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Posted : January 16, 2010
Hi Andrew. Most commercial aircraft seem to drop the cabin pressure to around 0.75 bar. If the cabin is filled with air this would give a ppO2 of c. 0.15 which seems dangerously low. So, do they top the air up with O2 so you're breathing Nitrox, or is the density of the O2 - still 21% - sufficient to keep you breathing happily? Regards, Rob

Answer
Posted : January 16, 2010
One for the airlines Rob!!!

Its a ballance of fuel as the cabins are technically designed to be kept at 1 atm, with a bleed off system so for argument sake a % per hour is replaced with air from outside... To prevent decompression of the cabin, this is replaced at pressure... some airlines take this from inside the engine (before combustion) some have other means... both take a hit on fuel and hence it is economics as to how much of a drop this goes to...

I could look up the FAA/CAA/EASA guidelines but no way do the airlines take it ppO2 as low as you suggest - just think of the litigation for DVTs alone...

I would have to do serious research - but i would guess they all pitch this around ppO2 of around 0.17 to 0.18...

Happy to look at this further if you want... just reply and ask....

Above all your point is exactly why most training agencies for Diving recommend 24hrs before flying and even longer like 36hrs + if you have been diving on every day for the preceeding week and doing deco....

cheers and regards Andrew
Answer provided by ADM Diving

Andrew Dawson-Maddocks
Additional Comment

Posted : January 18, 2010 by - ADM Diving
Minor tweeks above...

You pose a very interesting question. For the flight engineers and manufacturers amongst you I have deliberately not gone into Boyles Law etc, nor also the fine detail of the automatic bleed system and supercharges, refrigeration and heat exchanges etc etc etc

This is an intersting question - as just sat there you can happily allow the PPO2 to go down lower than my first answer, so long as you hydrate well. The rate of change of this pressure change to 2400m will be carefully controlled to prevent passanger discomfort and any sudden (temporary) altitude issues.

I guess as i said - if you want more - i'll find it. my other company designs and builds flight data and voice cockpit recorders so we have the necessary links to find out...

cheers Andrew.
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