pressure points

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pressure points

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36 FLIGHT SAFETY AUSTRALIA MARCH–APRIL 2006
FLYING OPERATIONS
Loss of pressure is a big killer. The
internationally respected Air Safety
Network lists six major regular public
transport (RPT) accidents involving
depressurisation since 1970, with a combined
death toll of 719.
The most recent – and infamous – was
the crash of Helios Airways Flight 522 on
August 14 last year, when all 121 on board
perished. The Boeing 737-31S ran out of
fuel and slammed into a hill 40 km north
of Athens after it had failed to pressurise.
Most likely, the crew lost consciousness as
the aircraft climbed above 10,000 ft.
In Australia, in September 2000, a
Beech Super King Air, registered VH-SKC,
claimed the lives of seven miners and their
pilot when it crashed on a property in Burketown
in remote north Queensland.
The men were travelling to Western Australia’s
Goldfields from Perth., a distance of
some 600 km. After flying five hours and
3,000 km across Western Australia and the
Northern Territory, the aircraft ran out of
fuel. Although the cause of the accident
may never be known, the ATSB found that
the aircraft was, “… probably unpressurised
for a significant part of its climb and cruise
for undetermined reasons. The pilot and
passengers were incapacitated, probably
due to hypobaric hypoxia because of the
high cabin altitude”.
No-one is immune. In 1981 I also nearly
joined the long list of pressurisation victims.
I was taking a Cessna 425 (twin PT6 powered
cabin-class Cessna) from Bankstown
to Essendon for some maintenance. I felt
very comfortable in the aircraft, because I
was part of the crew that flew it from the
US to Sydney. I lodged a flight plan, filled
the aircraft’s fuel tanks and departed for
Essendon. I was by myself, so early in the
climb I engaged the autopilot, preset the
cruise altitude for the cleared FL250, and
dealt with ATC and the busy traffic out of
the Sydney area.
During the climb I became aware that
something was not right. I was a little
flushed and slow, but at first I could not work
out what the problem was. Then, remembering
the symptoms of hypoxia from my
military decompression chamber training,
I checked the cabin altitude. I was shocked
to see the cabin at 16,000 ft, and climbing.
Straight away I slapped on an oxygen mask.
Once I felt my senses restored, I contacted
ATC and asked for descent to a lower level.
Troubleshooting on descent I found the
“pressurise/depressurise” switch (hard to
see under the cabin dump knob) in the
POINTS
Hundreds have lost their lives as a result of
pressurisation problems. John McGhie explains the
basics of how the pressure system works and what to
do when there is a problem.
Out of it: As soon as there is an indication of
a pressurisation problem you need to don
an oxygen mask. Otherwise you risk loss
of judgement and other even more serious
effects due to hypoxia.
Rob Fox
MARCH–APRIL 2006 FLIGHT SAFETY AUSTRALIA 37
FLYING OPERATIONS
“depressurise” position. My mistake – I had
missed it on the pre-flight! I reset the switch,
reset the cabin altitude, and climbed back to
my planned cruise level.
I was lucky. If I had not felt uneasy and
remembered my experience in the decompression
chamber the aircraft would have
continued the climb. I would have lapsed
into unconsciousness and the aircraft
would have continued on its way until it ran
out of fuel, probably somewhere south of
Tasmania.
If you want to avoid becoming a pressurisation
statistic, particularly if you are new to
flying pressurised aircraft, it is vital that you
understand the basics of why and how the
system works, and the importance of correct
handling of pressurisation problems.
Pump up the pressure: Let’s start with the
basics. Luckily for us, the ratio of oxygen to
the other gasses remains constant at 21 per
cent, even as we climb and the atmospheric
pressure drops from its sea level value.
However, as the atmospheric pressure
drops the pressure of oxygen also drops –
until your lungs struggle to transfer enough
oxygen to the bloodstream. That’s when
you suffer the effects of lack of oxygen – the
dreaded hypoxia. There are several key altitudes
involved in staving off hypoxic effects:
above 10,000 ft you need supplemental oxygen
to maintain bodily functions, above
30,000 ft this supplemental oxygen needs
to be 100 per cent of the gas you breathe,
and above 40,000 ft the oxygen needs to be
delivered to your lungs under pressure.
Because the ratio of oxygen remains the
same at different altitudes in the atmosphere,
a supply of compressed air will
allow you to continue to function. This is
done by pressurising the cockpit and cabin
areas of the aircraft. You can then function
without the impediment of oxygen masks.
A back-up supply of oxygen is provided
should there be problems with the pressurisation
system.
For passenger comfort the aircraft cabin
should remain no higher than around 8,000
ft, and for safety not above 10,000 ft. Cabin
temperature also needs to be maintained at
a comfortable level of about +25º C.
For pressurised flight you need two
things: an enclosed cabin, in the form of a
pressure vessel, and a way to compress air
to raise the pressure inside the chamber, in
effect lowering the altitude of the cabin to a
safe level.
