Cabin Decompression and Hypoxia
by Mark Wolff -- Source: PIA Air Safety Publication
At the hypobaric chamber at the RAAF base
in Edinburgh several
hundred air force pilots each year
get to check out their reactions to
depressurization and the effects of
The chamber is set to an
altitude of 25,000 feet, which gives a
time of useful consciousness of
around three to five minutes.
Up to ten pilots at a time sit in
the chamber tensely-waiting for the
depressurization, which starts at
8,000 feet and moves to 25,000 feet in
just 10 seconds. Each clutches a
checklist of tasks they are to perform. Each is determined to remain
conscious and capable for as long as
After about two minutes one of
the subjects is asked to repeat back a
number. Inevitably the subject is
unable to do so. In fact, most don't
remember being asked.
Trying to go through the
checklist the pilots tend to exhibit
one of two kinds of behaviour; they
are either "page flickers" or
"fixators". The page flickers will just
sit there mindlessly flipping through
the checklists while the fixators will just
stare at one page. They are "passengers in their own bodies".
These two quite different
behavioural responses to rapid
depressurization hint at the variation
in individual responses to lack of
oxygen, or hypoxia.
Hypoxia is a threat to safety
for all pilots operating pressurised
aircraft and for unpressurised
aircraft that fly at an altitude of
10,000 feet or above --- the legal ceiling
above which oxygen must be used
by flight crew members in
Some individuals with reduced
lung function will become hypoxic
well below this level. This includes
people with emphysema, industrial
lung disease, certain forms of
anaemia, ischaemic heart disease
and even mild degrees of heart
If you smoke, you may have
already reduced your oxygen intake by
a significant factor. Avoid smoking
before and during flight.
The Nature of Hypoxia:
term hypoxia translates from the
Latin to mean below normal (hypo)
oxygen (oxia). It is a physiological
state in which tissues are deprived of
adequate oxygen, and organs such as
the brain, eyes, ears, lungs and heart
are adversely affected.
When an aircraft undergoes
rapid decompression above around
35,000 feet, the time of useful consciousness for crew may be 30
seconds or less, depending on the
altitude (see table).
Time of Useful Consciousness
30 minutes or more
Pressure and Altitude
At lower altitudes the time of useful consciousness maybe longer, but subtle effects may still impair your functioning.
The more rapid the decompression, the faster the symptoms of hypoxia will
Crew surprise and perhaps
lack of familiarity with decompression can contribute to dangerous delays in appropriate response. Research by the US Air Force shows 80 per cent of pilots with no experience of decompression wait as long as 15 seconds to respond correctly to a loss of cabin pressure.
Because of the insidious effects
of hypoxia on judgement and reasoning, the correct response to loss of
cabin pressure is always to don the
oxygen mask - immediately. That's
the only way you can be sure that
you will make the right choices.
The death of US golfer Payne
Stewart in October 1999 and two
recent Australian incidents - one in a
RAAF King Air and the other in a
civilian King Air - have put the
spotlight on the issue of hypoxia.
While the RAAF incident is known
to have involved hypoxia, we may
never know the contributing factors
to the civilian accidents.
In the RAAF incident, shortly
after the aircraft was levelled at the
planned cruising level of FL250, the
right-hand seat passenger noticed
that the pilot was acting erratically
while manipulating the Global
Positioning System (GPS). Soon
after, the pilot slumped over the
controls and turned a curious shade
Fortunately the passenger (who
was a pilot but was not endorsed on
type) was able to take control of the
aircraft. He descended
and, after having some
trouble locating the
declared an emergency.
He was extremely lucky
- he should first have
donned his own oxygen
mask to ensure he was
able to function correctly.
reports from US investigators reveal the cockpit
voice recorder on Payne
contained no voices, but
that there were sounds
consistent with various
alarms (cabin altitude /
low pressure, stall
is that the accident may
have been related to
decompression early in
the flight and that the
pilots and passengers
may have been incapacitated by the low level of
oxygen. Pilots of military aircraft
assigned to follow the Learjet after it
failed to respond to ATC transmissions and climbed above its assigned
altitude said the windows were
covered with ice and that there was
no sign of flight control movement.
The flight ended when the
aircraft dived into the ground at the
time that the Learjet's fuel supply
would have been exhausted.
Symptoms and Signs:
of hypoxia are similar
to alcohol. Like
alcohol, there can be
a personality change.
The first signs
include both mental
and physical effects.
Mentally there can be
a loss of judgement,
This can be accompanied by an increase in
reaction time and a
kind of mental
similar to the fixation
on the GPS unit
experienced by the
RAAF King Air pilot.
You may even become euphoric.
muscular incoordination and an impairment of colour, night
and peripheral vision.
Hearing also deteriorates. You may
experience hot flushes
and turn bluish at
or hyperventilation is
one of the early
judgement, you may
not notice loss of
vision and hearing or other physical or mental signs. It's
the opposite of "You don't know
what you've got until you lose it".
More like, "You don't know what
you've lost until you get it back".
Simple tasks become extraordinarily difficult and performance
As hypoxia continues, you
become semiconscious. After you
lose consciousness entirely, you have
only minutes to live, depending on
There are many factors which
affect the tolerance to hypoxia. The
faster the rate of ascent, the quicker
Apart from smoking and lung
disease, you maybe more susceptible
if you are ill, stressed, unfit, fatigued, under the influence of drugs
or are suffering from a hangover.
