直升机飞行手册Rotorcraft flying handbook

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situation where the r.p.m. is low even though you are

using maximum throttle. This is usually the result of

Figure 11-9. In a low G condition, improper corrective action

could lead to the main rotor hub contacting the rotor mast.

The contact with the mast becomes more violent with each

successive flapping motion. This, in turn, creates a greater

flapping displacement. The result could be a severely

damaged rotor mast, or the main rotor system could separate from the helicopter.

11-11

the main rotor blades having an angle of attack that has

created so much drag that engine power is not sufficient to maintain or attain normal operating r.p.m.

If you are in a low r.p.m. situation, the lifting power of

the main rotor blades can be greatly diminished. As soon

as you detect a low r.p.m. condition, immediately apply

additional throttle, if available, while slightly lowering

the collective. This reduces main rotor pitch and drag. As

the helicopter begins to settle, smoothly raise the collective to stop the descent. At hovering altitude you may

have to repeat this technique several times to regain normal operating r.p.m. This technique is sometimes called

“milking the collective.” When operating at altitude, the

collective may have to be lowered only once to regain

rotor speed. The amount the collective can be lowered

depends on altitude. When hovering near the surface,

make sure the helicopter does not contact the ground as

the collective is lowered.

Since the tail rotor is geared to the main rotor, low main

rotor r.p.m. may prevent the tail rotor from producing

enough thrust to maintain directional control. If pedal

control is lost and the altitude is low enough that a

landing can be accomplished before the turning rate

increases dangerously, slowly decrease collective pitch,

maintain a level attitude with cyclic control, and land.

SYSTEM MALFUNCTIONS

The reliability and dependability record of modern

helicopters is very impressive. By following the

manufacturer’s recommendations regarding periodic

maintenance and inspections, you can eliminate most

systems and equipment failures. Most malfunctions or

failures can be traced to some error on the part of the

pilot; therefore, most emergencies can be averted before

they happen. An actual emergency is a rare occurrence.

ANTITORQUE SYSTEM FAILURE

Antitorque failures usually fall into two categories.

One focuses on failure of the power drive portion of the

tail rotor system resulting in a complete loss of antitorque. The other category covers mechanical control

failures where the pilot is unable to change or control

tail rotor thrust even though the tail rotor may still be

providing antitorque thrust.

Tail rotor drive system failures include driveshaft failures, tail rotor gearbox failures, or a complete loss of

the tail rotor itself. In any of these cases, the loss of

antitorque normally results in an immediate yawing of

the helicopter’s nose. The helicopter yaws to the right

in a counter-clockwise rotor system and to the left in a

clockwise system. This discussion assumes a

helicopter with a counter-clockwise rotor system. The

severity of the yaw is proportionate to the amount of

power being used and the airspeed. An antitorque

failure with a high power setting at a low airspeed

results in a severe yawing to the right. At low power

settings and high airspeeds, the yaw is less severe. High

airspeeds tend to streamline the helicopter and keep it

from spinning.

If a tail rotor failure occurs, power has to be reduced in

order to reduce main rotor torque. The techniques

differ depending on whether the helicopter is in flight

or in a hover, but will ultimately require an autorotation.

If a complete tail rotor failure occurs while hovering,

enter a hovering autorotation by rolling off the

throttle. If the failure occurs in forward flight,

enter a normal autorotation by lowering the collective

and rolling off the throttle. If the helicopter has

enough forward airspeed (close to cruising speed) when

the failure occurs, and depending on the helicopter

design, the vertical stabilizer may provide enough directional control to allow you to maneuver the helicopter to

a more desirable landing sight. Some of the yaw may be

compensated for by applying slight cyclic control opposite the direction of yaw. This helps in directional

control, but also increases drag. Care must be taken not

to lose too much forward airspeed because the streamlining effect diminishes as airspeed is reduced. Also,

more altitude is required to accelerate to the

correct airspeed if an autorotation is entered into at a

low airspeed.

A mechanical control failure limits or prevents control of tail rotor thrust and is usually caused by a

stuck or broken control rod or cable. While the tail

rotor is still producing antitorque thrust, it cannot be

controlled by the pilot. The amount of antitorque

depends on the position where the controls jam or

fail. Once again, the techniques differ depending on

the amount of tail rotor thrust, but an autorotation is

generally not required.

LANDING—STUCK LEFT PEDAL

Be sure to follow the procedures and techniques

outlined in the FAA-approved rotorcraft flight manual for the helicopter you are flying. A stuck left

pedal, such as might be experienced during takeoff or

climb conditions, results in the helicopter’s nose

yawing to the left when power is reduced. Rolling off

the throttle and entering an autorotation only makes

matters worse. The landing profile for a stuck left

pedal is best described as a normal approach to a

momentary hover at three to four feet above the

surface. Following an analysis, make the landing. If

the helicopter is not turning, simply lower the

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helicopter to the surface. If the helicopter is turning

to the right, roll the throttle toward flight idle the

amount necessary to stop the turn as you land. If the

helicopter is beginning to turn left, you should be

able to make the landing prior to the turn rate

becoming excessive. However, if the turn rate

becomes excessive prior to the landing, simply

execute a takeoff and return for another landing.

