is
within the allowable range referred to as the center of gravity envelope.
A more forward center of gravity requires a nose up pitching moment
obtained through reduced tail plane lift, which is compensated for by more wing
lift. This creates more induced drag and leads to an increase in fuel consumption.
It is better to have the center of gravity as far aft as possible. As a rearward shift
in CG position deteriorates the dynamic stability of the aircraft, the CG envelope
defines an aft limit.
4.1.2 AUTOMATIC CENTER OF GRAVITY MANAGEMENT
AIRBUS has created a trim tank transfer system that controls the center of
gravity of the airplane. This system is installed on some A300 and A310 aircraft
and all A330 and A340 aircraft. When an airplane with a trim tank is in cruise, the
system optimizes the center of gravity position to save fuel by reducing the drag
on the airplane. The system transfers fuel to the trim tank (aft transfer) or from
the trim tank (forward transfer). This movement of fuel changes the center of
gravity position. The crew can also manually select forward fuel transfer.
The Fuel Control and Management Computer (FCMC) calculates the center of
gravity of the airplane from various parameters including input values (Zero Fuel
Weight or Gross Take-off Weight and the associated CG) and the fuel tank
contents. It continuously calculates the CG in flight. From this calculation, the
FCMC decides the quantity of fuel to be moved aft or forward in flight to maintain
the CG between the target value and 0.5% forward of the target band.
Usually one initial aft fuel-transfer is carried out late in the climb to bring the
CG within this band. During the flight there are several smaller forward
movements as the fuel burn moves the CG more aft. Finally a forward transfer is
made as the aircraft nears its destination to bring the CG within the landing CG
range.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 9 -
4.1.3 INFLUENCE ON FUEL CONSUMPTION
The following graph shows the change in fuel consumption, expressed in
terms of specific range (nm per kg of fuel), for both a Forward (20%) and an Aft
(35%) CG position compared to a mid CG position of 27% at cruise Mach.
This graph, which is for the A310-203, shows the advantage of flying at aft
CG. Also shown are the optimum altitude lines and these show the effects of CG
to be constant at these altitudes, with almost no variation with aircraft weight.
Other aircraft have similar shape curves with similar optimum altitude
characteristics (except the A320 family). The following table summarizes the
effect of CG on specific range at the optimum altitude :
Aircraft Type Aft CG(35-37%) Fwd CG(20%)
A300-600 +1.7% -0.9%
A310 +1.8% -1.8%
A330 +0.5% -1.3%
A340 +0.6% -0.9%
For the A300/A310 reference CG is 27% and aft CG is 35%.
For the A330/A340 reference CG is 28% and aft CG is 37%.
Specific Range variation with CG position
-3%
-2%
-1%
0%
1%
2%
3%
290 310 330 350 370 390 410 430
Flight Level
SR Variation
140t
140t
130t
130t
110t
110t
90t
90t
Weight 35%
27%
20%
CG
OPT FL
OPT FL
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 10 -
At maximum altitude, the change in fuel consumption given in the table is
larger by up to 1%. However no benefit is obtained, as the specific range (SR) is
lower at aft CG at maximum altitude than at mid CG at optimum altitude.
For aircraft that are not fitted with automatic center of gravity management,
not all these advantages may be realized because of the normal forward and
rearward shift of CG in flight due to fuel burn. In addition loading these aircraft at
max fuel to an aft CG could prove difficult.
The A320 family does not show the same SR variation with CG as the other
aircraft. The aft CG produces worst SR at FL290, crossing over to show an
improvement at higher flight levels. The SAR variation is much smaller also. This
is due to a complex interaction of several aerodynamic effects. The SAR can be
considered effectively constant with CG position. Loading is therefore not critical
for fuel economy for the A320 family.
In order to assess the overall impact of CG variation on fuel burn, it must be
assessed on a complete sector. The following table shows increases in fuel
consumed with a more forward CG. It is expressed as kg per 1000nm sector per
10% more forward CG for the max variation case (high weight, high flight level)
with no in flight CG shift. The fuel increment in kg is also given for the Forward
(20%) position, compared with the Aft (35 or 37%) position, for a typical sector.
Fuel Burn Increase with a more Forward CG
Aircraft types Fuel increment
KG/1000nm/10%CG
Typical Sector distance
(nm)
Fuel increment per
sector (kg)
A300-600 240 2000nm 710
A310 110 2000nm 330
A319/A320/A321 Negligible 1000nm Negligible
A330-200 70 4000nm 480
A330-300 90 4000nm 600
A340-200 90 6000nm 900
A340-300 80 6000nm 800
A340-500 150 6000nm 1550
A340-600 130 6000nm 1300
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 11 -
4.2 TAKEOFF WEIGHT
4.2.1 INTRODUCTION
Another way to save fuel is to avoid excess take-off weight, which consists of
the operating empty weight of the aircraft plus the payload plus the fuel.
In addition accurate knowledge of weight is an important factor needed to
ensure that fuel burn predictions are met. This gives pilots confidence in the flight
plans thus reducing the tendency to carry excess fuel.
4.2.2 OVERLOAD EFFECT
The specific range, flying at given altitude, temperature and speed depends
on weight. The heavier the aircraft, the higher the fuel consumption.
In addition, fuel savings can be made during climb since the aircraft would
reach its optimal flight level earlier if it were lighter.
The effect of overloading with respect to the in-flight weight is shown on the
following graph, for an excess load of 1% of MTOW (2600kg) in cruise for an
A340-313 This shows the increase in specific range penalty with both weight and
altitude. Maximum and optimum altitudes are shown together with selected sub
optimum flight levels representing the choice of a FL below the Optimum instead
of above it. For example, at 220t the optimum altitude is just under FL 350. If we
select FL 330 1% extra MTOW will decrease the specific range by just under 1.2%
Specific Range Penalty for Excess Weight of 1% MTOW
A340-313 ISA MN 0.82
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
120 140 160 180 200 220 240 260 280 300
Aircraft Weight - Tonnes
Decrease in SR
Optimum
Altitude
Maximum
Altitude
Selected Sub
Optimum FL
FL410 FL390
FL370
FL350 FL330
FL310
FL290
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 12 -
The characteristic curves for the other aircraft types have a similar shape.
Calculating the weight effect on specific range on all Airbus aircraft in accordance
with the lower boundary of typical flight levels gives an average reduction of 1%
of SR for a weight increase of 1% of Maximum Take-off Weight. The scatter in this
value is generally within .2%.
At the higher altitudes, obtainable at lower weights, the previous picture shows
that the SR reduction can increase to 1.5%
Overloading affects not only the trip fuel but also the reserves and requires
increased fuel uplift for a specific mission. The following table shows the effect of
1 tonne/1000nm and also 1% of basic MTOW for a typical sector, both at optimum
altitude, assuming maximum passengers and some freight.
Although the A320 family show considerably lower fuel burn penalties than
the other aircraft, the total fuel penalty is of a similar order due to the high
number of sectors per day. It can readily be seen that a 1% weight penalty has a
significant impact on fuel costs when looked at on a yearly basis for a fleet of
aircraft.
4.2.3 AIRCRAFT OPERATING WEIGHT
The operating empty weight of an aircraft is defined as the manufacturer’s
weight empty plus the operator’s items. The latter include the flight and cabin
crew and their baggage, unusable fuel, engine oil, emergency equipment, toilet
chemicals and fluids, galley structure, catering equipment, seats, documents, etc.
The OEW of new aircraft, even in the same fleet, can vary significantly, due
to specification changes, build differences and normal scatter. Also aircraft
Aircraft
types
Payload Weight
Increase
Stage Fuel Penalty
1000nm/t
Fuel penalty
per sector
Extra
Reserves
A300-600 31000 kg 1705 kg 2000 Nm 93 kg 320 kg 100 kg
A310-300 26560 kg 1500 kg 2000 Nm 59 kg 240 kg 90 kg
A318 14650 kg 640 kg 1000 Nm 31 kg 30 kg 30 kg
A319 13000 kg 590 kg 1000 Nm 38 kg 50 kg 40 kg
A320 17200 kg 735 kg 1000 Nm 43 kg 60 kg 45 kg
A321 19100 kg 890 kg 1000 Nm 48 kg 55 kg 50 kg
A330-200 29800 kg 2300 kg 4000 Nm 49 kg 460 kg 100 kg
A330-300 29800 kg 2300 kg 4000 Nm 47 kg 440 kg 100 kg
A340-200 29000 kg 2535 kg 6000 Nm 74 kg 1130 kg 170 kg
A340-300 29000 kg 2535 kg 6000 Nm 87 kg 1330 kg 230 kg
A340-500 35700 kg 3680 kg 6000 Nm 64 kg 1410 kg 210 kg
A340-600 42250 kg 3650 kg 6000 Nm 65 kg 1420 kg 210 kg
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 13 -
generally get heavier all through their operational life. This is due to repair
schemes, service bulletins, equipment upgrades, dirt, rubbish and moisture
accumulation and unnecessary equipment and supplies.
This variation in weight requires regular monitoring for flight planning
purposes. In general most weight growth is inevitable and it cannot be controlled
at the operational level. However the airline has to be sensitive to these problems
and efforts have to be made in order to avoid excess weight, such as dirt, rubbish
and unnecessary equipment and supplies. It should be noted that 100kg of excess
weight requires an additional 5000kg of fuel per year per aircraft.
4.2.4 PAYLOAD
The most important part of the take-off weight from an airlines point of view
is the payload (passengers and freight). Generally the weight of passengers,
carry-on baggage and checked bags are defined in the operating rules by the
authorities such as the JAA or the FAA. Most operators use standard weights
although other values may be used if they can be statistically demonstrated
through surveys. In general there is not much an operator can do to change the
situation. However they should be aware of the rules and their validity. If the
weights do not seem appropriate then an operator should consider conducting a
survey.
As each freight consignment is weighed, the only influence it can have on
fuel economy is its location and hence the aircraft CG.
4.2.5 EMBARKED FUEL
Fuel is loaded onto the aircraft to be used as follows:
1. Start-up Fuel
2. Taxi Fuel
3. Trip Fuel
4. Reserve Fuel
5. Fuel for Transportation
6. APU Fuel
In order to avoid unnecessary fuel weight, the flight must be planned very
precisely to calculate the exact fuel quantity to be embarked. Flight planning
should be based on aircraft performance monitoring by taking into account
performance factors derived from specific range variations. In addition the
planning should be based on the appropriate optimized techniques using the best
achievable routing and flight levels.
More detailed information on this subject is given later in this brochure.
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 14 -
4.3 FLIGHT PLANNING
The fundamental requirement for achieving fuel economy and reduction of
operating costs is a quality Flight Planning System.
A good flight planning system will produce an optimized route, in terms of
track, speeds and altitudes, which meets the operator’s economic criteria. This
track and vertical profile must be normally achievable in operation, given the
constraints of ATC, climb rates, descent rates, etc.
Climb, cruise and descent techniques and cruise flight levels should be
optimized in accordance with the operator’s criteria, for both the sector and the
diversion. This is covered in much more detail in this brochure.
It will be based on good quality data (temperature, wind, aircraft weight,
payload, fuel uplift, etc)
It will be use the correct aircraft performance and will include an
individual aircraft performance factors derived from an ongoing aircraft
performance monitoring (APM) program (see “Getting to Grips with Aircraft
Performance Monitoring”).
Having established the climb, cruise and descent techniques, it should be
verified from time to time that the aircrews are using these techniques
The fuel reserves will be based on a policy that aims at obtaining the
minimum values required within the regulations.
Within JAR OPS, there are several definitions of Contingency fuel,
depending on diversion airfields, fuel consumption monitoring, etc. Full details can
be found in “Getting to Grips with Aircraft Performance”, but briefly the fuel is the
greater of two quantities:
1. 5 minutes hold fuel at 1500 feet above destination at ISA
2. One of the following quantities:
5% of trip fuel,
3% of trip fuel with an available en route alternate airport
15 minutes hold fuel at 1500 feet above the destination at ISA
20 minutes trip fuel, based upon trip fuel consumption.