In most cases the outer walls of the cabin
area (the aircraft skin) can form the side
walls of the pressure vessel. The front and
back of the pressure vessel usually take the
form of bulkheads as the extremities of the
aircraft are not needed for passenger use.
Often these unpressurised areas will contain
services such as hydraulic and electrical
systems. Some smaller aircraft may have
The forces on a normal entry
door at maximum differential can
be over five tonnes.
Helios tragedy: On August 14 last year, Helios Flight 522 enroute from Cyprus to Prague, crashed
into a hilly area near Athens after losing pressure. The accident serves as a stark reminder of the
dangers of hypoxia.
Courtesy: Quinn Savit
AAP
38 FLIGHT SAFETY AUSTRALIA MARCH–APRIL 2006
FLYING OPERATIONS
baggage holds in these unpressurised areas,
leading to some interesting effects with
passenger personal items such as shampoo
bottles!
To maintain a cabin altitude of 8,000 ft at
an aircraft altitude of 35,000 ft requires a differential
in pressure between the inside and
outside of about 7.0 psi or 50 kPa (usually
measured on a cabin pressure differential
gauge). To contain this pressure differential,
the aircraft structure must be strong enough
to withstand high outward loads from the
internal pressure.
Additionally, the cycle of pressure – in
which the aircraft moves from being unpressurised
on the ground to pressurised in the
climb and cruise – will impose cyclic fatigue
loading on the structure.
It was this cyclic load that led to serious
problems in the early days of jet air transport.
In 1953 the problem surfaced when a
De Havilland Comet crashed in mysterious
circumstances shortly after takeoff. Two
similar accidents followed soon afterwards,
causing British authorities to ground the
entire fleet of the world’s first jet airliner.
The cause was eventually traced to
fatigue cracks around the square windows
following repeated pressurisation and
depressurisation. The problem is now well
understood.
Several methods have been used to
“pump up” the cabin pressure. In the early
days, dedicated compressors – or blowers
– were used. In jet and turbo prop aircraft,
the engines provide a source of air suitable
for pressurisation by tapping the compressor
sections of the engines. Similarly, piston
engines using turbo chargers now provide
a source of air suitable for pressurisation of
the cabin, and aircraft such as the Cessna
421 use this method very successfully.
Air from jet-engine compressors, or turbo
chargers, is normally very hot and needs to
be cooled before use in the cabin. Air entering
the cabin must also be regulated and
conditioned; passengers do not appreciate
raw engine bleed air in their faces! This can
be done by combinations of air-to-air intercoolers
or, as used on most jet aircraft, by
passing the air through air-cycle machines,
which can also provide a source of cold air
for cooling in hot conditions. Freon cooling
systems, similar to car air conditioning, are
also used in some aircraft.
Control: Once a source of regulated and
conditioned air entering the cabin is assured,
you need to control the pressure in the
cabin to safe and comfortable levels. This is
the job of the pressurisation controller and
associated outflow valves. Using a relatively
constant inward flow – that is, from the
bleed-air source – the pressure differential
is controlled by allowing excess air pressure
to escape outside the cabin through outflow
valve(s). Preset safety features of these
valves, or a dedicated safety valve, prevent
excess pressure building up in the cabin.
There are many variations of controllers
– you should check your flight manual for
details. The simpler controllers use air pressure
and a source of vacuum to operate the
valves to maintain a preset cabin altitude or
cabin rate of climb. The actual cabin altitude,
pressure differential and cabin rate of
climb and descent are shown on dedicated
gauges.
Theoretically, you would want a perfectly
sealed pressure vessel to ease the amount of
air required from the engines for pressurisation.
Practically you must have access holes
in the structure such as doors, windows,
cargo hatches, control cable runs, electrical
cables and hydraulic system pipes.
Various methods are used to seal these
access areas, such as inflatable seals around
doors, or seals around control cables. Ideally
doors and hatches will be of a “plug” type
where the increase in pressure as you climb
will tend to force the door onto the structure
to maintain a good seal. In smaller aircraft,
doors will normally open outwards and so
need to be held in place with pins that seat
in the door surround.
The forces on a normal entry door at maximum
differential can be over five tonnes,
so some form of safety mechanism has to
be used to prevent the door from opening in
flight. Failure to correctly set the pressurisation
value for landing can lead to problems
resulting from pressure retained in the
cabin, such as a sudden release of pressure
when the safety valve opens on landing or
the possibility of a sudden and violent opening
of a door.
Pressurisation systems are simple to set
and use provided the flight manual procedures
are followed. You set the pressurisation
controller for the desired cruising level
before takeoff and normally you select “ON”
for the air source (bleed air). Before descent
you set the destination altitude, usually plus
a small margin to ensure a depressurised
cabin. You then monitor the cabin rate of
descent and altitude to ensure passenger
comfort during descent and arrival.