11-12

LANDING—STUCK NEUTRAL OR RIGHT PEDAL

The landing profile for a stuck neutral or a stuck right

pedal is a low power approach or descent with a

running or roll-on landing. The approach profile can

best be described as a steep approach with a flare at the

bottom to slow the helicopter. The power should be low

enough to establish a left yaw during the descent. The

left yaw allows a margin of safety due to the fact that

the helicopter will turn to the right when power is

applied. This allows the momentary use of power at the

bottom of the approach. As you apply power, the helicopter rotates to the right and becomes aligned with the

landing area. At this point, roll the throttle to flight idle

and make the landing. The momentary use of power

helps stop the descent and allows additional time for

you to level the helicopter prior to closing the throttle.

If the helicopter is not yawed to the left at the conclusion

of the flare, roll the throttle to flight idle and use the

collective to cushion the touchdown. As with any

running or roll-on landing, use the cyclic to maintain the

ground track. This technique results in a longer ground

run or roll than if the helicopter was yawed to the left.

UNANTICIPATED YAW / LOSS OF TAIL

ROTOR EFFECTIVENESS (LTE)

Unanticipated yaw is the occurrence of an uncommanded yaw rate that does not subside of its own

accord and, which, if not corrected, can result in the

loss of helicopter control. This uncommanded yaw rate

is referred to as loss of tail rotor effectiveness (LTE)

and occurs to the right in helicopters with a counterclockwise rotating main rotor and to the left in helicopters with a clockwise main rotor rotation. Again, this

discussion covers a helicopter with a counter-clockwise

rotor system and an antitorque rotor.

LTE is not related to an equipment or maintenance malfunction and may occur in all single-rotor helicopters

at airspeeds less than 30 knots. It is the result of the tail

rotor not providing adequate thrust to maintain directional control, and is usually caused by either certain

wind azimuths (directions) while hovering, or by an

insufficient tail rotor thrust for a given power setting at

higher altitudes.

For any given main rotor torque setting in perfectly

steady air, there is an exact amount of tail rotor thrust

required to prevent the helicopter from yawing either

left or right. This is known as tail rotor trim thrust. In

order to maintain a constant heading while hovering,

you should maintain tail rotor thrust equal to trim thrust.

The required tail rotor thrust is modified by the effects

of the wind. The wind can cause an uncommanded yaw

by changing tail rotor effective thrust. Certain relative

wind directions are more likely to cause tail rotor thrust

variations than others. Flight and wind tunnel tests

have identified three relative wind azimuth regions that

can either singularly, or in combination, create an LTE

conducive environment. These regions can overlap,

and thrust variations may be more pronounced. Also,

flight testing has determined that the tail rotor does not

actually stall during the period. When operating in

these areas at less than 30 knots, pilot workload

increases dramatically.

MAIN ROTOR DISC INTERFERENCE

(285-315°)

Refer to figure 11-10. Winds at velocities of 10 to 30

knots from the left front cause the main rotor

vortex to be blown into the tail rotor by the relative

wind. The effect of this main rotor disc vortex causes

the tail rotor to operated in an extremely turbulent environment. During a right turn, the tail rotor experiences

a reduction of thrust as it comes into the area of the

main rotor disc vortex. The reduction in tail rotor thrust

comes from the airflow changes experienced at the tail

rotor as the main rotor disc vortex moves across the tail

rotor disc. The effect of the main rotor disc vortex

initially increases the angle of attack of the tail rotor

blades, thus increasing tail rotor thrust. The increase in

the angle of attack requires that right pedal pressure be

added to reduce tail rotor thrust in order to maintain the

same rate of turn. As the main rotor vortex passes the

tail rotor, the tail rotor angle of attack is reduced. The

reduction in the angle of attack causes a reduction in

thrust and a right yaw acceleration begins. This acceleration can be surprising, since you were previously

adding right pedal to maintain the right turn rate. This

thrust reduction occurs suddenly, and if uncorrected,

develops into an uncontrollable rapid rotation about the

mast. When operating within this region, be aware that

the reduction in tail rotor thrust can happen quite

suddenly, and be prepared to react quickly to counter

this reduction with additional left pedal input.

Figure 11-10. Main rotor disc vortex interference.

300°

330°

285°

270°

240°

210° 150°

120°

90°

60°

30°

15 Knots

20 Knots

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10 Knots

0°

360°

Region of Disc

Vortex Interference

315°

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WEATHERCOCK STABILITY

(120-240°)

In this region, the helicopter attempts to weathervane

its nose into the relative wind. [Figure 11-11] Unless a

resisting pedal input is made, the helicopter starts a

slow, uncommanded turn either to the right or left

depending upon the wind direction. If the pilot allows a

right yaw rate to develop and the tail of the helicopter

moves into this region, the yaw rate can accelerate

rapidly. In order to avoid the onset of LTE in this

downwind condition, it is imperative to maintain positive control of the yaw rate and devote full attention to

flying the helicopter.

Figure 11-11. Weathercock stability.

TAIL ROTOR VORTEX RING STATE

(210-330°)

Winds within this region cause a tail rotor vortex ring

state to develop. [Figure 11-12] The result is a non-uniform, unsteady flow into the tail rotor. The vortex ring

state causes tail rotor thrust variations, which result in

yaw deviations. The net effect of the unsteady flow is

an oscillation of tail rotor thrust. Rapid and continuous

pedal movements are necessary to compensate for the

rapid changes in tail rotor thrust when hovering in a left

crosswind. Maintaining a precise heading in this region

is difficult, but this characteristic presents no significant problem unless corrective action is delayed.

However, high pedal workload, lack of concentration

and overcontrolling can all lead to LTE.

When the tail rotor thrust being generated is less than

the thrust required, the helicopter yaws to the right.

