How can this be?

August 23, 2005 8:12 PM

Science or Airplane safety question: How can an airplane partially decompress at 14000 feet, knocking the crew out (not killing them) and then when it goes up to 34000 feet the problem does not worsen?

You guessed it, I am referring to the recent accident with the Cypriot plane in the Athens area.

That plane reportedly undergone gradual decompression at 14000 feet to the point that the crew (at least) was incapacitated *but not dead*. Then the plane continued ascending to 34000 feet where obviously the decompression was continuing and I assume worsened. But none of the passengers died. Which means that the plane somehow "adjusted" its compression a little bit to better counter the extra decompression (albeit still partially). Otherwise, the crew would have gone kaput after 2+ hours at this altitude. See, the oxygen masks cannot last for more than half an hour, right?

This decompression story bugs me. You too?
Can you point out the/a fallacy in my previous arguments? Pretty please?

posted by carmina to grab bag (8 comments total)

Once you're above the weather it's safer? Crew woke up? Robot pilot?
posted by Napierzaza at 8:24 PM on August 23, 2005

The story bugs me, but not for the same reasons you cite.

Aircraft pressurizations systems have a number of settings that are used to control cabin pressure. Cabin altitude is usually regulated to be somewhere around 7-8000 feet when the aircraft is at cruising altitude. Among the settings are such arcana as max cabin climb rate, max cabin descent rate, max pressure differential (the difference between outside and inside pressure), heating and cooling controls, etc. Not to put too fine a point on it, they are complex, and as subject to mismanagement as any other complex system, and I can think of somewhere between 6 and 12 ways a failure could occur, and I'm nowhere near close to an expert on pressuruzation systems.

Just basic things thet need to be considered are the comfort of the crew and pax, the difference in elevation between flight origination and landing points, etc. Imagine setting your rate controller to have the cabin come down to sea level for landing, just as your wheels touch the ground and the pressure equalization valve opens- in Denver. Can you say "Ow! My ears!"
I knew you could.

My point is, is that commercial pilots are trained to manage many complex systems, and that systems management is the primary reason they are trained so thoroughly. Flying the airplane is the easy part. As a private pilot with about 150 hours total time, I had a friend in Miami who ran the L-1011 sims for Eastern Airlines. One Sunday I met him at work, and after an hour or so I could land the Tristar sim as easily as I could my 172. Actually, I think it was easier :) Flying the plane itself is second nature to any pilot- it's the systems that seem to get us every time (If not our own stupidity).

Modern pressurization systems have automatic rate controllers and computerized profiles that follow some programmed schedule for the flight plan, I imagine. No telling from my seat what sort of equipment was on the aircraft involved, of course, although I'm sure there's some guy on airliners.net that can tell you the serial number of the rate controller.

There are a number of troubling aspects to this crash- but the very least of it was that people were physiologically alive at the end. Everyone on board was certainly hypoxic and unconscious for most of the time the airplane was above 14000 feet or so after the O2 ran out. You make the assumption that death would have occurred, but there's no way for certain to tell what the cabin altitude was, hence no way to tell with any precision when death would likely occur. Yes, at 34,000 ft. all would likely be dead after 2.5 hrs, but there's no likely reson to assume the cabin was at that altitude. Perhaps the cabin only got up to 20,000 ft. Most of what we are hearing right now falls into the area of wild speculation, and should be treated as such.
posted by pjern at 9:24 PM on August 23, 2005

The speed of a plane can drive the level of compression inside it (part of a plane's pressurization system works by having air intakes at the front being larger than the venting at the rear). If the plane doesn't reach full speed until after it's reached cruising altitude (where it no-longer needs to spend so much energy climbing up to altitude), maybe that would explain it if the adjustment systems had completely locked-up - thinner air but greater speed. I don't know how/why it decompressed in the first place though, so this is all conjecture.

On preview, solopsist's explanation makes more sense :-)
posted by -harlequin- at 9:41 PM on August 23, 2005

Salon.com's "Ask the Pilot" guy said this, in an article on Friday [site subscription required to read it all, sorry]:

Doubtless those clicking in are curious about last Sunday's mysterious 737 crash in Greece, and the one in Venezuela on Aug. 16. You can expect a full report next week. There's plenty to talk about, but for now details are sketchy and conjecture rife. As these things go, it's prudent to hold off until more is known. Until then, it's best to season your intake of news with as much salt as you can possibly tolerate. The usual suspects, from the Associated Press to the network anchormen, have truly outdone themselves over the past several days, propagating the usual myths about cabin pressurization and engine failure. Dispatches from Greece in particular have been depressingly outrageous.

Without a subscription, you can still check salon.com to see what articles are posted, and (I think) you can get a free day-pass (one-time?). So you might want to wait and check.
posted by WestCoaster at 9:44 PM on August 23, 2005

I liked the part about how bodies were 'frozen solid'. Anyone who's ever tried to freeze something larger than a small steak will realize how ridiculous it is to postulate that could happen in an hour or two. In reality, the outside of the bodies had cooled a bit and rigor mortis had set in.

Agree with WestCoaster's pilot-guy that the investigators need some time to do their work and produce their report.
posted by ikkyu2 at 5:44 AM on August 24, 2005

Thank you for the responses.

WestCoaster the gradual air pressurization failure is in the preliminary report of the investigation.

ikkyu2, the "frozen solid" theory was debunked by the pathologists from day one (and in a rather sarcastic way). Of course media loved such a sensational story...

Solopsist's response is very useful for me because it explains how cabin pressure was kept at hypoxic levels but I still wonder whether people can survive (albeit unconscious) at such conditions for 2+ hours. ikkyu2, if I am not mistaken, you are a doctor, right? Do you know?
posted by carmina at 9:16 AM on August 24, 2005

Nice explanation solopsist and harlequin. As many climbers who have reached the summit of Everest without Oxygen have shown, the human body can survive even 28,000 ft for long periods of time. One's mental function at such altitude is very limited, especially without prior acclimation, which would make correcting the problem tenuous.

However, since most of the confusion about this incident seems to center around the question about why they did not all die during their time at altitude, I think that the pieces of this thread start to make sense of that situation.

It will be fun to see how close the collective consciousness of the green comes to the final conclusion.
posted by RMALCOLM at 10:59 AM on August 24, 2005

carmina: severe hypoxia and anoxia can be tolerated for long periods if the body temperature is cool. If core temperature can be lowered to 28 degrees Celsius, for instance, pediatric cardiac surgeons feel that 43 minutes of cardiac arrest are an acceptable risk to the brain on day 1 of life. Heart survival can be much longer - the brain is a throwaway organ and would not have had time to necrose catastrophically even if most of its tissue was destroyed.

Clearly these people were cold - it's -70 degrees at altitude and there's a heckuva wind chill.

My guess, however, is that the pressurization system had not failed completely, but instead had failed partially, as it apparently had done on a previous flight of this airplane. It's hard to say what the cabin pressure might have been - pathologic examination of the corpses won't reveal it. Possibly the flight data recorder tracks this parameter, but I don't believe so. The clever NTSB, or NTSB-equivalent in Greece, though, may be able to discern some tell-tale signs.
posted by ikkyu2 at 12:15 AM on August 25, 2005