There is a high degree of
individual variation in the response
to hypoxia. Some people may
hyperventilate and turn blue immediately; others may not. In a small number of individuals, unconsciousness may occur before any other symptom. (See footnotes for variability of symptoms)
your oxygen mask immediately,
select 100 percent oxygen if you
have differential settings, then
descend to 10,000 feet or below, terrain
permitting. You may find that you
feel worse immediately after putting
your oxygen mask on. Do not take it
off. This is called the oxygen paradox and you will feel better after
about one minute.
Breathe at a normal rate and
depth. Declare an emergency, and
land as soon as possible.
After recovery from an episode
of hypoxia, some symptoms may
persist. These include headache,
fatigue and lethargy.
Note that if you have been in
an aircraft which has been decompressed, you should not fly again the
same day because you will increase
your risk of decompression illness.
Decompression illness (or the bends)
can be incapacitating, particularly if
nitrogen bubbles enter the brain.
You should also use oxygen if
you detect fumes or smoke. Again,
set the oxygen at 100 per cent in
order to prevent any toxins from the
fumes or smoke entering the system.
The key to prevention is twofold. First, you need to
follow your flight manual prompts to
accurately set and monitor cabin
As soon as the cabin pressure
drops below recommended levels
you should take preventative action.
If warning systems indicate
problems with cabin pressure, you
must immediately don your oxygen
mask and descend if terrain permits.
It pays to know your equipment, because it can take some time
to put the oxygen mask on. Make
sure you know how to use the
masks, practise using them, and time
yourself in putting them on.
There are traps with checklists
which you must guard against. You
should understand your pressurization and oxygen systems, and fill in the gaps in your checklists so that
you are sure of what to do in an
For example, if you are over
ocean and have an uncontained
engine failure which leads to depressurization, you will want 20,000 feet or so to retain range. But do you have
enough oxygen? Do you know how
to calculate that?
Sixty per cent of corporate jet
depressurizations are caused by
uncontained engine failures. Most
others are caused by doors or
windows departing the aeroplane.
Even if you set up and use
checklists properly, things can still
A checklist is a skill based
action which means it is a stored
pattern of preprogrammed activity.
The greater the skill level, the
greater the chance of "strong, but
wrong" error. All it takes is a change
to a well practised routine and a
missed attentional check. The
intention may be correct, but the
action may be wrong.
You could get it wrong through
inattention, jisti action or preoccupation. That's why checklists need to be monitored carefully as they are the last line of defence.
If you are halfway through a
checklist, and are interrupted, go
back to the beginning and start
again. Be sure, however, not to
introduce new errors by operating
things like switches which have
already been activated correctly.
The one thing that you should
not do first is to start working out
what's gone wrong. That is, you
should not be problem solving and
planning on line.
This is hard to do. If you know
the aircraft well and you have a
pressurization warning going off, but
the cabin pressure indicator seems
OK, you should not be trying to
work out which is correct. You must
immediately put on the oxygen
mask, descend and then look to
you should not
work the problem
early is that hypoxia
interferes with your
ability to solve
problems and limits
your time of useful
the oxygen right
first, then ponder
If you go into
mode you will lose
valuable time. You
should take the
course of least
From a human
factors point of
view, once you
notice a pressurization warning, you
need to quickly don your oxygen
mask. So your well-practised rule
should always be:
- Pressurization warning.
- Don mask.
- Descend (terrain permitting).
- Solve the problem.
Variability of Symptoms:
Larger aircraft are not immune.
Cabin alerts are fallible and
hypoxia symptoms are
insidious and variable.
Even if cabin altitude
alerts function correctly,
time of useful consciousness may
be less than expected for a given
altitude. In 1995, a US Navy P3C
departing Japan suffered a rapid
decompression to cabin altitude of
24,000 feet in 10 seconds.
Despite the captain's immediate directions, it took the flying
pilot some time to don oxygen
equipment and initiate descent. He
then had difficulty remembering the
emergency descent procedure. The
non-flying pilot made several radio
calls without response, before
others realised she had not replaced
her headset after donning her
The flight engineer became
fixated with the uncorrectable
pressurization problem and the
captain placed his mask on him.
The captain, sitting behind the
flying pilot, noticed that his fingernails were blue and decided to
check the crew aft. They had
One member felt light-headed,
experienced tingling, started to
walk aft but had to sit down.
A spare pilot noticed the pressure
change, discussed rapid decompression and hypoxia with another
member, felt light headed and lost
colour vision. He felt nervous as he
had no oxygen mask.
The navigator felt tingling and was
disoriented. He noticed misting in the
cabin, became claustrophobic,
hyperventilated and fixated on his
Another crewman felt his ears pop, felt
cold, dizzy, confused, disoriented and
sat down until another member administered oxygen.
Another felt cold, short of breath and
suffered an upset stomach.
Another attempted to grab a walkaround oxygen bottle but became
confused and remembers searching for a
toolbox key for a ratchet to unfasten the
One passenger became nervous, laid
down, began shaking and had blurred
Another passenger saw the misting,
thought there was a fire, turned blue,
had ear pain and noticed everything in
The spare flight engineer was fascinated when the coffee pot lid exploded
and coffee sprayed everywhere. He
walked to the flight station, became
exhausted, disoriented, saw bright
flashing lights and had to be administered oxygen.
Another passenger started cleaning the
spilled coffee, felt ill, couldn't figure out
how to open the toilet door and vomited
Following this incident, the US
Navy recommended "Don Oxygen Mask" as the first item in the Rapid Decompression and Cabin Pressure Light On emergency checklists.