When hovering in left crosswinds, you must concentrated on smooth pedal coordination and not allow an

uncontrolled right yaw to develop. If a right yaw rate

is allowed to build, the helicopter can rotate into the

wind azimuth region where weathercock stability then

accelerates the right turn rate. Pilot workload during a

tail rotor vortex ring state is high. Do not allow a right

yaw rate to increase.

Figure 11-12. Tail rotor vortex ring state.

LTE AT ALTITUDE

At higher altitudes, where the air is thinner, tail rotor

thrust and efficiency is reduced. When operating at

high altitudes and high gross weights, especially while

hovering, the tail rotor thrust may not be sufficient to

maintain directional control and LTE can occur. In this

case, the hovering ceiling is limited by tail rotor thrust

and not necessarily power available. In these conditions gross weights need to be reduced and/or

operations need to be limited to lower density altitudes.

REDUCING THE ONSET OF LTE

To help reduce the onset of loss of tail rotor effectiveness, there are some steps you can follow.

1. Maintain maximum power-on rotor r.p.m. If the

main rotor r.p.m. is allowed to decrease, the antitorque thrust available is decreased proportionally.

2. Avoid tailwinds below an airspeed of 30 knots. If

loss of translational lift occurs, it results in an

increased power demand and additional antitorque pressures.

3. Avoid out of ground effect (OGE) operations and

high power demand situations below an airspeed

of 30 knots.

4. Be especially aware of wind direction and velocity

when hovering in winds of about 8-12 knots. There

are no strong indicators that translational lift has

been reduced. A loss of translational lift results in

an unexpected high power demand and an

increased antitorque requirement.

Region Where Weathercock

Stability Can Introduce Yaw Rates

360°

0°

15 Knots

10 Knots

5 Knots

17 Knots

30°

60°

90°

120°

150°

180°

210°

240°

270°

300°

330°

17 Knots

15 Knots

10 Knots

5 Knots

0°

180°

150°

30°

120°

60°

90°

210°

240°

270°

300°

330°

360°

Region of

Roughness

Due toTail

Rotor Vortex

Ring State

F

11-14

5. Be aware that if a considerable amount of left

pedal is being maintained, a sufficient amount of

left pedal may not be available to counteract an

unanticipated right yaw.

6. Be alert to changing wind conditions, which may

be experienced when flying along ridge lines and

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around buildings.

RECOVERY TECHNIQUE

If a sudden unanticipated right yaw occurs, the following recovery technique should be performed. Apply full

left pedal while simultaneously moving cyclic control

forward to increase speed. If altitude permits, reduce

power. As recovery is effected, adjust controls for

normal forward flight.

Collective pitch reduction aids in arresting the yaw rate

but may cause an excessive rate of descent. Any large,

rapid increase in collective to prevent ground or

obstacle contact may further increase the yaw rate and

decrease rotor r.p.m. The decision to reduce collective

must be based on your assessment of the altitude

available for recovery.

If the rotation cannot be stopped and ground contact is

imminent, an autorotation may be the best course of

action. Maintain full left pedal until the rotation stops,

then adjust to maintain heading.

MAIN DRIVE SHAFT FAILURE

The main drive shaft, located between the engine and

the main rotor gearbox, transmits engine power to the

main rotor gearbox. In some helicopters, particularly

those with piston engines, a drive belt is used instead of

a drive shaft. A failure of the drive shaft or belt has the

same effect as an engine failure, because power is no

longer provided to the main rotor, and an autorotation

has to be initiated. There are a few differences,

however, that need to be taken into consideration. If the

drive shaft or belt breaks, the lack of any load on the

engine results in an overspeed. In this case, the throttle

must be closed in order to prevent any further damage.

In some helicopters, the tail rotor drive system

continues to be powered by the engine even if the main

drive shaft breaks. In this case, when the engine

unloads, a tail rotor overspeed can result. If this happens, close the throttle immediately and enter an

autorotation.

HYDRAULIC FAILURES

Most helicopters, other than smaller piston powered

helicopters, incorporate the use of hydraulic actuators

to overcome high control forces. A hydraulic system

consists of actuators, also called servos, on each flight

control; a pump, which is usually driven by the main

rotor gearbox; and a reservoir to store the hydraulic

fluid. A switch in the cockpit can turn the system off,

although it is left on under normal conditions. A

pressure indicator in the cockpit may be installed to

monitor the system.

An impending hydraulic failure can be recognized by a

grinding or howling noise from the pump or actuators,

increased control forces and feedback, and limited

control movement. The corrective action required is

stated in detail in the appropriate rotorcraft flight

manual. However, in most cases, airspeed needs to be

reduced in order to reduce control forces. The hydraulic

switch and circuit breaker should be checked and

recycled. If hydraulic power is not restored, make a

shallow approach to a running or roll-on landing. This

technique is used because it requires less control force

and pilot workload. Additionally, the hydraulic system

should be disabled, by either pulling the circuit breaker

and/or placing the switch in the off position. The

reason for this is to prevent an inadvertent restoration

of hydraulic power, which may lead to overcontrolling

near the ground.

In those helicopters where the control forces are so

high that they cannot be moved without hydraulic

assistance, two or more independent hydraulic systems

may be installed. Some helicopters use hydraulic accumulators to store pressure that can be used for a short

time while in an emergency if the hydraulic pump fails.

This gives you enough time to land the helicopter with

normal control.