The last 3 options require airworthiness approval and the last 2 options
require fuel consumption monitoring with fuel based on results. What we can
conclude is that depending on the flight distance, there is a lowest contingency
fuel.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 15 -
The following graphs show the different contingency fuel quantities for
different distances for an A320.
The graphs for other members of the A320 family are similar and indicate that
below about 500nm, the contingency fuel is set by the minimum 5-minute hold
value. Above about 1000nm, contingency fuel can be reduced to 3% of trip fuel if
there is an en-route alternate available. If not, reductions can be made above
about 2000nm by using the 15-minute destination hold option, which always
requires less fuel than the 20 minute trip fuel option.
The graphs for the other aircraft show different characteristics because of their
longer-range capability.
The A340-600 picture, on the following page, indicates that with no enroute
alternate the 15-minute destination hold requirement enables the
contingency fuel to be reduced above 2150nm. An en-route alternate will give
more benefit until 3500nm, beyond which the 15-minute destination hold
minimises the contingency fuel requirement. The A340-500 is similar.
The A300, A310, A330 and other A340’s have slightly different critical
distances as follows:
5% trip fuel/15-minute hold 1700 to 1900nm.
3% trip fuel/15-minute hold 2800 to 3200nm
However these will also vary with weight, winds, temperature, etc so
the limiting reserve should always be checked. Each aircraft type will show critical
sector distances beyond which a change in contingency policy will yield benefits.
Contingency Fuel - A320-214
0
100
200
300
400
500
600
700
800
900
1000
500 1000 1500 2000 2500 3000
Sector Distance - nm
Fuel - kg
Minimum - 5 min Hold at 1500ft
15 min Hold at 1500ft
20 min trip fuel
3% trip fuel
5% trip fuel
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 16 -
One further method of reducing the contingency fuel is by using a Decision
Point or Redispatch Procedure. This involves the selection of a decision point
where the aircraft can either continue to the destination as the remaining fuel is
sufficient, or it can reach a suitable proximate diversion airport. More details are
given in “Getting to Grips with Aircraft Performance”.
To minimize the alternate fuel, the alternate airports should be chosen as
near as possible to the destination.
Both the JAA and FAA do not require the alternate fuel reserve in certain
cases, depending on meteorological conditions and the suitability of the airport.
More details are given in “Getting to Grips with Aircraft Performance”.
Another part of the reserves is the extra fuel, which is at the Captain’s
discretion.
There are many reasons why this extra fuel is necessary. It could be due to
uncertain weather conditions or availability of alternate and destination airfields,
leading to a probability of re-routing. However it is often due to lack of confidence
in the flight planning and the natural desire to increase reserves.
This is the one area where a significant impact can be made through
accurate flight planning. With this in place, the aircrew will see that the flight
plans fuel burns are being achieved in practice. They will realize that the planned
reserves are adequate and that there is no need for more.
Contingency Fuel - A340-642
0
1000
2000
3000
4000
5000
6000
7000
2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000
Sector Distance - nm
Fuel - kg
Minimum - 5 min Hold at 1500ft
15 min Hold at 1500ft
20 min trip fuel
5% trip fuel
3% trip fuel
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 17 -
4.4 TAXIING
Good estimate of taxi times are required. Actual times need to be
monitored and standard estimates changed as necessary. Jet engine performance
is optimised for flight conditions, but all aircraft spend considerable time on the
ground taxiing from the terminal out to the runway and back. This time has
increased due to airport congestion, and increased airport size. This all leads to a
waste of precious time and fuel.
Only using one engine for taxiing twin-engine aircraft, or two engines for
four-engine aircraft can give benefits in fuel burn. Such procedures need to be
considered carefully, and operators have to define their field of application.
Airbus provides standard procedures in the Flight Crew Operating Manual
(FCOM) for such operations. The following factors regarding one or two engine out
taxi should be considered carefully prior to its incorporation in the operators
standard operating procedures:
1. This procedure is not recommended for high gross weights
2. This procedure is not recommended for uphill slopes or slippery
runways
3. No fire protection from ground staff is available when starting engine
(s) away from the ramp
4. Reduced redundancy increases the risk of loss of braking capability and
nose wheel steering.
5. FCOM procedures require not less than a defined time (from 2 to 5
minutes depending on the engine) to start the other engine(s) before
take off. On engines with a high bypass ratio, warm-up time prior to
applying maximum take off thrust has a significant effect on engine
life.
6. Mechanical problems can occur during start up of the other engine(s),
requiring a gate return for maintenance and delaying departure time.
7. FCOM procedures require APU start before shutting down the engine
after landing, to avoid an electrical transient.
8. FCOM procedures require not less than a defined time before shutting
down the other engine(s) after landing. On engines with a high bypass
ratio, the cool-down time after reverse operation, prior to shut down
has a significant effect on engine life.
9. If an operator decides to use one or two engine out taxi, then FCOM
recommendations about which engine(s) to use should be followed.
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 18 -
As engine-out taxi requires more thrust per engine to taxi and maneuver,
caution must be exercised to avoid excessive jet blast and FOD. More thrust is
necessary for breakaways and 180 degrees turns.
On twin-engine aircraft slow and/or tight taxi turns in the direction of the
operating engine may not be possible at high gross weight.
Single engine taxi may also be considered at low weights to avoid
excessive use of the brakes to control the acceleration tendency with all engines.
This brake use would be detrimental to carbon brake life.
The following table gives an indication of the advantages of engine out taxi
for 8 of the 12 minutes total taxi time, leaving 4 minutes warm up time.
Fuel savings with Engine out taxi
Aircraft types 12 minutes taxi
(all engines)
12 minutes taxi
(8 with engine out)
Engine Out taxi
savings
A300-600 300kg 200kg 100kg
A310 240kg 160kg 80kg
A318 120kg 80kg 40kg
A319 120kg 80kg 40kg
A320 138kg 92kg 46kg
A321 162kg 108kg 54kg
A330 300kg 200kg 100kg
A340-200/300 300kg 200kg 100kg
A340-500/600 420kg 280kg 140kg
For engine out or all engines taxi, the use of a slow taxi speed
costs fuel and time. A burst of power should be used to get the aircraft to
taxi speed, then the power should be reduced to idle. However 30kt should
not be exceeded.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 19 -
4.5 FUEL FOR TRANSPORTATION
The normal message regarding fuel burn is that it is more economical to carry
the minimum amount required for the sector. However there are occasions
when it is economic to carry more fuel. This is when the price of fuel at the
destination airfield is significantly higher than the price at the departure
airfield.
However, since the extra fuel on board leads to an increase in fuel
consumption the breakeven point must be carefully determined.
K is the transport coefficient:
The addition of one tonne to the landing weight, means an addition of K
tonnes to the take-off weight.
For example, if K=1.3 and 1300 kg fuel is added at the departure, 1000 kg
of this fuel amount will remain at the destination. So carrying one tonne of fuel
costs 300 kg fuel more.
The extra-cost of the loaded fuel at departure is
Fuel weight x departure fuel price (ΔTOW x Pd = ΔLW x K x Pd)
The cost saving of the transported fuel is
Transported fuel x arrival price (ΔLW x Pa)
The cost due to a possible increase in flight time is
Flight time increase x cost per hour (ΔT x Ch)
It is profitable to carry extra fuel if the cost saving exceeds the extra fuel loaded
cost plus the extra time cost.
(ΔLW x Pa) >(ΔLW x K x Pd) + (ΔT x Ch)
That is to say:
ΔLW ( Pa - K x Pd) - (ΔT x Ch) > 0
LW
K TOW
Δ
= Δ
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 20 -
Therefore, if ΔT=0, it is profitable to carry extra fuel if the arrival fuel price to
departure fuel price ratio is higher than the transport coefficient K.
Pa > K
Pd
Thus carrying extra fuel may be of value when a fuel price differential exists
between two airports. Graphs in the FCOM assist in determining the optimum fuel
quantity to be carried as a function of initial take-off weight (without additional
fuel), stage length, cruise flight level and fuel price ratio. The following graph is
an example for an A320.
However the needs for accurate fuel planning is necessary to avoid arriving at the
destination airport overweight. This could result in the economic benefit being
eroded or negated due to extra hold time or circuits.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 21 -
4.6 AUXILIARY POWER UNIT
The Auxiliary Power Unit (A.P.U.) is a self-contained unit which
makes the aircraft independent of external pneumatic and electrical power supply
and environmental conditioning.
A.P.U. fuel consumption obviously represents very little in comparison with
the amount of fuel for the whole aircraft mission. Nevertheless, operators have to
be aware that adopting specific procedures on ramp operations can help save fuel
and money.
On the ground, A.P.U. fuel consumption depends on the A.P.U.
type load and the ambient conditions. The minimum is when the APU is in the
RTL(ready to load) condition. As additional loads, such as Electrical Loads(EL) and
Environmental Conditioning System (ECS), are connected, the fuel consumption
increases as shown in the following table (ISA, SL conditions).
Aircraft Type APU
Model
RTL RTL
Max EL
Min ECS
Max EL
Max ECS
Max EL
A320 family 36-300 70 kg/hr 85 kg/hr 105 kg/hr 125 kg/hr
A320 Family 131-9A 75 kg/hr 95 kg/hr 115 kg/hr 125 kg/hr
A330, A340 331-350 120 kg/hr 140 kg/hr 175 kg/hr 210 kg/hr
A340-500/600 331-600 160 kg/hr 180 kg/hr 225 kg/hr 290 kg/hr
A.P.U. specific procedures to save fuel have to be defined by the
operators. One extra minute of A.P.U. operation per flight at 180 kg/hr fuel flow,
means an additional 3000 kg per year per aircraft. This will also result in
increased maintenance costs.
They have to choose between using ground equipment (Ground Power Unit,
Ground Climatisation Unit, Air Start Unit) and the A.P.U. This choice depends on
several parameters and each operator needs to determine the benefits at each
airport and at each turnaround.
Such parameters can include length of turnaround, ambient conditions, cost
of ground connections, time delay to get connected, suitability and quality of
ground equipment, passenger load, local noise restrictions, etc.
For a long turnaround or night stop the G.P.U. is the best choice as time
considerations are not prevailing. It saves both fuel and A.P.U. life. So operators
are advised to use ground equipment if of a good quality, and to try to conclude
agreements with airport suppliers to get preferential prices.
However, for a short turnaround (45 minutes on average), the use of
A.P.U. may be preferable to limit A.P.U. start cycles and improve reliability, even
if it is not fully used during the turnaround. It is better to operate with A.P.U. at
RTL than to shut it down and perform a new start cycle soon after shut down.
Lack of suitable ground power may also require the use of APU. The use of APU
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 22 -
may also be preferable to avoid excessive hook up charges or to reduce
turnaround time.
Some airport regulations restrict the use of the APU to a defined time prior to
departure time and after the arrival.
For extremely short turnarounds, complete engine shut down would have
a cyclic cost impact, and therefore the turnaround could be made without APU.
However a main engine can sometimes not meet the ECS demand in high load
conditions (hot days).
The disconnection of ground equipment supplies and the start of A.P.U. must
be coordinated with A.T.C. pushback/slot requirements. A one-minute
anticipation in each A.P.U. start will lead to a significant amount of fuel saving
during a year (2000 to 4000 kg depending on A.P.U. types).
Engine start up should also, if possible, be carefully planned in conjunction
with A.T.C. If pushback is delayed, it is preferable to wait and use A.P.U. for air
conditioning and electrical requirements. Engine start time is critical, and the
engines should not be started until ready to go.
