Re: Cabin Depressurization

Date:         06 Jan 2000 01:26:07 
From:         Mary Shafer <shafer@rigel.dfrc.nasa.gov>
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"P. Wezeman" <pwezeman@blue.weeg.uiowa.edu> writes:

> 	Following the ValueJet fire and crash in Florida there was
> an article published in a magazine, it might have been the Atlantic
> Monthly, in which the author stated that at any given time there
> was a maximum atainable level of safety in aviation. There was
> a point of diminishing returns in adding safety features and
> equipment, since the safety equipment could itself malfunction,
> as had happened in the case of the ValueJet.
>
> 	The author also stated that emergency oxygen systems
> on airliners had never saved a single life. Is this true? Any
> counterexamples? I know of at least three explosive decompressions
> where most of the passengers survived: the DC-10 where the cargo
> door blew out over Canada, the Aloha Air 737 where the top of the
> forward fuselage came off, and the 747 that lost a cargo door
> over the Pacific. Did oxygen systems help in any of these cases?

This was in the Atlantic Monthly and can still be accessed, I
believe.  The author is the son (or possibly the grandson) of the man
who wrote the definitive book on how to fly an airplane ("Stick and
Rudder").  Their surname is Langewiesche (the elder is Wolfgang and I
think the Atlantic author is Richard).

I read this same article and was similarly surprised by the claim, so
I did a little looking around.  I checked the FAA, NTSB, and a couple
of non-US accident databases.  I have been able to find no case of the
emergency oxygen system on an airliner saving a single life or the
absence of one causing a fatality.  That Aloha Air 737 was too low for
it to be needed, as I recall (it occurred right after takeoff, at the
beginning of the beverage service, which starts at 10,000 ft MSL).
The two cargo door losses depressurized the airplanes slowly enough
that the emergency descent got them down before anyone died of hypoxia
or anoxia.  The closest I could come to an emergency O2-related
accident was one where the masks didn't drop as they should have, and
there were no fatalities.  I've got a query into a friend at the FAA,
though.

However, there are two important caveats to make.  The first is that
the absence of evidence isn't evidence of absence.  The second is that
complex accidents (say, for example, where the aircraft broke up in
flight and fell to the ground) are excluded; the O2 may have kept
someone alive long enough to die from the ground impact, for example,
but there's no evidence of this.

The DC-10 at Paris that crashed when the cabin floor collapsed onto
the control cables, jamming them, because of a decompression in the
cargo compartment, is an example.  It was just high enough for
the pressure differential to cause the structural failure, but not for
long enough for supplemental O2 to do anything more than keep
passengers conscious all the way to the ground, if (and that's a big
_if_) the masks deployed and the passengers were able to overcome g
forces and don them.

Airliners usually fly long flights at or below 40,000 ft.  Although
the time of useful consciousness is fairly short, death takes quite a
bit longer.  The emergency descent will get the airplane down into
breathable air long before people run out of O2.  Obviously, the
amount of O2 in the air increases constantly through the descent,
stringing out the time of useful consciousness.  After all, it's
something like more than twice as long at 30,000 ft as it is at
40,000 ft.

Of course, there have undoubtedly been cases where the bottled O2 on
board an airliner has saved a life, but this isn't the mass emergency
"masks will deploy" O2 system but just a standard O2 bottle and mask,
carried for individual use.  The O2 is there for people having heart
attacks or difficulty breathing or some other problem that O2 will
help.  That is, it's there for an individual emergency, not an
aircraft emergency.

None of this applies to military aircraft, particularly the aircraft
with canopies, not windows.  There are innumerable cases of the O2
system (either in normal or emergency mode) saving all aboard.
However, the assumption is that the pressurization is going to fail,
as the US military requires fighter and attack crew to use O2
throughout the flight.  Aircraft with canopies have latches that hold
the canopy down to the canopy rail, with a tube that fills with air
and seals the gap.  A faulty latch or leaky tube can render a cockpit
unpressurizable, which is why the crew of such aircraft are on O2 all
the time.  In addition, there's an O2 bottle in every ejection seat,
to tide the crew over during the freefall until the baro altimeter
takes control and throws the crewmember out of the seat and opens the
parachute at about 15,000 ft MSL.  In more extreme cases, with
aircraft cruising high enough that there are other dangers, the crew
wear pressure suits, even though the cockpit is pressurized as
described.

--
Mary Shafer    http://www.dfrc.nasa.gov/People/Shafer/mary.html
shafer@rigel.dfrc.nasa.gov     Of course I don't speak for NASA
Lead Handling Qualities Engineer, SR-71/LASRE
NASA Dryden Flight Research Center, Edwards, CA
For non-aerospace mail, use shafer@ursa-major.spdcc.com please