GOVERNOR FAILURE

Governors automatically adjust engine power to maintain rotor r.p.m. when the collective pitch is changed. If

the governor fails, any change in collective pitch

requires you to manually adjust the throttle to maintain

correct r.p.m. In the event of a high side governor

failure, the engine and rotor r.p.m. try to increase above

the normal range. If the r.p.m. cannot be reduced and

controlled with the throttle, close the throttle and enter

an autorotation. If the governor fails on the low side,

normal r.p.m. may not be attainable, even if the throttle

is manually controlled. In this case, the collective has

to be lowered to maintain r.p.m. A running or roll-on

landing may be performed if the engine can maintain

sufficient rotor r.p.m. If there is insufficient power,

enter an autorotation.

ABNORMAL VIBRATIONS

With the many rotating parts found in helicopters, some

vibration is inherent. You need to understand the cause

and effect of helicopter vibrations because abnormal

vibrations cause premature component wear and may

even result in structural failure. With experience, you

learn what vibrations are normal versus those that are

abnormal and can then decide whether continued flight

is safe or not. Helicopter vibrations are categorized into

low, medium, or high frequency.

11-15

LOW FREQUENCY VIBRATIONS

Low frequency vibrations (100-500 cycles per minute)

usually originate from the main rotor system. The

vibration may be felt through the controls, the airframe,

or a combination of both. Furthermore, the vibration

may have a definite direction of push or thrust. It may

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be vertical, lateral, horizontal, or even a combination.

Normally, the direction of the vibration can be determined by concentrating on the feel of the vibration,

which may push you up and down, backwards and

forwards, or from side to side. The direction of the

vibration and whether it is felt in the controls or the

airframe is an important means for the mechanic

to troubleshoot the source. Some possible causes

could be that the main rotor blades are out of track or

balance, damaged blades, worn bearings, dampers out

of adjustment, or worn parts.

MEDIUM AND HIGH FREQUENCY VIBRATIONS

Medium frequency vibrations (1,000 - 2,000 cycles per

minute) and high frequency vibrations (2,000 cycles

per minute or higher) are normally associated with outof-balance components that rotate at a high r.p.m., such

as the tail rotor, engine, cooling fans, and components

of the drive train, including transmissions, drive shafts,

bearings, pulleys, and belts. Most tail rotor vibrations

can be felt through the tail rotor pedals as long as there

are no hydraulic actuators, which usually dampen out

the vibration. Any imbalance in the tail rotor system is

very harmful, as it can cause cracks to develop and

rivets to work loose. Piston engines usually produce a

normal amount of high frequency vibration, which is

aggravated by engine malfunctions such as spark plug

fouling, incorrect magneto timing, carburetor icing

and/or incorrect fuel/air mixture. Vibrations in turbine

engines are often difficult to detect as these engines

operate at a very high r.p.m.

TRACKING AND BALANCE

Modern equipment used for tracking and balancing the

main and tail rotor blades can also be used to detect

other vibrations in the helicopter. These systems use

accelerometers mounted around the helicopter to detect

the direction, frequency, and intensity of the vibration.

The built-in software can then analyze the information,

pinpoint the origin of the vibration, and suggest the

corrective action.

FLIGHT DIVERSION

There will probably come a time in your flight career

when you will not be able to make it to your destination.

This can be the result of unpredictable weather conditions,

a system malfunction, or poor preflight planning. In any

case, you will need to be able to safely and efficiently

divert to an alternate destination. Before any crosscountry flight, check the charts for airports or suitable

landing areas along or near your route of flight. Also,

check for navaids that can be used during a diversion.

Computing course, time, speed, and distance information in flight requires the same computations used

during preflight planning. However, because of the

limited cockpit space, and because you must divide

your attention between flying the helicopter, making

calculations, and scanning for other aircraft, you should

take advantage of all possible shortcuts and rule-ofthumb computations.

When in flight, it is rarely practical to actually plot a

course on a sectional chart and mark checkpoints and

distances. Furthermore, because an alternate airport is

usually not very far from your original course, actual

plotting is seldom necessary.

A course to an alternate can be measured accurately

with a protractor or plotter, but can also be measured

with reasonable accuracy using a straightedge and the

compass rose depicted around VOR stations. This

approximation can be made on the basis of a radial

from a nearby VOR or an airway that closely parallels

the course to your alternate. However, you must

remember that the magnetic heading associated with

a VOR radial or printed airway is outbound from

the station. To find the course TO the station, it may

be necessary to determine the reciprocal of the

indicated heading.

Distances can be determined by using a plotter, or by

placing a finger or piece of paper between the two and

then measuring the approximate distance on the

mileage scale at the bottom of the chart.

Before changing course to proceed to an alternate, you

should first consider the relative distance and route of

flight to all suitable alternates. In addition, you should

consider the type of terrain along the route. If circumstances warrant, and your helicopter is equipped with

navigational equipment, it is typically easier to navigate to an alternate airport that has a VOR or NDB

facility on the field.

After you select the most appropriate alternate, approximate the magnetic course to the alternate using

a compass rose or airway on the sectional chart. If time

permits, try to start the diversion over a prominent

ground feature. However, in an emergency, divert

promptly toward your alternate. To complete all

plotting, measuring, and computations involved before

diverting to the alternate may only aggravate an

actual emergency.

Once established on course, note the time, and then

use the winds aloft nearest to your diversion point to

calculate a heading and groundspeed. Once you have

calculated your groundspeed, determine a new arrival

time and fuel consumption.