The following table assuming typical engine fuel flows, shows extra fuel
consumption by using one engine instead of the A.P.U. for 1 minute, assuming
maximum electrical load and minimum ECS:
Extra fuel when using Engine instead of APU
Aircraft Type A.P.U. type
Engine FF
kg/hr/eng
APU FF
kg/hr
Extra Fuel for
1 minute
A300 GE 331-250 520 150kg 6kg
A310 GE 331-250 520 150kg 6kg
A320 family CFM 36-300 300 105kg 3kg
A330 GE 331-350 520 175kg 6kg
A330 RR 331-350 720 175kg 9kg
A340 CFM 331-350 300 175kg 2kg
A340 RR 331-600 480 275kg 4kg
In overall economic terms, the benefits of APU operation are not just
confined to fuel usage. The hourly maintenance costs of an APU are cheaper than
the aircraft powerplant, so reducing ground running time on the engines can
significantly reduce the operating costs.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 23 -
4.7 AERODYNAMIC DETERIORATION
Some of the most severe penalties in terms of fuel consumption are caused by
increased drag resulting from poor airframe condition. Normal aerodynamic
deterioration of an aircraft over a period of time can include the incomplete
retraction of moving surfaces, damaged seals on control surfaces, skin roughness
and deformation due to bird strikes or damage caused by ground vehicles,
chipped paint, mismatched doors and excessive gaps. Each deterioration incurs a
drag increase, and this increased drag is accompanied by increased fuel
consumption.
This subject is covered fully in the brochure “Getting Hands-On Experience with
Aerodynamic Deterioration”.
The following table gives the highest deterioration effect in each category for the
three aircraft families as increased sector fuel consumption in Kg, based on typical
utilization figures.
Category Condition A300/310 A320
Family
A330/340
Misrigging Slat 15mm 90 60 270
Absence of Seals Flap (chordwise) 30 14 90
Missing Part
(CDL)
Access Door 50 13 150
Mismatched
Surface
Fwd Cargo Door
10mm step for
1m
20 11 80
Door seal leakage Fwd Pax Door
5cm
2 1 5
Skin Roughness 1 m2 21 13 105
Skin Dents Single 2 1 2
Butt joint gaps Unfilled 0.2 0.1 0.6
Butt Joint Gaps Overfilled 3 2 7
External Patches 1 m2 3mm high 6 3 16
Paint Peeling 1 m2 leading
edge slat
12 8 57
Sector Distance 2000nm 1000nm 4000/6000nm
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 24 -
5. IN FLIGHT PROCEDURES
When an aircraft arrives at the end of the runway for take-off, it is the
flying techniques (speed, altitude, configuration, etc) that have the biggest
influence on fuel economy. Disciplined flight crews adhering to a flight plan based
on the operator’s priorities can save much fuel and/or costs.
This part intends to give recommendations to flight crews on the means to
save fuel during the flight. It reviews the different phases of the flight, that is to
say:
• Take-off and Initial Climb
• Climb
• Cruise
• Descent
• Holding
• Approach
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 25 -
5.1 TAKE-OFF AND INITIAL CLIMB
5.1.1 INTRODUCTION
There are many variations in take-off technique that can directly affect the
fuel burn. In general the effects are very dependent on the airframe/engine
combination as well as aircraft weight, airfield altitude and temperature. The
following fuel effects are representative values.
5.1.2 BLEEDS
For take-off, full bleeds can be used or one can consider selecting packs off
or APU bleed on to improve take-off performance. Selecting packs off without
APU will also improve fuel burn. The normal procedure would then be to select
pack 1 on after climb thrust is selected and pack 2 on after flap retraction. This
has the effect of reducing fuel burn by 2-3 kg on an A320 increasing to 5-10 kg
on an A340-500/600.
With APU bleed the engine fuel burn will be decreased by the same amount.
However with APU used from pushback with 12minutes taxi, the additional APU
fuel burn is 30kg for an A320 and 60-70kg for an A340.
In economic terms, the APU fuel and maintenance cost is largely offset due
to decreased engine maintenance costs bleeds off (higher flex temp).
5.1.3 CONFIGURATION
This effect is very dependent on the variables mentioned in the introduction,
plus the choice of VR and V2. However the trend is always the same , with high
flap/slat configurations (more extended) using more fuel than the lowest setting.
Typical penalties/takeoff of higher flap settings compared with the low flap
settings Conf 1+F are shown below (note that for the A300/A310 Conf 1+F, Conf
2 and Conf 3 corresponds to the Flap 0,15 and 20 configuration respectively).
Aircraft Conf 2 Conf 3
A300/A310 1- 5kg 15kg
A320 3-5kg 8-13kg
A330 12kg 24kg
A340 30kg 50kg
These figures assume Full take-off thrust. The advantage of Conf 1+F
increase with reduced power take-offs.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 26 -
5.1.4 SPEEDS
During a non limiting full power take-off, the use of the higher speeds
appropriate to flex thrust instead of optimized speeds appropriate to the actual
temperature can reduce the fuel burn by up to 8kg.
5.1.5 FLEX THRUST
Compared to a full thrust take-off, flex thrust will generally increase fuel
burn. The increased time at low level offsets the slight reduction in fuel flow
induced by the lower thrust. Typical increases are as follows:
Aircraft Conf 1+F Conf 2 Conf 3
A300/A310 10kg 10kg 10kg
A320 1kg 5kg 5kg
A330 0 0 0
A340 5kg 20kg 25kg
5.1.6 NOISE FLIGHT PATHS
The effect of an ICAO type A noise flight path, with climb thrust selected at
800ft and clean up delayed until 3000ft is generally to increase fuel burn
compared to the standard take-off with power reduced at 1500ft. The actual
distance to a fixed height, say 5000ft, varies very little with configuration. The
main effect is the different altitude – speed history experienced by the engines.
Typical values are as follows:
Aircraft Conf 1+F Conf 2 Conf 3
A320 -4kg +5kg +2kg
A330 +100kg +100kg +115kg
A340 +90kg +130kg +125kg
5.1.7 COURSE REVERSAL
In the event that a course reversal is required after take-off, then much
distance can be saved using a lower initial climb speed. Suppose ATC require an
aircraft to maintain runway heading to 6000ft. A lower climb speed will achieve
this altitude earlier and thus reduce the ground distance and fuel burnt.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 27 -
5.2 CLIMB
5.2.1 INTRODUCTION
Depending on speed laws, the climb profiles change. The higher the speed,
the lower the climb path, the longer the climb distance.
Climb profiles
Climbs are normally performed in three phases on a constant IAS/Mach climb
speed schedule at max climb thrust, as follows:
• 250 KT indicated air speed (IAS) is maintained until flight level 100, then
the aircraft accelerates to the chosen indicated air speed (e.g. “300kts);
• constant indicated air speed is maintained until the crossover altitude;
• constant Mach number is maintained until top of climb;
Cruise level
High speed
Low speed
Typical Climb Law
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
200 250 300 350 400 450 500
Speed - ktas
Altitude - ft
Crossover
altitude
Tropopause
250 kias
300 kias
M No 0.8
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 28 -
The crossover altitude is the altitude where we switch from constant IAS
climb to the constant Mach number climb. It only depends on the chosen IAS and
Mach number, and does not depend on ISA variation.
During climb, at constant IAS, the true air speed (TAS) and the Mach
number increase. Then, during climb at constant Mach number, the TAS and the
IAS decrease until the tropopause.
To correctly evaluate the effects of climb techniques, climb and cruise flight
must be viewed in relation to each other. A short climb distance for example
extends the cruise distance; a low climb speed requires more acceleration to
cruise speed at an unfavourable high altitude. One has therefore to consider
sectors that cover acceleration to climb speed, climb, acceleration to cruise speed
and a small portion of the cruise to the same distance.
5.2.2 THE EFFECT OF CLIMB TECHNIQUE ON FUEL BURN
This evaluation has been made for all Airbus types, based on a climb to
35000ft, acceleration and cruise to a fixed distance. The assumed cruise speed
was 0.78 for the A320 family and 0.8 for the rest.
The reference climb technique is the standard technique given in each FCOM,
and is summarized below:
Aircraft types Speed law
A300-600 250kts/300kts/M0.78
A310 (GE) 250kts/300kts/M0.79
A310 (PW) 250kts/300kts/M0.80
A318/A319/A320/A321 250kts/300kts/M0.78
A330 250kts/300kts/M0.80
A340-200/300 250kts/300kts/M0.78
A340-500/600 250kts/320kts/M0.82
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 29 -
The following chart shows the variation of fuel burn with climb technique
over a given climb + cruise distance.
This shows that there is an optimum climb speed and max climb Mach
number that produces the lowest fuel burn. This happens to be the standard
technique (300kt/0.78). Climbing at 320kt/0.82 will burn 1% more fuel.
However the following chart shows that this is obtained at the expense of
time.
Effect of Climb Technique on Fuel to 120nm
A300B4-605R ISA F/L 350 Weight 140000kg
-0.2%
0.0%
0.2%
0.4%
0.6%
0.8%
1.0%
1.2%
1.4%
1.6%
260 270 280 290 300 310 320 330 340 350
Climb Speed - kias
Fuel difference - %
0.76
0.78
0.80
Mach No
FMG
0.82
CI "0"
CI "100"
Effect of Climb Technique on Time to 120nm
A300B4-605R ISA F/L 350 Weight 140000kg
-4%
-3%
-2%
-1%
0%
1%
2%
3%
4%
5%
6%
260 270 280 290 300 310 320 330 340 350
Climb Speed - kias
Time difference - %
0.80
0.78
0.76
Mach No
FMGS
0.82
CI "0"
CI "100"
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 30 -
This time difference plot has the same characteristics for all Airbus aircraft,
with the best time being obtained at the highest climb speed and max climb Mach
number. Note that although a slow climb speed gets the aircraft to cruise altitude
earlier, this requires more acceleration to cruise speed and more cruise to a given
distance, making it slower overall.
The fuel difference plot characteristics vary with aircraft type. The A310,
A321 and A330 show similar characteristics to the A300 with a best fuel climb
speed of about 290 to 300 knots.
The A318, A319 and A320 show better fuel burn at the lower speed range
(260 to 280 knots)
The A340 shows better fuel burn at the higher speed range (310-330 knots).
The A310 and A340 are similar to the A300 in showing minimum fuel at a
max climb Mach number of 0.78. In fact 0.8 is better for the A340-500/600.
However the A320 family and A330 benefit from the lower Mach No of 0.76.
Thus the A320 family benefits from low climb speeds and the A340 from high
climb speeds. This difference arises from the different behavior during climb of
twin-engine and four-engine aircraft. Indeed, twin-engine aircraft have a higher
thrust than four engine aircraft, as they must satisfy some take-off climb
requirements with only one engine operative, compared with 3 engines operative
on the quads. This enables them to have a higher rate of climb than four engine
aircraft and reach cruise flight levels quicker.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 31 -
5.2.3 CORRELATION OF FUEL BURN & TIME WITH CLIMB TECHNIQUE
The following chart shows the differences in fuel and time to climb and
cruise to a fixed distance with varying climb speed and max climb Mach number
relative to the standard technique.
This chart shows that the fastest technique (330/0.82) uses the least time
(-3.2%) and the most fuel (+1.5%) whereas the slowest technique (270/0.76)
uses the most time (+4.5%) and nearly the most fuel (+1.4%). The least fuel is
obtained using a 300/0.78 climb technique. Variation of climb technique can cause
a total variation of 1.5% and climb time by 8% for this aircraft.
Also plotted on the charts are lines representing the speeds selected by the
FMGS for various cost indices (CI). The left hand point of each line represents a CI
of zero (fuel cost priority) and the right hand point represents a CI of 100 (flight
time priority). It should be noted how the FMGS line approximates to the lower
boundary of the time - fuel difference plot.
The chart on the following page is for the A320 and shows completely
different characteristics.
