Oxygen system ATR72

[numpty64]

I wonder if any kind engineer could help out?
I have just started training on the ATR72 and I was reading before the course that the fixed oxygen supply in an emergency is 15 mins for the flight deck and 30 mins for the cabin. Could anyone explain how this is supplied and is the difference due to the the cabin supply being mixed with ambient?
I know this will be in the manuals but we don't have them as yet.
Cheers

[moo]

Not sure I totally understand your question numpty, but the oxy system on pressurised aircraft is supplied from a number of oxygen cylinders located somewhere within the aircraft. I can't answer specifically for the ATR but the 747-400 ones are located in the sidewall fwd freight lining. On this aircraft the crew and pax oxygen are two seperate systems. The passenger system has a ringmain running in the PSU rail (alongside the reading light and attendant call panels overhead) This is normally unpressurised until a decompression or a manual selection from a flight deck switch activates a valve down in the freights and pressurises the lines with oxy. The dropdown doors which allow the masks out in the cabin are have a small pressure switch which react to high cabin altitude (can't remember the figure) or will drop if the pilot moves that guarded switch in the flight deck to ON.
The crew oxygen line is pressurised constantly, its there, right at the mask whenever it is needed. The -400 ones wrap around your head and can be put on in seconds. On the front of them, there is a selector switch which allows the crew to select either mixed oxygen or 100% pure (i.e. straight from the cylinder into your gob!)
Under normal operation, the oxygen has no part to play in keeping the pax alive and well. Ambient air is ducted from the compressor of the engine, passed through various precoolers and a/c packs etc. and is fed into the cabin with plenty of oxygen partial pressure.

hope that helps!

[Compass Call]

What I don't understand about this question is that the flight deck crew only get 15 mins supply and the pax get 30 mins. This would mean that the flight deck crew are liable to be unconscious long before the pax(assuming smoke in the aircraft). I would have thought that these values should have been the other way around. Also I would have thought that when the crew bottle pressure fell below a set figure the pax bottle would give priority to the crew. After all, the pax stand a better chance of survival if the crew are alert and able to land the aircraft!

CC

[avoman]

I have looked it up to make sure.
The ATR oxy system has been designed to supply 3 cockpit crew and 10% of the passengers during: 4 minutes for descent 25000 to 14000 feet; continuation of flight between 14000 and 10000 feet for 26 minutes.
One oxygen bottle with one regulator and one shutoff supplies flight deck masks and numerous ports in the cabin. The cabin supply can be isolated with a further shutoff to prioritise supply to the flight deck.
Therefore there is the same duration of supply to both compartments.
There is no oxygen generation system.
Cabin crew have portable bottles for independent supply.

[mono]

Heh?

:confused:

So you mean to say that 90% of the pax get no oxygen during the emergency descent?????

Maybe I better read that again.

:uhoh:

[avoman]

mono, correct!
Obviously this is sufficient for certification. These turboprops do not fly as high as the jets. Furthermore rapid rates of descent are possible.

[A Very Civil Pilot]

At the sharp end we have a bottle that provides for the flight deck. The on/off valve is to the right of the FO seat. Make sure it's on! The pax access this supply, not by drop down masks as in a jet, but by opening each hatch individually. There are only about 13 masks intotal, so the CC dish them out to whoever needs them most (old, ill etc).

Pax only need oxygen supplies above FL140, so a descent from FL245 wont take all that long.