11-16

You must give priority to flying the helicopter while

dividing your attention between navigation and

planning. When determining an altitude to use while

diverting, you should consider cloud heights, winds,

terrain, and radio reception.

LOST PROCEDURES

Getting lost in an aircraft is a potentially dangerous

situation especially when low on fuel. Helicopters have

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an advantage over airplanes, as they can land almost

anywhere before they run out of fuel.

If you are lost, there are some good common sense

procedures to follow. If you are nowhere near or cannot

see a town or city, the first thing you should do is climb.

An increase in altitude increases radio and navigation

reception range, and also increases radar coverage. If

you are flying near a town or city, you may be able to

read the name of the town on a water tower or even land

to ask directions.

If your helicopter has a navigational radio, such as a

VOR or ADF receiver, you can possibly determine

your position by plotting your azimuth from two or

more navigational facilities. If GPS is installed, or you

have a portable aviation GPS on board, you can use it

to determine your position and the location of the

nearest airport.

Communicate with any available facility using

frequencies shown on the sectional chart. If you are

able to communicate with a controller, you may be

offered radar vectors. Other facilities may offer

direction finding (DF) assistance. To use this

procedure, the controller will request you to hold

down your transmit button for a few seconds and

then release it. The controller may ask you to change

directions a few times and repeat the transmit

procedure. This gives the controller enough information to plot your position and then give you vectors to a suitable landing sight. If your situation

becomes threatening, you can transmit your problems on the emergency frequency 121.5 MHZ and

set your transponder to 7700. Most facilities, and

even airliners, monitor the emergency frequency.

EMERGENCY EQUIPMENT AND

SURVIVAL GEAR

Both Canada and Alaska require pilots to carry survival

gear. However, it is good common sense that any time

you are flying over rugged and desolated terrain, consider carrying survival gear. Depending on the size and

storage capacity of your helicopter, the following are

some suggested items:

• Food that is not subject to deterioration due to

heat or cold. There should be at least 10,000 calo-

ries for each person on board, and it should be

stored in a sealed waterproof container. It should

have been inspected by the pilot or his representative within the previous six months, and bear a

label verifying the amount and satisfactory condition of the contents.

• A supply of water.

• Cooking utensils.

• Matches in a waterproof container.

• A portable compass.

• An ax at least 2.5 pounds with a handle not less

than 28 inches in length.

• A flexible saw blade or equivalent cutting tool.

• 30 feet of snare wire and instructions for use.

• Fishing equipment, including still-fishing bait

and gill net with not more than a two inch mesh.

• Mosquito nets or netting and insect repellent

sufficient to meet the needs of all persons aboard,

when operating in areas where insects are likely

to be hazardous.

• A signaling mirror.

• At least three pyrotechnic distress signals.

• A sharp, quality jackknife or hunting knife.

• A suitable survival instruction manual.

• Flashlight with spare bulbs and batteries.

• Portable ELT with spare batteries.

Additional items when there are no trees:

• Stove with fuel or a self-contained means of providing heat for cooking.

• Tent(s) to accommodate everyone on board.

Additional items for winter operations:

• Winter sleeping bags for all persons when the

temperature is expected to be below 7°C.

• Two pairs of snow shoes.

• Spare ax handle.

• Honing stone or file.

• Ice chisel.

• Snow knife or saw knife.

12-1

Attitude instrument flying in helicopters is essentially

visual flying with the flight instruments substituted for

the various reference points on the helicopter and the

natural horizon. Control changes, required to produce a

given attitude by reference to instruments, are identical

to those used in helicopter VFR flight, and your

thought processes are the same. Basic instrument training is intended as a building block towards attaining an

instrument rating. It will also enable you to do a 180°

turn in case of inadvertent incursion into instrument

meteorological conditions (IMC).

FLIGHT INSTRUMENTS

When flying a helicopter with reference to the flight

instruments, proper instrument interpretation is the

basis for aircraft control. Your skill, in part, depends on

your understanding of how a particular instrument or

system functions, including its indications and limitations. With this knowledge, you can quickly determine

what an instrument is telling you and translate that

information into a control response.

PITOT-STATIC INSTRUMENTS

The pitot-static instruments, which include the airspeed

indicator, altimeter, and vertical speed indicator, operate on the principle of differential air pressure. Pitot

pressure, also called impact, ram, or dynamic pressure,

is directed only to the airspeed indicator, while static

pressure, or ambient pressure, is directed to all three

instruments. An alternate static source may be included

allowing you to select an alternate source of ambient

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pressure in the event the main port becomes blocked.

[Figure 12-1]

AIRSPEED INDICATOR

The airspeed indicator displays the speed of the helicopter through the air by comparing ram air pressure

from the pitot tube with static air pressure from the

static port—the greater the differential, the greater the

speed. The instrument displays the result of this pressure differential as indicated airspeed (IAS).

Manufacturers use this speed as the basis for determining helicopter performance, and it may be displayed in

knots, miles per hour, or both. [Figure 12-2] When an

indicated airspeed is given for a particular situation,

you normally use that speed without making a correction for altitude or temperature. The reason no correc-

tion is needed is that an airspeed indicator and aircraft

performance are affected equally by changes in air density. An indicated airspeed always yields the same

performance because the indicator has, in fact, compensated for the change in the environment.