Effect of Climb Technique on fuel and time to 120nm
A300B4-605R ISA F/L 350 Weight 140000kg
-6%
-4%
-2%
0%
2%
4%
6%
-1% 0% 1% 2% 3%
Fuel %
Time %
Mach No
FMGS
0.76
0.78
0.8
280
300
320
330
Climb Speed
KIAS
0.82 270
50kg
1 minute
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 32 -
The different mach numbers all coalesce together and the FMGS line forms the
common boundary. Climb speed increases from the left to the right. Least fuel is
obtained using a 0.76/280 technique. Mach No has little influence, but increasing
speed from 280 to 330kias decreases time by 6% and increases fuel by 6%.
Completely different characteristics are also shown in the next chart (A340-642).
Effect of Climb Technique on fuel and time to 200nm
A320-214 ISA F/L 350 Weight 70000kg
-6%
-4%
-2%
0%
2%
4%
6%
-2% -1% 0% 1% 2% 3% 4% 5% 6%
Fuel %
Time %
Mach No
FMGS
0.76
0.8
100 kg
1 minute
270 kias
280 kias
300 kias
320 kias
330 kias
Effect of Climb Technique on fuel and time to 160nm
A340-642 ISA F/L 350 Weight 320000kg
-2%
0%
2%
4%
6%
8%
10%
-1% 0% 1% 2% 3% 4% 5%
Fuel %
Time %
Mach No
FMGS
0.76
0.78
0.8
0.82 100kg
1 minute
270 kias
280 kias
300 kias
320 kias
330 kias
Climb
Speed
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 33 -
This shows a common technique is good for both fuel burn and time. The optimum
is 320/0.80. There is little Mach No effect, but reducing the speed to 270 kias will
increase fuel by 4% and time by 8%. Because the optimum technique is good for
both fuel and time, there is a single FMGS point for all cost indeces.
Earlier versions of the A340 showed that some marginal time benefit could be
gained by climbing faster. However this would have affected the flight levels
achieved. Consequently there is no variation of FMGS climb speed with cost index
for all the A340 family.
Appendix A presents some examples of time - fuel charts for other Airbus aircraft.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 34 -
5.2.4 CLIMB TECHNIQUE COMPARISON TABLES
The following tables show, for various Airbus aircraft, the climb time and
fuel variations for a fixed distance, to FL 350, relative to a 300kias reference
speed.
Effect of Climb Speed on Fuel
Effect of Climb Speed on Time
It can be seen from the tables how the optimum techniques are very
dependant on the aircraft type, and that a 10kt climb speed change can have a
significant impact.
Aircraft Climb ΔFuel – kg
Mach No. 270KT 280 KT 300 KT 320 KT 330 KT
A300 0.78 +40 +15 0 +5 +10
A310 0.79 +5 0 +5 +15
A318/A319/A320 0.78 -15 0 +30 +70
A321 0.78 -10 0 +25 +60
A330 0.80 +15 +5 0 +20 +35
A340–200 0.78 +45 +20 0 +10 +25
A340-300 0.78 +105 +50 0 -5 +20
A340-500/600 0.82 +135 0 -5 -10
Aircraft Climb ΔTime – minutes
Mach No. 270KT 280 KT 300 KT 320 KT 330 KT
A300 0.78 +0.8 +0.5 0 -0.3 -0.4
A310 0.79 +0.5 0 -0.5 -0.6
A318/A319/A320 0.78 +0.5 0 -0.4 -0.8
A321 0.78 +0.8 0 -0.6 -1.0
A330 0.80 +0.9 +0.6 0 -0.4 -0.7
A340–200 0.78 +1.4 +0.8 0 -0.6 -0.8
A340-300 0.78 +1.5 +0.9 0 -0.6 -1.0
A340-500/600 0.82 +0.8 0 -0.6 -0.8
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 35 -
5.2.5 DERATED CLIMB
In order to reduce engine maintenance costs, there are derated climb
options available on the A330 and A340 aircraft. There are two levels of derate,
D1 and D2. At a certain altitude the derate is washed out such that at Max Climb
rating is achieved generally before 30000ft. The following shows a typical derate
thrust variation picture, but this will vary with engine and temperature.
However this derate will result in more fuel and time required to reach the
same distance. The effect is dependant on aircraft weight, temperature and cruise
flight level. The following table gives some typical penalties in ISA conditions to
35000ft.
Derate D1 Derate D2
Aircraft Weight
(kg)
Fuel
Increase
Time
Increase
Fuel
Increase
Time
Increase
A330-203 190000 5kg 0.5 min 20kg 0.6 min
A330-223 190000 20kg 0.2 min 40kg 0.5 min
A330-343 190000 20kg 0.2 min 40kg 0.5 min
A340-212 240000 65kg 0.9 min 120kg 1.5 min
A340-313 240000 140kg 0.8 min 225kg 1.4 min
A340-313E 240000 140kg 1.0 min 335kg 1.4 min
A340-642 340000 270kg 0.6 min 445kg 1.0 min
Derated Climb - Net Thrust Reduction
-25
-20
-15
-10
-5
0
0 10000 20000 30000 40000
Altitude / ft
Net Thrust Difference to MCL %
Derated Climb 1
Derated Climb 2
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 36 -
5.3 CRUISE
5.3.1 INTRODUCTION
The cruise phase is the most important phase regarding fuel savings. As it is
the longest for long haul aircraft, it is possible to save a lot of fuel. So discipline
must be exercised particularly in this phase.
The two variables that most influence cruise fuel consumption are the cruise
speed (IAS or Mach Number) and the altitude or flight level. The following shows
their influence on a single sector assuming standard climb and descent
procedures.
The correct selection of the cruise parameters is therefore fundamental in
minimizing fuel or operating cost. This chart shows the normal laws that aircraft
consume less fuel when flown slower or when flown higher. However there are
limits to these laws. Flying lower than the maximum range speed will increase the
block fuel, as will flying higher than an optimum altitude.
Block Fuel and Time for various Flight Levels and Mach numbers
A330-223 ISA 3000nm Payload 30000kg JAR Reserves
35000
37000
39000
41000
43000
45000
360 380 400 420 440 460 480 500 520 540 560
Block Time - minutes
Block Fuel - kg
Green Dot
Max Speed
Range
Long
Range
M 0.78
M 0.80
M 0.82
Flight
Level
250
270
290
310
330
350
370
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 37 -
5.3.2 CRUISE ALTITUDE OPTIMISATION
In examining SR changes with the altitude at a constant Mach number, it is
apparent that, for each weight, there is an altitude where SR is maximum. This
altitude is referred to as “optimum altitude”.
Optimum Altitude Determination at Constant Mach Number
When the aircraft flies at the optimum altitude, it is operated at
the maximum lift to drag ratio corresponding to the selected Mach number.
High Speed Polar Curve
When the aircraft flies at high speed, the polar curve depends on the indicated Mach
number, and decreases when Mach increases. So, for each Mach number, there is a different
value of (CL/CD)max, that is lower as the Mach number increases.
M=0.84
M = 0.82
M = 0.86
M <0.76
Pressure
Altitude
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 38 -
When the aircraft is cruising at the optimum altitude for a given Mach, CL is
fixed and corresponds to (CL/CD)max of the selected Mach number. As a result,
variable elements are weight and outside static pressure (Ps) of the optimum
altitude. The formula expressing a cruise at optimum altitude is:
constant
P
Weight
s
=
In the FCOM Flight Planning Chapters the optimum altitude is
shown on the Cruise Level chart for 2 or more speeds. This chart also shows the
Maximum Altitudes as limited by performance and buffet. A typical FCOM chart
showing the variation of optimum altitude with weight for one speed is shown
below.
It should be noted that the influence of airspeed on optimum altitude is not
very significant in the range of normal cruise speeds.
In order to minimize fuel burn, the aircraft should therefore be flown at the
optimum altitude. However this is not always possible. Performance limitations
such as rate of climb or available cruise thrust can lead to a maximum altitude
below the optimum, as can buffet limitations. At low weights, the optimum
altitude may be above the maximum certificated altitude. In addition, Air Traffic
Control restrictions can affect the flown flight level.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 39 -
The following table shows the specific range penalty of not flying at optimum
altitude, assuming a cruise Mach No of 0.8. It should be noted that each
airframe/engine combination has different values. It should be noted that these
are average values and there are slight variations with different weight/optimum
altitude combinations.
Specific Range Penalty for not flying at Optimum Altitude
Aircraft +2000ft -2000ft -4000ft -6000ft
A300B4-605 2.0% 0.9% 3.4% 9.3%
A310-324 1.9% 1.4% 4.4% 9.3%
A318-111 0.7% 1.6% 5.0% 10.0%
A319-132 1.0% 3.0% 7.2% 12.2%
A320-211 ** 1.1% 4.7% 9.5%
A320-232 1.4% 2.1% 6.2% 12.0%
A321-112 2.3% 1.4% 4.6% 15.2%
A330-203 1.8% 1.3% 4.2% 8.4%
A330-343 3.0% 1.0% 3.2% 7.2%
A340-212 1.4% 1.5% 4.0% 8.0%
A340-313E 1.5% 1.6% 5.2% 9.5%
A340-642 1.6% 0.6% 2.2% 5.1%
** Above Maximum Altitude
Generally if one flies within 2000ft of optimum altitude, then the specific
range is within about 2% of the maximum. However fuel burn-off is an important
consideration.
Consider an A340-313E at a weight such that the optimum altitude is
33000ft. If the aircraft flies at FL 310 the SR penalty is 2.1% for the weight
considered. However after a fuel burn of 20800kg, during which the aircraft would
have traveled 1400nm the optimum altitude increases to 35000ft and the penalty
is now 5.2%.
There is also an effect on block time due to the different altitudes. The true
air speed increases/decreases 4kts, or just under 1% for each 2000ft lower/higher
cruise altitude.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 40 -
5.3.2.1 CROSS-OVER ALTITUDE VERSUS OPTIMUM ALTITUDE
It has been previously shown that the TAS is the maximum at the crossover
altitude. One can wonder whether it is profitable to stay at this altitude, instead of
climbing to the first optimum altitude.
Assuming the standard climb laws, the crossover altitude can be derived. The
standard speed laws are tabulated in paragraph 5.2.2.
The next table shows the effect of flying at the crossover altitude instead of
optimum flight levels. The 1st optimum flight level has been chosen for the short
sectors, whereas longer sectors assume step climbs with FL 310, 350 and 390
being available. This assumes ISA conditions and a take-off weight for a typical
sector with max passengers and some freight (2500kg for the A320 family and
5000kg for the other aircraft).
Aircraft type Sector
Distance
Cross-over
altitude
Optimum
Flight Levels
Gained
time (min)
Increase in fuel
consumption
A300B4-605R 2000nm 29000 ft 310/350 7 1190kg
A310-324 2000nm 30000ft 350/390 3 2160kg
A318-111 1000nm 29000 ft 370 3 740kg
A319-112 1000nm 29000 ft 370 3 650kg
A320-214 1000nm 29000 ft 350 2 580kg
A320-232 1000nm 29000 ft 340 2 440kg
A321-211 1000nm 29000 ft 330 2 350kg
A330-203 4000nm 31000 ft 350/390 9 5040kg
A330-223 4000nm 31000 ft 350/390 9 5780kg
A330-343 4000nm 31000 ft 350/390 10 6380kg
A340-212 6000nm 29000 ft 310/350/390 17 10900kg
A340-313 6000nm 29000 ft 310/350/390 14 8410kg
A340-313E 6000nm 29000 ft 310/350/390 17 9310kg
A340-500/600 6000nm 29000 ft 310/350/390 18 2430kg
This table shows that flying at crossover altitude increases the fuel burn
significantly for a relatively small reduction in block time.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 41 -
5.3.2.2 CRUISE OPTIMISATION WITH STEPPED CLIMB
5.3.2.2.1 Introduction
It has been shown that flying at non-optimum altitudes can cause significant
fuel penalties, and that the effect of fuel burn is to increase the optimum altitude.