Care must be taken when you depart
from – and arrive at – aerodromes with different
altitudes. When you depart a high
aerodrome for a low one, the aircraft’s pressure
will climb and the cabin pressure will
The most critical action is
to maintain the level of oxygen
in the body. You must don a
working oxygen mask as soon
as a pressurisation problem is
suspected
Altitude (ft) Approximate time of
useful consciousness
18,000 20-30 minutes
22,000 10 minutes
25,000 3-5 minutes
28,000 2-3 minutes
30,000 1-2 minutes
35,000 30 seconds to 1 minute
40,000 15-20 seconds
43,000 9-12 seconds
50,000 9-12 seconds
21%
1%
78%
Black-out: Without sufficient oxygen the brain
shuts down quickly. At 35-40,000 ft you have
onlyseconds of useful consciousness. Note that
reactions to hypoxia vary widely, and time are
indicative only.
USEFUL CONSCIOUSNESS THE AIR YOU BREATHE
Constant: The ratio of oxygen to other gasses in
the atmosphere remains roughly constant at 21
per cent, regardless of altitude.
MARCH–APRIL 2006 FLIGHT SAFETY AUSTRALIA 39
FLYING OPERATIONS
go down – a unique situation.
Before opening any door after landing at
your destination, you must make sure that
the differential pressure is at zero. A simple
check is to try to open the pilots DV window
– if it opens with no problem, the pressure
must be at zero.
Problem management: As with any aircraft
system, problems can be encountered with
pressurisation systems. In all cases, the aircraft
manufacturer’s emergency and abnormal
procedures must be followed explicitly,
promptly and correctly. This is the only way
to ensure a safe outcome to any pressurisation
problem.
The most critical action is to maintain the
level of oxygen in the body. You must don a
working oxygen mask as soon as a pressurisation
problem is suspected. This is essential
because your time of useful consciousness is
limited (see chart).
Problems can be broadly categorised into
air leaks and air source problems. Leaks can
occur slowly, such as when door seals do not
work correctly, or suddenly such as a major
component failure. Often door and hatch
failures result from incorrect closing procedures.
You should treat door and hatch closing
as an important part of the aircraft preflight
checklist.
A small leak can be handled by the system
and its only effect may be the noise created by
the escaping air. Large or sudden leaks will
cause a rise in cabin altitude, with the rate of
the rise depending on the size of the leak. A
major leak can lead the cabin altitude to rise
quickly to the same value as the aircraft altitude
(the term “explosive decompression” is
used to describe the extreme case of this).
This causes considerable distress to crew
and passengers, and the effects on ears and
sinuses can be painful. Aircraft capable of
flight above 25,000 ft provide passengers
with automatically presented oxygen masks.
Problems can also occur due to the source
of the bleed air. Internal engine oil leaks can
contaminate the bleed air leading to fumes
and even smoke in the cabin. Again, the first
action when you notice any smoke or fumes
should be to don an oxygen mask and select
100 per cent oxygen to ensure you have an uncontaminated
breathing source. You should
then follow the manufacturer’s instructions
to ensure a safe outcome. This will normally
require the shutting off of the source(s) of
bleed air to shut off the smoke or fumes.
You need to be careful to correctly operate
the pressurisation system. Failure to
turn on the pressurisation air source can
result in the onset of subtle hypoxia during
climb. It is critical to check that the cabin is
pressurised before a climb above 10,000 ft.
You should check the pressurisation every
5,000 ft during climb, checking that the
cabin pressure gauge shows an appropriate
differential pressure and that the cabin altitude
is less than the aircraft altitude.
Pressurised aircraft regularly operate
in the rarefied air above 10,000 ft. Understanding
the environment in which we
operate, and the aircraft systems that
allow us to operate in this area, can lead
to many thousands of hours of safe and
comfortable flying.
John McGhie is an authorised testing officer and
former CASA flying operations inspector. For further
information check out, “Oxygen first”, a video about
hypoxia. It’s available from CASA’s online store at
www.casa.jsmcmillan.com.au.
150 200 250 300
0 5 10 15 x 104
0 .5 1.0 1.5
200 250 300 350
K (temp)
0
10
20
30
40
50
60
70
80
90
100
Geometric altitude in kilometres
Temperature
Speed of sound
Density
Pressure
N/m2(pressure)
kg/m3(density)
m/sec(speed of sound)
Mesopause
Stratopause
Tropopause
Troposphere
Mesophere
Stratosphere
Thermosphere
Struggle: As atmospheric pressure drops the pressure of oxygen also drops until your lungs struggle
to transfer enough oxygen to the bloodstream. Above 10,000 ft you need supplemental oxygen to
maintain bodily functions, above 30,000 ft this supplemental oxygen will need to be 100 per cent of the
gas you breathe, and above 40,000 ft the oxygen will need to enter your lungs under pressure.
ALTITUDE, DENSITY AND PRESSURE

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pressure points