INSTRUMENT CHECK—During the preflight, ensure

that the pitot tube, drain hole, and static ports are unobstructed. Before liftoff, make sure the airspeed indicator

is reading zero. If there is a strong wind blowing directly

at the helicopter, the airspeed indicator may read higher

Pitot

Heater Switch

Pitot

Tube

Airspeed

Indicator

Vertical

Speed

Indicator

(VSI) Altimeter

Drain

Opening

Static Port

ON

OFF

Alternate Static Source

ALT

STATIC AIR

PULL ON

Figure 12-1. Ram air pressure is supplied only to the airspeed

indicator, while static pressure is used by all three instruments. Electrical heating elements may be installed to prevent ice from forming on the pitot tube. A drain opening to

remove moisture is normally included.

Diaphragm

Static Air Line

Ram Air

Pitot Tube

Figure 12-2. Ram air pressure from the pitot tube is directed

to a diaphragm inside the airspeed indicator. The airtight

case is vented to the static port. As the diaphragm expands

or contracts, a mechanical linkage moves the needle on the

face of the indicator.

12-2

than zero, depending on the wind speed and direction.

As you begin your takeoff, make sure the airspeed indicator is increasing at an appropriate rate. Keep in mind,

however, that the airspeed indication might be unreliable below a certain airspeed due to rotor downwash.

ALTIMETER

The altimeter displays altitude in feet by sensing pressure changes in the atmosphere. There is an adjustable

barometric scale to compensate for changes in atmospheric pressure. [Figure 12-3]

The basis for altimeter calibration is the International

Standard Atmosphere (ISA), where pressure, temperature, and lapse rates have standard values. However,

actual atmospheric conditions seldom match the standard values. In addition, local pressure readings within

a given area normally change over a period of time, and

pressure frequently changes as you fly from one area to

another. As a result, altimeter indications are subject to

errors, the extent of which depends on how much the

pressure, temperature, and lapse rates deviate from standard, as well as how recently you have set the altimeter.

The best way to minimize altimeter errors is to update

the altimeter setting frequently. In most cases, use the

current altimeter setting of the nearest reporting station

along your route of flight per regulatory requirements.

INSTRUMENT CHECK—During the preflight, ensure

that the static ports are unobstructed. Before lift-off, set

the altimeter to the current setting. If the altimeter indicates within 75 feet of the actual elevation, the altimeter

is generally considered acceptable for use.

VERTICAL SPEED INDICATOR

The vertical speed indicator (VSI) displays the rate of

climb or descent in feet per minute (f.p.m.) by measuring how fast the ambient air pressure increases or

decreases as the helicopter changes altitude. Since the

VSI measures only the rate at which air pressure

changes, air temperature has no effect on this instrument. [Figure 12-4]

There is a lag associated with the reading on the VSI,

and it may take a few seconds to stabilize when showing rate of climb or descent. Rough control technique

and turbulence can further extend the lag period and

cause erratic and unstable rate indications. Some aircraft are equipped with an instantaneous vertical speed

indicator (IVSI), which incorporates accelerometers to

compensate for the lag found in the typical VSI.

INSTRUMENT CHECK—During the preflight, ensure

that the static ports are unobstructed. Check to see that

the VSI is indicating zero before lift-off. During takeoff,

check for a positive rate of climb indication.

SYSTEM ERRORS

The pitot-static system and associated instruments are

usually very reliable. Errors are generally caused when

the pitot or static openings are blocked. This may be

caused by dirt, ice formation, or insects. Check the pitot

and static openings for obstructions during the preflight.

It is also advisable to place covers on the pitot and static

ports when the helicopter is parked on the ground.

The airspeed indicator is the only instrument affected by a

blocked pitot tube. The system can become clogged in two

Aneroid

Wafers

Altimeter

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Setting Window

Altitude

Indication

Scale

10,000 ft

Pointer

1,000 ft

Pointer

100 ft Pointer

Altimeter Setting

Adjustment Knob

Crosshatch

Flag

A crosshatched

area appears

on some altimeters

when displaying

an altitude below

10,000 feet MSL.

Static Port

Figure 12-3. The main component of the altimeter is a stack of

sealed aneroid wafers. They expand and contract as atmospheric pressure from the static source changes. The mechanical linkage translates these changes into pointer movements on

the indicator.

Diaphragm

Direct Static

Pressure

Calibrated

Leak

Figure 12-4. Although the sealed case and diaphragm are

both connected to the static port, the air inside the case is

restricted through a calibrated leak. When the pressures are

equal, the needle reads zero. As you climb or descend, the

pressure inside the diaphragm instantly changes, and the

needle registers a change in vertical direction. When the

pressure differential stabilizes at a definite ratio, the needle

registers the rate of altitude change.

12-3

ways. If the ram air inlet is clogged, but the drain hole

remains open, the airspeed indicator registers zero, regardless of airspeed. If both the ram air inlet and the drain hole

become blocked, pressure in the line is trapped, and the

airspeed indicator reacts like an altimeter, showing an

increase in airspeed with an increase in altitude, and a

decrease in speed as altitude decreases. This occurs as

long as the static port remains unobstructed.

If the static port alone becomes blocked, the airspeed

indicator continues to function, but with incorrect readings. When you are operating above the altitude where

the static port became clogged, the airspeed indicator

reads lower than it should. Conversely, when operating

below that altitude, the indicator reads higher than the

correct value. The amount of error is proportional to

the distance from the altitude where the static system

became blocked. The greater the difference, the greater

the error. With a blocked static system, the altimeter

freezes at the last altitude and the VSI freezes at zero.

Both instruments are then unusable.

Some helicopters are equipped with an alternate static

source, which may be selected in the event that the main

static system becomes blocked. The alternate source generally vents into the cabin, where air pressures are slightly

different than outside pressures, so the airspeed and

altimeter usually read higher than normal. Correction

charts may be supplied in the flight manual.