The ideal scenario would be to follow the optimum altitude as in the climbing
cruise, but A.T.C. constraints, performance and buffet limits do not make this
possible. However, by changing the cruise level with step climbs, as the aircraft
gets lighter the aircraft will remain as close as possible to the optimum altitude.
5.3.2.2.2 Choice of Profile
Several parameters such as weather conditions, ATC requirements, may
influence any decision made by the crew with regard to three fundamental
priorities: maneuverability, passenger comfort, and economics.
This pertains to the choice of the cruise flight level that can be made
according to the three following climb profiles as shown below for an A340-642:
The Low profile initiates the step climb at the weight where the next
available flight level is also the optimum flight level at that weight. Consequently
the flight levels are always at or below the optimum. This has the advantage of
better maneuverability margins and generally a better speed as it is closer to the
crossover altitude.
Step Climb Profiles
Even Flight Levels Non RVSM
26000
28000
30000
32000
34000
36000
38000
40000
42000
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Distance from Brake Release - nm
Altitude - ft
Low Profile
Mid Profile
High Profile
Optimum Altitude
Maximum Altitude
Maximum Altitude Optimum
Altitude
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 42 -
The high profile initiates the step climb at the weight where the next
available flight level is also the maximum flight level at that weight. The flight
levels are mainly above the optimum and the aircraft will have decreased
maneuverability and fly slower.
The mid profile initiates the step climb at the weight where the specific
range at the next available flight level is better than that at the current flight
level. This enables the flight profile to remain as close as practically possible to
the optimum flight level. It is this technique that is recommended for best fuel
economy, and is also very close to that required for best economics.
It is interesting to note that, in this case, the Mid profile step climb is made
1140nm before the Low Profile step climb and 1520nm after the High profile step
climb.
The situation changes with odd flight levels:
Because of the different available flight levels, the step climbs are initiated
some 1500nm further than the even flight level step climb points. However the
relative merits of each profile remains the same.
With Reduced Vertical Separation Minima (RVSM) the difference between
flight levels reduces from 4000 to 2000ft and this enables the aircraft profile to
remain much closer to the optimum. In addition the high profile (depending on
the aircraft) remains much higher than the optimum, increasing the fuel penalty.
This profile is shown on the following page.
Step Climb Profiles
Odd Flight Levels Non RVSM
26000
28000
30000
32000
34000
36000
38000
40000
42000
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Distance from Brake Release - nm
Altitude - ft
Low Profile
Mid Profile
High Profile
Optimum Altitude
Maximum Altitude
Maximum Altitude Optimum
Altitude
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 43 -
Thus pilots are advised to perform step climbs around the optimum altitudes.
To facilitate this, the optimum weight for climb to the next flight level is given in
most FCOM’s (not A300/A310). An example is shown below.
Step Climb Profiles
Odd Flight Levels RVSM
26000
28000
30000
32000
34000
36000
38000
40000
42000
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Distance from Brake Release - nm
Altitude - ft
Low Profile
Mid Profile
High Profile
Optimum Altitude
Maximum Altitude
Maximum Altitude Optimum
Altitude
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 44 -
On all Airbus FMS-equipped aircraft, the optimum altitude (OPT FL) and
the maximum flight level (MAX FL) are displayed on the MCDU progress page. The
recommended maximum altitude in the FMGC ensures a 0.3g buffet margin, a
minimum rate of climb of 300ft/min at MAX CLIMB thrust and a level flight at MAX
CRUISE thrust. Depending on weight and type, it is 2000 to 4000ft above the
optimum altitude.
Typical cruise distances between 2000 foot altitude steps are shown in the
following table:
Type Distance - nm
A300 1000 - 1100
A310 1150 - 1250
A320 1200 - 1300
A330 1500 - 1650
A340 1500 - 1650
A340-500/600 1600 - 1700
For sector lengths greater than these, where ATC restrictions do not allow a
change in cruise altitude from the initial requested level, the initial request should
be the highest compatible with the maximum cruise altitude.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 45 -
5.3.2.3 DELAYS IN ALTITUDE CHANGES
Let’s consider an aircraft that is at flight level 330, which has, at that weight,
an optimum flight level of 370. If it does not climb to FL 370 for ATC or other
reasons, it will consume more fuel. The following table shows the difference in fuel
burn for a 500nm still air cruise, when cruising at FL 330 instead of FL 370.
Aircraft
Type
Fuel Increase
(kg)
Fuel Increase
(%)
A300B4-605R 238 5.2
A310-324 221 5.3
A318-111 150 6.2
A319-132 184 7.9
A320-211 158 6.2
A320-232 187 7.9
A321-112 155 5.5
A330-203 324 5.5
A330-343 342 5.6
A340-212 393 6.2
A340-313E 378 6.0
A340-500/600 336 4.1
Thus delaying a climb to a higher altitude has a significant impact on fuel
burn.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 46 -
5.3.2.4 OPTIMUM ALTITUDES ON SHORT STAGES
For short stages, the choice of cruise flight level is often restricted due to the
necessary climb and descent distance. Airbus philosophy assumes a minimum 5
minute cruise sector, because a climb followed immediately by the descent is not
appreciated by pilots, passengers or ATC.
If the stage length is of sufficient length that the optimum flight level can be
reached, but the cruise is of short duration, then the benefits at this flight level
will be marginal. It may even be worthwhile to cruise at one flight level lower, as
the increased climb consumption offsets any reduced cruise consumption.
In the FCOM there is a chart showing the optimum altitude on a short stage.
An example is shown below.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 47 -
5.3.3 CRUISE SPEED OPTIMISATION
Having been given a flight level which may be a requested optimum altitude
or one imposed by air traffic control, speed is the only remaining parameter that
requires selection. The following picture shows the variation of Specific Range with
Mach Number for different aircraft weights at a fixed altitude.
The Mach number, which gives the best specific range, can be determined. It
is called the maximum range cruise Mach (MMR). Nevertheless, for practical
operations, a long-range cruise procedure is defined which gains a significant
increase in speed compared to MMR with only a 1% loss in specific range. Like the
MMR speed, the MLRC speed also decreases with decreasing weight, at constant
altitude.
A more detailed explanation of this can be found in “Getting to Grips with
Aircraft Performance”
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 48 -
The following chart shows the typical variation of the Long Range Cruise
Mach Number with aircraft weight for various flight levels. Also plotted on this
chart is the optimum altitude line. This shows that there is not much variation in
the long-range cruise mach number at these altitudes.
It would therefore be possible to fly a constant Mach number procedure
instead of the variable LRC speed procedure. In order to save fuel however, the
exact LRC speed should be maintained.
Long Range Cruise Mach Number
A330-243 ISA
0.72
0.74
0.76
0.78
0.8
0.82
0.84
160 170 180 190 200 210 220 230 240
Aircraft Weight (1000kg)
Mach Number
270
290
310
330
350
370
390
410
Flight
Optimum
Altitude
The Long Range Cruise Speed can be found in the Cruise tables in the FCOM.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 49 -
5.3.4 WIND INFLUENCE
Wind can have a significant influence on fuel burns. Nowadays,
meteorological forecasts are very reliable and its integration into the FMS provides
accurate information to crews. Hence the latter best perform flight planning with a
view towards fuel savings.
The effect of the wind on trip time and fuel is shown on the following chart,
which gives fuel consumption and time for a 2000nm sector, with respect to flight
levels, Mach number and wind (tailwind positive) for a fixed take-off weight.
This plot graphically shows the magnitude of the significant changes in fuel
consumption and time due to winds. FCOM Tables show the equivalent still air
distances for any ground distance/wind combination.
However the winds can affect the performance optimization as well as
changing the effective still air distance. The MMR (or MLRC) value varies with
headwind or tailwind, due to changes in the SR.
The effect of a tailwind is to increase the ground speed, and therefore the
SR, by the ratio of ground speed to airspeed. A given wind speed therefore has a
larger effect at the lower airspeeds, which changes the optimum speed.
A321
FL 350
FL 330
FL 310
FL 290
FL 330
FL 350
FL 310
FL 290
11000
12000
13000
14000
15000
16000
17000
18000
3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 5.7
Time (hour)
Fuel consumption (kg)
M 0.8
M 0.78 M 0.76
M 0.8
M 0.78
M 0.76
M 0.8
M 0.78
M 0.76
M 0.8
M 0.78
M 0.8 M 0.76
M 0.78
M 0.76
-60 kts
-30 kts
30 kts
60 kts
0 kts
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 50 -
The following chart shows the Maximum Range Mach number versus wind
variations.
MMR and wind influence
This shows that
Tailwinds increase the specific range and lower the speeds
Headwinds decrease the specific range and raise the speeds.
The wind speed can be different at different altitudes. For a given
weight, when cruise altitude is lower than optimum altitude, the specific range
decreases. Nevertheless, it is possible that, at a lower altitude with a favorable
wind, the ground specific range improves. When the favorable wind difference
between the optimum altitude and a lower one reaches a certain value, the
ground-specific range at lower altitude is higher than the ground-specific range at
optimum altitude. As a result, in such conditions, it is more economical to cruise
at the lower altitude.
There is information in the most FCOM’s (not A300/310) to indicate the
amount of favorable wind, necessary to obtain the same ground-specific range at
altitudes different from the optimum. If the wind is more favorable then it is
beneficial to fly lower. The following shows such a page:
Given weight
and Pressure
Altitude
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 51 -
IN FLIGHT PERFORMANCE
CRUISE
3.05.15 P 7
SEQ 020 REV 24
WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE
GIVEN : Weight : 68000 kg (150 000 lb)
Wind at FL350 : 10 kt head
FIND : Minimum wind difference to descend to FL310 : (26 − 3) = 23 kt
RESULTS : Descent to FL310 may be considered provided the tail wind at this
altitude is more than (23 − 10) = 13 kt.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 52 -
5.3.5 MANAGED MODE
The flight management system (FMS) optimizes the flight plan for winds,
operating costs and suggests the most economical cruise altitude and airspeed,
depending on the cost index chosen by the airline. An airline that wants to save
fuel has to choose a low cost index. The next part intends to highlight the impact
of the cost index on fuel consumption and on trip time. More complete information
can be found in the “Getting to Grips with Cost Index” brochure.
5.3.5.1 ECONOMIC MACH NUMBER
Long-range Cruise Mach number was considered as a minimum
fuel regime. If we consider the Direct Operating Cost instead, the Economic
Mach number (MECON), can be introduced.
Direct Operating Costs (DOC) are made up of fixed, flight-time
related and fuel-consumption related costs. As a result, for a given trip, DOC can
be expressed as:
DOC C C F C T C F T = + .Δ + .Δ
where CC = fixed costs
CF = cost of fuel unit ΔF = trip fuel
CT = time related costs per flight hour ΔT = trip time
As DOCs are calculated per nautical mile, it is possible to plot fuelrelated
costs, flight-time related costs, and direct operating costs based on Mach
number .
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 53 -
Minimum fuel costs correspond to the Maximum Range Mach
number. The minimum DOC corresponds to a specific Mach number, referred to as
Econ Mach (MECON).
ECON
ECON
weight constant FL M
constant weight M
= ⇒
FL = ⇒
􀃊 􀃊
􀃌 􀃌
The MECON value depends on the time and fuel cost ratio. This ratio is called
cost index (CI), and is usually expressed in kg/min or 100lb/h:
F
T
C
C
=
Cost of fuel
Cost Index (CI) = Cost of time
Depending on the cost index, the predicted aircraft and atmospheric
conditions, the optimum altitude and the economic Mach number are computed.
From then on, fuel consumption depends only of the chosen cost index.
The following chart shows the economic Mach number variation with flight
level for different cost indices.
This shows the general trend, common with all aircraft, of increasing
economic Mach number with flight level.
