On Wed, 31 Dec 2003 20:48:28 GMT,
(Aviation) wrote:


Someone also pointed out my goof about "holding" your breath
upon going from cabin (8000 ft pressure) to ambient (25-35,000
ft pressure). In estimating how much time the average civilian
passenger could go without TAKING a breath of good air (14,000
ft or below), I used the HOLD your breath estimate. Assuming
there is 3-5 minutes of mask-oxygen and one minute of "holding"
the last breath, they've got 3-6 minutes to get down to breathable
(14,000 ft?) air and then below. For the movie Executive Decision,
they were cruising at 39,000 ft. so they'd have to dive 25,000 ft
to 14,000 ft in 5 minutes, 5,000 ft/minute, average. Doable?

No, the length of time you can hold your breath on the ground doesn't
help. You will outgas O2 from your blood into your lungs and then
exhale it as part of your response to the RD, because the pressure
will be so greatly reduced. You will begin using the O2 mask with a
serious deficit and you'll continue to blow off O2, even with the
mask, until you get down to the altitude where the partial pressure is
large enough. Many people can't hold their breath for a minute,
either.

Airline pilots have quick-don masks that supply O2 at a higher rate
than do the cabin masks. The cockpit O2 is a separate supply from the
cabin O2, with more O2 at higher pressure. To be blunt, it's much
more important to keep the pilots conscious, of course.

Langeweische wrote, in his article about the ValuJet accident, that no
passenger had ever been saved by cabin O2. I don't know where he got
that information, but he's pretty good about checking statements like
that.

Mary

--
Mary Shafer Retired aerospace research engineer