GYROSCOPIC INSTRUMENTS

The three gyroscopic instruments that are required for

instrument flight are the attitude indicator, heading

indicator, and turn indicator. When installed in helicopters, these instruments are usually electrically powered.

Gyros are affected by two principles—rigidity in space and

precession. Rigidity in space means that once a gyro is

spinning, it tends to remain in a fixed position and resists

external forces applied to it. This principle allows a gyro to

be used to measure changes in attitude or direction.

Precession is the tilting or turning of a gyro in response to

pressure. The reaction to this pressure does not occur at

the point where it was applied; rather, it occurs at a point

that is 90° later in the direction of rotation from where the

pressure was applied. This principle allows the gyro to

determine a rate of turn by sensing the amount of pressure created by a change in direction. Precession can also

create some minor errors in some instruments.

ATTITUDE INDICATOR

The attitude indicator provides a substitute for the natural horizon. It is the only instrument that provides an

immediate and direct indication of the helicopter’s

pitch and bank attitude. Since most attitude indicators

installed in helicopters are electrically powered, there

may be a separate power switch, as well as a warning

flag within the instrument, that indicates a loss of

power. A caging or “quick erect” knob may be

included, so you can stabilize the spin axis if the gyro

has tumbled. [Figure 12-5]

HEADING INDICATOR

The heading indicator, which is sometimes referred to

as a directional gyro (DG), senses movement around

the vertical axis and provides a more accurate heading

reference compared to a magnetic compass, which has

a number of turning errors. [Figure 12-6].

Bank Index

Gyro

Gimbal

Rotation

Roll

Gimbal

Pitch

Gimbal

Horizon

Reference

Arm

Figure 12-5. The gyro in the attitude indicator spins in the

horizontal plane. Two mountings, or gimbals, are used so

that both pitch and roll can be sensed simultaneously. Due to

rigidity in space, the gyro remains in a fixed position relative

to the horizon as the case and helicopter rotate around it.

Adjustment Gears

Adjustment

Knob

Gimbal

Rotation

Gimbal Gyro

Main

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Drive Gear

Compass

Card Gear

Figure 12-6. A heading indicator displays headings based on

a 360° azimuth, with the final zero omitted. For example, a 6

represents 060°, while a 21 indicates 210°. The adjustment

knob is used to align the heading indicator with the magnetic

compass.

12-4

Due to internal friction within the gyroscope, precession is common in heading indicators. Precession

causes the selected heading to drift from the set value.

Some heading indicators receive a magnetic north reference from a remote source and generally need no

adjustment. Heading indicators that do not have this

automatic north-seeking capability are often called

“free” gyros, and require that you periodically adjust

them. You should align the heading indicator with the

magnetic compass before flight and check it at 15-

minute intervals during flight. When you do an in-flight

alignment, be certain you are in straight-and-level,

unaccelerated flight, with the magnetic compass showing a steady indication.

TURN INDICATORS

Turn indicators show the direction and the rate of turn.

A standard rate turn is 3° per second, and at this rate

you will complete a 360° turn in two minutes. A halfstandard rate turn is 1.5° per second. Two types of

indicators are used to display this information. The

turn-and-slip indicator uses a needle to indicate direction and turn rate. When the needle is aligned with the

white markings, called the turn index, you are in a

standard rate turn. A half-standard rate turn is indicated when the needle is halfway between the indexes.

The turn-and-slip indicator does not indicate roll rate.

The turn coordinator is similar to the turn-and-slip

indicator, but the gyro is canted, which allows it to

sense roll rate in addition to rate of turn. The turn coordinator uses a miniature aircraft to indicate direction,

as well as the turn and roll rate. [Figure 12-7]

Another part of both the turn coordinator and the turnand-slip indicator is the inclinometer. The position of

the ball defines whether the turn is coordinated or not.

The helicopter is either slipping or skidding anytime

the ball is not centered, and usually requires an adjustment of the antitorque pedals or angle of bank to correct it. [Figure 12-8]

INSTRUMENT CHECK—During your preflight, check

to see that the inclinometer is full of fluid and has no

air bubbles. The ball should also be resting at its lowest

point. Since almost all gyroscopic instruments installed

in a helicopter are electrically driven, check to see that

the power indicators are displaying off indications.

Turn the master switch on and listen to the gyros spool

up. There should be no abnormal sounds, such as a

grinding sound, and the power out indicator flags

should not be displayed. After engine start and before

liftoff, set the direction indicator to the magnetic compass. During hover turns, check the heading indicator

for proper operation and ensure that it has not precessed significantly. The turn indicator should also

indicate a turn in the correct direction. During takeoff,

check the attitude indicator for proper indication and

recheck it during the first turn.

MAGNETIC COMPASS

In some helicopters, the magnetic compass is the only

direction seeking instrument. Although the compass

appears to move, it is actually mounted in such a way

that the helicopter turns about the compass card as the

card maintains its alignment with magnetic north.

COMPASS ERRORS

The magnetic compass can only give you reliable

directional information if you understand its limitations

and inherent errors. These include magnetic variation,

compass deviation, and magnetic dip.