Economic Cruise Mach Number
A310-324 ISA 130000kg
0.73
0.74
0.75
0.76
0.77
0.78
0.79
0.8
0.81
0.82
290 300 310 320 330 340 350 360 370 380
Flight Level
ECON Mach
Optimum
F/L
Maximum
F/L
0
20
40
60
80
100
Cost Index
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 54 -
The charts also show large economic Mach number changes with flight
level for low cost indices, whereas it is rather constant for high cost indices. The
economic Mach is very sensitive to the cost index when flying below the optimum
altitude.
The effect of weight variation at a fixed flight level is shown below.
The charts show that for high cost indices, the economic Mach number stays
fairly constant throughout the flight. Nevertheless, for a low cost index, the
economic Mach number reduces significantly as the weight reduces. This is quite
normal as low cost indices favor fuel consumption at the expense of time.
Moreover, we notice that for low cost indices, a small cost index increment has a
far-reaching influence on the economic Mach number, and hence on flight time.
These trends are typical of all aircraft.
Economic Cruise Mach number
A310-324 ISA F/L 350
0.7
0.72
0.74
0.76
0.78
0.8
0.82
90 95 100 105 110 115 120 125 130 135 140
Aircraft Weight - tonnes
ECON Mach
0
20
40
60
80
100
Cost
Index
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 55 -
5.3.5.2 TIME/FUEL RELATIONSHIP
To know whether the fuel economies at low cost indices are worthwhile, the
impact of cost index on time has to be considered. The following graph show both
trip fuel and time for different flight levels and cost indices. The shape of this
chart is typical of all types.
As it can be seen, it is not really advantageous to fly at very low cost indices
as fuel savings are not significant compared to time loss. Although using slightly
higher fuel, a slightly higher cost index gives significant time gains.
For instance, for the A319, increasing the cost index from 0 to 20 reduces
the block time by 15 minutes (5%) for a fuel burn increase of only 200kg (2%) on
a 2000nm sector.
A319
FL390
FL370
FL350
FL330
FL310
FL290
FL390
FL370
FL350
FL330
FL310
FL290
FL390
FL370
FL350
FL330
FL310
FL290
9000
9500
10000
10500
11000
11500
12000
12500
13000
4.2 4.4 4.6 4.8 5 5.2 5.4
time (hours)
fuel consumption (kg)
CI=0
CI=20
CI=40
CI=60
CI>100
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 56 -
5.3.6 EFFECT OF SPEED INCREASE ON MANAGED MODE
Flying at a given cost index rather than at a given Mach number provides the
added advantage of always benefiting from the optimum Mach number as a
function of aircraft gross weight, flight level and head/tailwind components.
This means the ECON mode (“managed” mode) can save fuel relative to
fixed Mach schedules (“selected” mode) and for an equivalent time.
One can wonder whether selecting a higher Mach number than the one
chosen by the FMS has a significant impact on fuel consumption. Imagine an
aircraft flying at flight level 370, in managed mode and at the optimum weight of
FL370. The FMS computes the optimum speed based on cost index, temperature
and wind. If the pilot selects another (higher) Mach number, the fuel consumption
will increase.
The following tables show the effect of such a speed increase.
We notice that although decreasing block times, the increase of Mach
number above the Optimum speed can result in significant increases in fuel burn.
Pilots hence have to be patient and should not change the Mach number even
when under the impression that the aircraft does not fly fast enough.
Moreover, when possible, the managed mode must be kept.
Economic Mach No + 0.005 Economic Mach No + 0.01
Fuel Penalty ΔTime Fuel Penalty ΔTime
Aircraft Sector Kg % Min Kg % Min
A300-605 2000 Nm 110 0.4 1 230 0.9 3
A310-324 2000 Nm 90 0.4 1 430 2.0 8
A318 1000 Nm 30 0.5 1 60 1.0 1
A319 1000 Nm 20 0.2 1 40 0.6 2
A320 1000 Nm 20 0.3 1 40 0.7 2
A321 1000 Nm 10 0.1 1 30 0.4 1
A330 4000 Nm 150 0.3 3 330 0.6 6
A340-212 6000 Nm 390 0.5 5 790 0.9 10
A340-313E 6000 Nm 380 0.4 5 900 1.0 10
A340-500 6000 Nm 1050 0.9 5 2540 2.1 9
A340-600 6000 Nm 820 0.7 4 2060 1.8 9
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 57 -
5.4 DESCENT
5.4.1 INTRODUCTION
Depending on the descent law, flight paths do vary in steepness.
Indeed, the higher the speed law, the steeper the flight path.
Descent profiles.
Descents are normally performed in three phases on a constant IAS/Mach
descent speed schedule, as follows:
• Constant Mach number is maintained until the crossover altitude
• Constant indicated air speed is maintained down to 10000ft
• 250 KT indicated air speed (IAS) is maintained below flight level 100, until
the aircraft decelerates for landing
The engine thrust is normally set to flight idle for the descent and the speed
is controlled by the aircraft attitude. In these conditions higher weights increase
the descent distance because of the reduction of descent gradient (which equals
[thrust-drag]/weight in stabilized flight). This also increases the descent fuel.
However a descent from high altitudes at low weight may lead to a gradient
of descent that results in an excessive cabin rate of descent. In these cases the
rate of descent is reduced by application of power, until a flight idle descent can
be continued. This results in what is known as the re- pressurization segment, and
this can reverse the weight-descent distance relationship.
To correctly evaluate the effects of descent techniques, cruise and
descent flight must be viewed in relation to each other. A short descent distance
for example extends the cruise distance. One has therefore to consider in addition
to the descent, a small portion of the cruise to the same distance.
High speed
Low speed
Cruise level
TOD
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 58 -
5.4.2 THE EFFECT OF DESCENT TECHNIQUES ON FUEL BURN
An evaluation has been made of the fuel burn to a constant distance, and
this now shows that the higher weights use less fuel. Lower speeds, although
requiring more fuel for the descent only requires less total fuel because of the
longer descent distance. This is shown in the following chart.
At a fixed weight, the following chart shows that the minimum fuel occurs
at a descent speed of 240kias to 280kias, dependant on flight level.
However there is a significant time penalty at these speeds.
Effect of Descent Technique on Fuel and Climb for 115nm
A310-324 ISA 110000Kg
17.5
18.0
18.5
19.0
19.5
20.0
20.5
21.0
21.5
300 350 400 450 500 550 600 650
Fuel - kg
Time - min
240
250
260
300
320
330
3
13
33
54
74
100
390 370 Initi3a5l 0Flight Level330 310
Cost
Index
Descent
Speed KIAS
280
Effect of Descent technique on Fuel and Time for 115nm
A310-324 ISA F/L 350
17.5
18.0
18.5
19.0
19.5
20.0
20.5
380 400 420 440 460 480 500 520 540
Fuel - kg
Time - min
90t
110t
130t
260
280
300
320
Aircraft
Weight
Descent
Speed kias
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 59 -
Note that the effect of the descent Mach number is very dependant on cruise
flight level and descent speed, but is relatively small compared to the descent
speed effect, and is not fully investigated here.
These descent charts are typical of the other Airbus aircraft. Generally they
show a minimum fuel speed of 260 to 280 kts for flight level 310, reducing to
240kts for flight level 390. The exceptions are the A318, A319, A320 and A330,
which show the minimum fuel at 240kias for all flight levels which is slightly lower
than the other aircraft at FL310.
Appendix B presents some examples of these descent charts for other Airbus
aircraft.
The following tables show, for various Airbus aircraft, the descent time and
fuel variations for a fixed distance, from FL 350, relative to a 300kias reference
speed.
* A300/A310/A320 330kias A330/A340 340kias
Type ΔFuel – kg
240KT 260 KT 280 KT 300 KT 320 KT 330/340KT
*
A300 -55 -60 -30 0 25 35
A310 -55 -60 -30 0 25 40
A318, 319, 320 -50 -40 -20 0 20 25
A321 -35 -40 -20 0 20 35
A330 -110 -105 -60 0 50 70
A340–200/300 -70 -90 -50 0 50 75
A340-500/600 -125 -130 -70 0 70 100
Type ΔTime – minutes
240 KT 260 KT 280 KT 300 KT 320 KT 330/340KT
*
A300 2.7 1.5 0.6 0 -0.4 -0.6
A310 2.4 1.4 0.6 0 -0.4 -0.6
A320 family 2.6 1.4 0.6 0 -0.4 -0.6
A330 3.5 2.0 0.8 0 -0.6 -0.8
A340–200/300 3.2 1.8 0.8 0 -0.6 -0.8
A340-500/600 3.3 1.9 0.8 0 -0.6 -0.8
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 60 -
Comparing these tables to the equivalent climb comparison tables in
chapter 5.2.4, it can be noticed that descent techniques often have a
greater effect on fuel and time than climb techniques.
5.4.3 MANAGED MODE DESCENT
The FMS computes the Top Of Descent (TOD) as a function of the cost index.
We notice that the higher the cost index:
The steeper the descent path (the higher the speed)
The shorter the descent distance
The later the top of descent.
Descent performance is a function of the cost index; the higher the cost
index, the higher the descent speed. But contrary to climb, the aircraft gross
weight and the top of descent flight level appear to have a negligible effect on the
descent speed computation.
It can be noticed that time to descent is more dependant on cost indices
than the time to climb.
On the Effect of Descent Technique on Fuel and Time chart, the cost index
has been annotated for each speed. It can be seen that the minimum fuel is at a
CI of 0, and the minimum time occurs with a high CI, as would be expected.
For the A300, A310 and A320 family the speed at zero cost index is about
250kias. For the A330/340 it is about 270kias. Max speed normally corresponds
with a high cost index of 60 to 120. Once more it can be seen how, in the
managed mode, the cost index is used to choose the balance between fuel burn
and flight time.
5.4.4 EARLY DESCENT
If the aircraft begins its descent too early, the aircraft would leave its optimal
flight level, where fuel consumption is at its best, and would have to cruise at a
lower altitude to arrive at the same point.
Cruise Flight Level
•
TOD
Early Descent
FL 100
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 61 -
Two descent situations were simulated:
• Descent commenced 15nm (or about 2 minutes) early followed by a leveloff
at FL100.
• Cruise continued from the early descent point until the optimum start of
descent, followed by the descent
At 10000ft, the cruise speed could be selected between LRC and max speed.
If in managed mode, one could continue at the same cost index, or select the
250kias below 10000ft limiting speed. The following table compares the two
options.
Aircraft 250KIAS at FL100 LRC at FL100
ΔFuel – kg ΔTime – min ΔFuel – kg ΔTime – min
A300-600 70 1.1 95 0.4
A310 70 1.1 90 0.3
A320 family 50 1.1 65 0.2
A330 80 1.2 100 0.5
A340-200/300 95 1.2 105 0.5
A340-500/600 135 1.2 125 0.5
Cruising faster at 10000ft reduces the time penalty at the expense of fuel.
After a long flight with an A340–500 or –600, starting the descent some
100nm early would not appear to be significant in the overall flight. However this
can result in a 900kg fuel burn increase and 8 minutes longer block time.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 62 -
5.5 HOLDING
5.5.1 INTRODUCTION
When holding is required, it is generally flown on a “race track pattern”,
composed of two straight legs plus two 180 degree turns. In a hold the distance
covered is not the primary objective. On the contrary, the knowledge of the
maximum holding time (maximum endurance) is a determining factor for any
diversion decision. As a result, it is important, during holding, to try to minimize
fuel by simply minimizing fuel flow.
For all aircraft, the minimum fuel consumption speed is very close
to the maximum lift-to-drag ratio (Green Dot) speed as shown below. As a result,
in clean configuration, the standard holding speed is selected equal to green dot
speed (GD).