MAGNETIC VARIATION

When you fly under visual flight rules, you ordinarily navigate by referring to charts, which are oriented

Figure 12-7. The gyros in both the turn-and-slip indicator and

the turn coordinator are mounted so that they rotate in a vertical plane. The gimbal in the turn coordinator is set at an angle,

or canted, which means precession allows the gyro to sense

both rate of roll and rate of turn. The gimbal in the turn-and-slip

indicator is horizontal. In this case, precession allows the gyro

to sense only rate of turn. When the needle or miniature aircraft

is aligned with the turn index, you are in a standard-rate turn.

Gyro

Rotation

Gimbal

Rotation

TURN-AND-SLIP

INDICATOR

Gimbal

Gimbal

Rotation

Gyro

Rotation

Canted Gyro

TURN

COORDINATOR

Horizontal

Gyro

Inclinometer

Figure 12-8. In a coordinated turn (instrument 1), the ball is

centered. In a skid (instrument 2), the rate of turn is too great

for the angle of bank, and the ball moves to the outside of the

turn. Conversely, in a slip (instrument 3), the rate of turn is

too small for the angle of bank, and the ball moves to the

inside of the turn.

12-5

to true north. Because the aircraft compass is oriented

to magnetic north, you must make allowances for the

difference between these poles in order to navigate

properly. You do this by applying a correction called

variation to convert a true direction to a magnet direction. Variation at a given point is the angular difference between the true and magnetic poles. The amount

of variation depends on where you are located on the

earth’s surface. Isogonic lines connect points where

the variation is equal, while the agonic line defines the

points where the variation is zero. [Figure 12-9]

COMPASS DEVIATION

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Besides the magnetic fields generated by the earth, other

magnetic fields are produced by metal and electrical

accessories within the helicopter. These magnetic fields

distort the earth’s magnet force and cause the compass

to swing away from the correct heading. Manufacturers

often install compensating magnets within the compass

housing to reduce the effects of deviation. These magnets are usually adjusted while the engine is running and

all electrical equipment is operating. Deviation error,

however, cannot be completely eliminated; therefore, a

compass correction card is mounted near the compass.

The compass correction card corrects for deviation that

occurs from one heading to the next as the lines of force

interact at different angles.

MAGNETIC DIP

Magnetic dip is the result of the vertical component of

the earth’s magnetic field. This dip is virtually nonexistent at the magnetic equator, since the lines of force

are parallel to the earth’s surface and the vertical component is minimal. As you move a compass toward the

poles, the vertical component increases, and magnetic

dip becomes more apparent at these higher latitudes.

Magnetic dip is responsible for compass errors during

acceleration, deceleration, and turns.

Acceleration and deceleration errors are fluctuations

in the compass during changes in speed. In the northern hemisphere, the compass swings toward the north

during acceleration and toward the south during deceleration. When the speed stabilizes, the compass

returns to an accurate indication. This error is most

pronounced when you are flying on a heading of east

or west, and decreases gradually as you fly closer to a

north or south heading. The error does not occur when

you are flying directly north or south. The memory

aid, ANDS (Accelerate North, Decelerate South) may

help you recall this error. In the southern hemisphere,

this error occurs in the opposite direction.

Turning errors are most apparent when you are turning

to or from a heading of north or south. This error

increases as you near the poles as magnetic dip becomes

more apparent. There is no turning error when flying

near the magnetic equator. In the northern hemisphere,

when you make a turn from a northerly heading, the

compass gives an initial indication of a turn in the

opposite direction. It then begins to show the turn in

the proper direction, but lags behind the actual heading. The amount of lag decreases as the turn continues,

then disappears as the helicopter reaches a heading of

east or west. When you make a turn from a southerly

heading, the compass gives an indication of a turn in

the correct direction, but leads the actual heading. This

error also disappears as the helicopter approaches an

east or west heading.

INSTRUMENT CHECK—Prior to flight, make sure that

the compass is full of fluid. During hover turns, the

compass should swing freely and indicate known headings. Since that magnetic compass is required for all

flight operations, the aircraft should never be flown

with a faulty compass.

INSTRUMENT FLIGHT

To achieve smooth, positive control of the helicopter

during instrument flight, you need to develop three

fundamental skills. They are instrument cross-check,

instrument interpretation, and aircraft control.

INSTRUMENT CROSS-CHECK

Cross-checking, sometimes referred to as scanning, is

the continuous and logical observation of instruments

for attitude and performance information. In attitude

instrument flying, an attitude is maintained by reference

to the instruments, which produces the desired result in

performance. Due to human error, instrument error, and

helicopter performance differences in various atmospheric and loading conditions, it is difficult to

establish an attitude and have performance remain

constant for a long period of time. These variables make

A

True

North Pole

Magnetic

North Pole

Agonic

Line

20°

20°

15°

15°

10° 5°

5°

0°

Isogonic Lines

17°

10°

Figure 12-9. Variation at point A in the western United States

is 17°. Since the magnetic north pole is located to the east of

the true north pole in relation to this point, the variation is

easterly. When the magnetic pole falls to the west of the true

north pole, variation is westerly.

12-6

it necessary for you to constantly check the instruments

and make appropriate changes in the helicopter’s attitude. The actual technique may vary depending on what

instruments are installed and where they are installed,

as well as your experience and proficiency level. For

this discussion, we will concentrate on the six basic

flight instruments discussed earlier. [Figure 12-10]

At first, you may have a tendency to cross-check

rapidly, looking directly at the instruments without

knowing exactly what information you are seeking.

However, with familiarity and practice, the instrument

cross-check reveals definite trends during specific

flight conditions. These trends help you control the

helicopter as it makes a transition from one flight

condition to another.

If you apply your full concentration to a single instrument,