Fuel Consumption
A330-343 170000kg 1500ft
0
2000
4000
6000
8000
160 180 200 220 240 260 280 300 320
Speed - Knots CAS
Fuel Consumption kg/hr
GD
Min Fuel Min Drag Max Range Long Range
Holding patterns may be quite limiting around certain airports due to
obstacle proximity. Therefore, green dot is sometimes too high, especially during
turn phases where the bank angle can be too significant. As it is not possible to
significantly reduce the speed below green dot in clean configuration, slats may
be extended and a holding done in CONF1 at “S” speed. (min slat retraction
speed Conf 1 to Conf clean).
At other airports, Air Traffic Control may require the hold to be performed at a
certain speed, and it may not be possible fully optimise the fuel burn. In order to
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 63 -
allow flexibility in planning and operations, the FCOM has four different holding
speed and configuration combinations, adapted to each type of aircraft.
The following table gives the configurations and speeds for each type.
Aircraft types
First Flap/slat
Configuration Clean configuration
A300-600 210kts S speed 240kts Green Dot
A310 170kts S speed 210kts Green Dot
A320 Family (CFM) 170kts S speed 210kts Green Dot
A320 Family (IAE) 170kts S speed 210kts
Green Dot +
20
A330 170kts S speed 210kts Green Dot
A340-200/300 210kts S speed 240kts Green Dot
A340-500/600 240kts S speed - Green Dot
For the A300/A310 the first configuration is flap 15, slat 0. For the other
aircraft this is Conf 1. Note that the fourth combination for the A340-500/600 is
Configuration 2 at 210kts.
5.5.2 VARIOUS CONFIGURATION / SPEED COMBINATIONS
The following graphs show the holding fuel flow variation with weight for the
four different holding configurations. This is done at an altitude of 10000ft.
Effect of Holding Technique on Fuel Flow
A300B4-605R ISA F/L 100
2500
3000
3500
4000
4500
5000
90 95 100 105 110 115 120 125 130
Aircraft Weight - tonnes Fuel Flow -
kg/h
Clean Green Dot
Clean 240 kt
15/0 'S' Speed
15/0 210kt
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 64 -
This graph is for an A300 and it shows the advantage of holding in a clean
configuration at the green dot speed. The clean configuration fixed speed of 240kt
is significantly higher than the green dot speed; hence the large increase in fuel
flow with this technique. The 15/0 configuration with a fixed speed of 210kt is also
significantly higher than the ‘S’ speed, hence higher fuel flows.
The large variation in fuel flow shows how important it is to use the right
configuration and speed, compatible with the other operational requirements.
The A340-200/300 schedules the same hold speeds as the A300, and the
graphs have a similar form with a large increase in fuel flow at low weights with
the fixed speed techniques. However at high weights the difference is much
smaller. There is also a large increase when using Conf 1. Once more holding
clean at green dot speed gives the lowest fuel flow.
The following graph is for the A310 and this shows completely different
characteristics because of the lower fixed speeds used in each configuration.
Each configuration shows very similar fuel flow, whichever speed technique is
used. The clean configuration green dot speed still represents the best single
choice for lowest fuel burn over the normal holding weight range.
The A330, which has the same hold speed schedules shows the same
characteristics, with clean configuration at green dot speed being marginally
better than clean configuration at 210kt over the normal holding weight range.
The A320 family shows a completely different set of characteristics as shown
in the graph on the next page.
Effect of Holding Technique on Fuel Flow
A310-324 ISA F/L 100
2500
3000
3500
4000
4500
5000
90 95 100 105 110 115 120 125 130
Aircraft Weight - tonnes
Fuel Flow - kg/h
Clean Green Dot
Clean 210kt
15/0 'S' Speed
15/0 170kt
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 65 -
The variation of different techniques is very weight sensitive. However it is
still the clean configuration at green dot speed that gives the lowest fuel flow. This
picture is typical of all the A320 family.
Finally the A340-500/600 has another set of configuration/speed
combinations and the following graph shows its different characteristics, but the
basic concept that the clean configuration and green dot is the best combination
still remains true.
Effect of Holding Technique on Fuel Flow
A320-214 ISA F/L 100
1400
1600
1800
2000
2200
2400
2600
2800
40 45 50 55 60 65 70
Aircraft Weight - tonnes
Fuel Flow - kg/h
Clean Green Dot
Clean 210kt
Conf 1 'S' Speed
Conf 1 170kt
Effect of Holding Technique on Fuel Flow
A340-642 ISA F/L 100
5500
6000
6500
7000
7500
8000
8500
9000
9500
10000
200 220 240 260 280
Aircraft Weight - tonnes
Fuel Flow - kg/h
Clean Green Dot
Conf 1 240kt
Conf 1 'S' Speed
Conf 2 210kt
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 66 -
There is also altitude to be considered, although it is often not the
operator’s decision what flight level to hold at. Altitude has different effects on the
fuel flow, depending on the airframe/engine combination. However, whatever the
altitude effect, it generally affects all techniques equally; generally the higher the
hold altitude the lower the fuel flow. This however is true only up to a certain
altitude and this varies with each type.
The following table shows this altitude effect for a hold in the clean
configuration at green dot speed. The holding fuel flow is compared with the
lowest for the flight levels considered for each type, and the difference expressed
as a percentage.
Flight Level 50 100 150 200 250 300 350 400
A300B4-605R 4 2 1 0 3 8 16
A310-324 11 5 2 0 0 5 9 23
A318-111 13 8 4 2 1 0 0 5
A319-112 19 11 3 1 0 1 0 4
A320-214 13 5 3 1 1 1 0 2
A320-232 7 5 5 5 2 0 4 11
A321-211 14 11 8 3 0 1 5
A330-203 2 1 0 0 2 4 8 18
A330-223 9 9 5 2 0 1 6 14
A340-343 10 5 1 0 0 2 7 16
A340-212 3 2 0 0 2 3 5
A340-313E 2 1 0 0 2 3 5
A340-642 6 2 0 1 2 3 4 11
In order to allow an assessment of the sensitivity of each aircraft type to
different hold techniques, the following table shows the extra fuel required to
hold for 15 minutes at 10000ft in the first flap configuration at ‘S’ speed,
compared to Conf clean at green dot speed.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 67 -
Effect of Holding in First flap Setting at ‘S’ speed compared with Clean at Green Dot speed
Aircraft types
Fuel Increase (kg)
Low Holding
Weight
Fuel Increase (kg)
High Holding
Weight
A300B4-605R 70 110
A300B4-622R 110 190
A310-324 70 135
A318 5 10
A319 10 30
A320 10 30
A321 30 50
A330-203 135 175
A330-223 175 205
A330-343 145 175
A340-212 170 230
A340-313 125 175
A340-642 130 150
The table shows that the green dot speed/clean configuration combination
enables significant savings to be made.
However, green dot speed increases with weight and can become higher
than the maximum recommended speeds, which are listed below:
Levels ICAO PAN-OPS FAA France
Up to 6,000 ft inclusive 230 KT 210 KT 200 KT 220 KT
Above 6,000 ft to 14,000 ft inclusive 230 KT 230 KT 230 KT 220 KT
Above 14,000 ft to 20,000 ft inclusive 240 KT 240 KT 265 KT 240 KT
Above 20,000 ft to 24,000 ft inclusive 265 KT 240 KT 265 KT 240 KT
Above 24,000 ft to 34,000 ft inclusive 265 KT 240 KT 265 KT 265 KT
Above 34,000 ft M 0.83 240 KT 265 KT M 0.83
If green dot is higher than these maximum recommended speeds, it is
advised to hold in configuration 1 at “S” speed below 20000ft: keeping clean
configuration coupled with a speed reduction would save fuel but would decrease
the speed margins which are especially important in turbulent conditions.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 68 -
5.5.3 LINEAR HOLDING
If holding is going to be necessary, linear holding at cruise flight level and at
green dot speed should be performed whenever possible since total flight time will
remain constant (cruise time is increased but holding time is reduced) and fuel
flow is lower at high flight levels.
If A.T.C. informs 15 minutes before reaching a fix and that 10 minutes
holding is expected. Two options are possible:
• The aircraft is flown 15 minutes at cruise speed and holds for 10 minutes at
green dot speed.
• The aircraft performs the cruise to reach the fix at green dot speed and
holds for the remaining time at the same speed.
10 minutes holding
at green dot speed
15 minutes at cruise
d
Holding at cruise
Holding for remaining
time at green dot speed
Cruise at green dot speed
Holding optimization
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 69 -
ATC restrictions may not permit a cruise speed reduction at the cruise flight
level, or permit a hold at the cruise flight level. The standard procedure would be
to continue to the top of descent at cruise speed and descend to a flight level to
join the stack. However if ATC permit a linear hold it can give significant fuel
savings.
However the amount of savings is very dependant on the characteristics of
the aircraft type. The increase in time in the cruise depends on how much slower
green dot speed is compared to the normal cruise speed. This increase was much
higher with the A320 than the A340. In addition, most aircraft, flying the same
cruise distance at green dot speed actually uses a little more fuel at these
altitudes. The following table shows the gains due to cruising slower and
spending less time in the hold at the cruise flight level.
Advantages of a 15min linear hold at cruise altitude at Green Dot speed
Aircraft type Weight kg
Cruise
Flight Level
Cruise
Speed
Fuel
savings kg
A300 120000 350 0.8 95
A310 110000 350 0.8 115
A318 50000 350 0.78 120
A319 50000 350 0.78 135
A320 60000 350 0.78 80
A321 70000 350 0.78 50
A330 180000 390 0.82 95
A340-200 200000 390 0.82 10
A340-300 200000 390 0.82 45
A340-500/600 270000 390 0.82 5
The high green dot speed for the A340 leads to very little advantage in linear
holding. However the other aircraft show significant benefits.
If the increase in cruise time can be used to reducing the time in the holding
pattern or stack, then the benefits will be similar to those shown in the table
above. However the constraints of ATC are unlikely to let these benefits accrue.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 70 -
5.6 APPROACH
5.6.1 FLIGHT PATH PRIOR TO GLIDE SLOPE INTERCEPTION
Procedures used in the approach phase can affect the amount of fuel
consumed in this phase of the flight. The glide slope can be intercepted either
horizontally between 1500ft and 2000ft or in a descending flight path above
2000ft. This latter method uses less fuel, but the amount is difficult to quantify, as
it depends on the exact flight paths in each case. However, the most important
feature of an approach is that it should be well executed, stabilized and safe.
None of these features should be compromised in an attempt to save fuel, and the
procedure flown should be that appropriate to the airport, runway, equipment,
conditions, etc.
5.6.2 LANDING GEAR EXTENSION
The standard procedure is that Gear Down is selected down when Conf 2 (or
flap 20 for A300/310) is achieved. The effect of extending the gear prior to this
point will increase fuel burn, but the amount is difficult to quantify without
knowing when the gear is extended. However, the most important feature of an
approach is that it should be well executed, stabilized and safe. The use of gear is
often one of the means of achieving this through speed control, and gear
extension should not be delayed to save fuel.
Getting to grips with Fuel Economy 6 - DETAILED SUMMARY
- 71 -
6. DETAILED SUMMARY
6.1 INTRODUCTION
In this brochure it can be seen that there are many ways of influencing the
fuel burn of an aircraft, but most depend on the way that the sector is planned
and flown. Maximising the fuel economy requires:
• Good flight planning based on good data.
• Correct aircraft loading (weight and cg).
• An aerodynamically clean aircraft.
• Optimal use of systems (APU, Bleed, Flaps/Slats, Gear, etc).
• Flight Procedures using speeds and altitudes appropriate to the
companies economic priorities.
• Use of the FMGS in the managed mode.
• Use of performance factors in flight planning and in the FMGS derived
from an ongoing aircraft performance monitoring program.
6.2 GENERAL GUIDELINES
6.2.1 PRE-FLIGHT PROCEDURES
• For most Airbus aircraft, an aft CG position saves fuel.
• Excess weight costs fuel. Minimize zero fuel weight and embarked fuel.
• A good flight planning system will minimise fuel through correct
optimisation.
• An aircraft performance measurement system and good flight planning
will give confidence in fuel burn reducing extra reserve.
• Keep A.P.U. running during short turnarounds to reduce A.P.U. start
cycles.
• Use ground power, when possible to save both fuel and A.P.U. life.
7 – CONCLUSIONS Getting to grips with Fuel Economy
- 72 -
• Do not start engines until ready to go.
• If considered operationally acceptable, taxi with one engine out.
• Keep the aircraft in an aerodynamically clean condition.
6.2.2 TAKE-FF AND INITIAL CLIMB
• Bleeds off fuel improvements normally negated by APU fuel burn
• Lower configurations do save fuel.
• Flex thrust cost fuel but saves engine costs.
• Noise flight paths cost fuel
6.2.3 CLIMB
• Climb as close as possible to the optimum climb law.
• Fast Climb speeds use more fuel (except A340)
6.2.4 CRUISE
• The best speed for fuel burn (very low cost index) is slow and has a
big time penalty.
• If possible, fly in managed mode at the cost index appropriate to the
airlines economic priorities.
• Flying faster than the FMGS economical Mach number costs fuel.
• Try to fly at optimum altitude. Chase the optimum altitude.
• Flying at the cross-over altitude is faster, but costs fuel.
• Step Climb around the optimum altitude (see FCOM).
• Avoid delays in initiating a step climb.
• For short stage lengths, fly at an appropriate altitude (see FCOM).
• Wind variations with altitude can give advantages in flying at lower
altitudes.
6.2.5 DESCENT
• Diminishing descent speed can allow significant fuel savings.
• Avoid early descents
Getting to grips with Fuel Economy 6 - DETAILED SUMMARY
- 73 -
6.2.6 HOLDING
• The best combination for fuel burn is clean configuration at green dot
speed.
• Manoeuvrability, speed or ATC restrictions may require a hold in
configuration 1 at S speed.
• If holding is to be anticipated, linear holding saves fuel.
6.2.7 APPROACH
• Avoid extending gear unnecessarily early.
6.3 FUEL SAVINGS
The following table gives examples of the savings possible through the
application of correct procedures and practices. The values represent typical
saving as there is variation dependant on the actual base case considered.
However these figures serve to illustrate the magnitude of savings being achieved
(or penalties being paid). The savings are expressed in kg of fuel for one flight,
with the sector length being representative for each aircraft.
Fuel Savings Possible in the Pre-Flight phase
Item Variation A300 A310 A320 A330 A340-
200/300
A340-
500/600
Sector 2000nm 2000nm 1000nm 4000nm 6000nm 6000nm
CG mid to aft 710 330 0 600 900 1550
Weight -1%
MTOW
380 250 100 800 1530 1920
EO Taxi 8
minutes
50 40 25 50 50 70
APU 3 min
Grd
Power
9 9 6 10 10 14
Ground
Idle
3 min
APU
18 18 9 15-24 3 9
Misrigged
Slat
15mm to
zero
90 90 60 270 270 270
Peeling
Paint
1sq m
slat to
zero
12 12 8 60 60 60
7 – CONCLUSIONS Getting to grips with Fuel Economy
- 74 -
Fuel Savings in the In Flight Phase
Item Variation A300 A310 A320 A330 A340-
200/300
A340-
500/600
Sector 2000nm 2000nm 1000nm 4000nm 6000nm 6000nm
TO Conf Max to min
F/S
15 15 10 24 - 50
Climb
Rating
Derate 2
to full
Climb
NA NA NA 30 120-320 445
Climb
Speed
330 to
300kias
10 15 70 35 25 -10
Cruise
Altitude
Optimum
to –2000’
65 80 80 100 95 135
Cruise
Altitude
Optimum
to +2000’
90 60 25 145 30 25
Cruise
Mach
Mecon+.01
to Mecon
230 430 40 330 900 2540
Delayed
Climb
CFP to
500nm
late
240 220 180 330 390 340
CG mid to aft 710 330 0 600 900 1550
Descent
Speed
Max to
300kt
35 40 30 70 75 100
Early
Descent
CFP to
2min early
70 70 50 80 95 135
Hold Green Dot
Clean Conf
190 135 30 205 230 130
Getting to grips with Fuel Economy 6 - DETAILED SUMMARY
- 75 -
6.4 ECONOMIC BENEFITS
It may be that 5 or 10 kg extra fuel per flight does not seem significant in
terms of the total fuel burn during the flight. However this saving accumulates
with every flight. Sometimes the savings for an A340 seem worthwhile compared
with the equivalent value for an A320, but the increased number of flight cycles
for an A320 can make this saving more significant than that of the A340. The only
way to assess the impact of any saving is to look at it over a given time span.
The economic impact calculations have assumed typical yearly utilisation
rates, average sector lengths and sectors per year as follows:
The following table shows the annual cost savings for one aircraft associated
with various fuel savings for each Airbus type based on the above utilisation
figures. Fuel is assumed to cost $1/us gallon (33cents/kg).
Utilisation A300 A310 A320 A330 A340
Flying
Hours/year
2600 3200 2700 2900 4700
Average
sector - nm
2000 2000 1000 4000 6000
Average flight
time - hr
4.5 4.6 2.4 8.5 13.8
No of
sectors/year
580 700 1125 340 340
Savings/flight A300 A310 A320 A330 A340
10kg $1920 $2310 $3720 $1120 $1120
50kg $9600 $11550 $18600 $5600 $5600
250kg $48000 $57750 $93000 $28000 $28000
1000kg $192000 $231000 $372000 $112000 $112000
7 – CONCLUSIONS Getting to grips with Fuel Economy
- 76 -
7. CONCLUSIONS
There are many factors that influence the fuel used by aircraft, and these are
highlighted in this report. The unpredictability of fuel prices, together with the fact
that they represent such a large burden to the airline has prompted Airbus to be
innovative in the field of fuel conservation. The relationship between fuel used and
flight time is such that sometimes compromise is necessary to get the best
economics. Whether in the field of design engineering or in flight operations
support, we have always maintained a competitive edge. Whether it is in short or
ample supply, we have always considered fuel conservation a subject worth
revisiting.
Fuel conservation affects many areas including flight planning, flight
operations and maintenance. Airbus is willing and able to support airlines with
operational support in all the appropriate disciplines. Despite the increasing
efficiency of modern aircraft it is a subject that demands continuous attention and
an airline that can focus on the subject, together with the Operation Support of
Airbus is best placed to meet the challenges of surviving and profiting in the harsh
airline environment of the 21st century.
Getting to grips with Fuel Economy 8 - APPENDIXES
- 77 -
8. APPENDICES
These appendices contain climb and descent graphs for some of the other
variants of Airbus aircraft. Each airframe/engine combination have different
characteristics. Even the weight variant can influence these characteristics. It is
therefore impossible to include all variants, but the selection shown will give an
idea of the sensitivities of fuel burn and time to technique.
8 - APPENDICES Getting to grips with Fuel Economy
- 78 -
APPENDIX A (CLIMB CHARTS)
The climb chart for the A300 is given in the main report (5.2.3)
The A310 shows similar characteristics and is shown below. Note that the
lower mach numbers (0.76, 0.78 and 0.8) show no variation.
The main report gives the climb chart for the A320 which is similar to the
A318 and A319. The A321 however shows more significant differences.
Effect of Climb Technique on fuel and time to 130nm
A310-324 ISA F/L 350 Weight 130000kg
-6.00%
-4.00%
-2.00%
0.00%
2.00%
4.00%
6.00%
-1.00% 0.00% 1.00% 2.00% 3.00% 4.00%
Fuel - %
Time %
FMGS
280
300
320
330
Climb Speed kias
270
100kg
1 minute
M 0.82
Effect of Climb Technique on fuel and time to 230nm
A321-211 ISA F/L 350 Weight 80000kg
-6.00%
-4.00%
-2.00%
0.00%
2.00%
4.00%
6.00%
-2.00% -1.00% 0.00% 1.00% 2.00% 3.00% 4.00% 5.00% 6.00%
Fuel %
Time %
M No
FMGS
0.76
0.8
100 kg
1 minute
0
100
Getting to grips with Fuel Economy 8 - APPENDIXES
- 79 -
The A330 aircraft show characteristics similar to the A321 and an example
is shown below.
The A340-200/300 series show characteristics approaching that of the
A340-500/600 but still shows minimum fuel at a speed lower than max, unlike the
A340-500/600 (see 5.2.3).
Effect of Climb Technique on fuel and time to 220nm
A340-313E ISA F/L 350 Weight 240000kg
-6.00%
-4.00%
-2.00%
0.00%
2.00%
4.00%
6.00%
-1.00% 0.00% 1.00% 2.00% 3.00%
Fuel %
Time %
M No
FMGS
0.76
0.78
0.8
280
300
320
340
Climb Speed kias
0.82
270
100kg
1 minute
Effect of Climb Technique on fuel and time to 150nm
A330-343 ISA F/L 350 Weight 190000kg
-6.00%
-4.00%
-2.00%
0.00%
2.00%
4.00%
6.00%
-1.00% 0.00% 1.00% 2.00% 3.00%
Fuel %
Time %
M No
FMGS
0.8 0.76
280
300
320
340
Climb Speed kias
0.82
270
100 kg
1 minute
0
100
8 - APPENDICES Getting to grips with Fuel Economy
- 80 -
APPENDIX B (DESCENT CHARTS)
The A300 shows similar characteristics to the A310 in the main report.
The A320 shown below has similar characteristics to the A318, A319 and
A321.
Effect of Descent Technique on Fuel and Time for 115nm
A300B4-605R ISA Weight 120000kg
17.5
18.0
18.5
19.0
19.5
20.0
20.5
21.0
21.5
22.0
250 300 350 400 450 500 550 600
Fuel - kg
Time - min
240
250
260
300
320
330
6
28
50
72
100
390 370 350 330 310
Initial Flight Level
Cost
Index
Descent
Speed KIAS
Effect of Descent Technique on Fuel and Time for 120nm
A320-214 ISA Weight 60000kg
16.0
17.0
18.0
19.0
20.0
21.0
22.0
23.0
200 250 300 350 400 450
Fuel - kg
Time - min
240
250
260
300 320
340
0
24
36
50
80
390 370 350 330 310
Initial Flight
Cost
Index
Descent
Speed KIAS
280
9
Getting to grips with Fuel Economy 8 - APPENDIXES
- 81 -
The following shows an example of an A330:
Most of the A340’s have similar speed characteristics to the A340-642
shown below. The A340-200 does however show an improvement in fuel for
speeds lower than 260kts.
Effect of Descent Technique on Fuel and Time for 160nm
A330-343 ISA Weight 180000kg
24.0
25.0
26.0
27.0
28.0
29.0
30.0
500 600 700 800 900 1000 1100
Fuel - kg
Time - min
240
250
260
300
320
340
48
310
390 370 I3n5i0tial Flight Le3v3e0l
Cost
Index
Descent
Speed
KIAS
280
30
CI=0 270kias
CI=60 315kias
CI=100 315kias
Effect of Descent Technique on Fuel and Time for 160nm
A340-642 ISA Weight 270000kg
24.0
25.0
26.0
27.0
28.0
29.0
30.0
700 800 900 1000 1100 1200 1300 1400 1500 1600
Fuel - kg
Time - min
240
250
260
300
320
340
78
310
370 350 330
390
Initial Flight Level
Cost
Index
Descent Speed
KIAS
280
21
CI=0 272kias
CI=100 308kias

sunenjoy

能看看吗？

xheleon

非常感谢楼主发布！！！！

bocome

study hard

leoyuxin

好的，谢谢楼主的分享。

kinran

留下了~~~~~~~~

kinran

留下了~~~~~~~~