HT From Germany, joined May 2005, 6511 posts, RR: 25 Posted (8 years 2 months 3 days 13 hours ago) and read 3697 times:
After a couple of flights the last days I´ve watched the usual safety demonstration over and over, however one question came up in me:
Everytime the use of pax-oxygenmasks is demonstrated it is said that "the bag may not inflate".
Q: Why is it so ? Which purpose does the bag have ?
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SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 69 Reply 3, posted (8 years 2 months 3 days 5 hours ago) and read 3642 times:
Quoting Bri2k1 (Reply 2): My understanding was that it's a rebreather system
Not exactly as that usually implies the presence of carbon dioxide scrubbers in the system.
Actually the system is constant-flow, but the flow rate is not very high - a liter a week or so. (kidding) So while you are not inhaling, oxygen will pool in the mask. When you do inhale the first part is high O2 content, but when that is gone the little flapper valve in the back of the cup opens and you are inhaling cabin air plus smoke or whatever else it contains.
If you breathe rapidly the bag will never inflate because the flow rate is not high enough to fill the cup, then inflate the bag.
If you are not breathing rapidly you obviously have not grasped the seriousness of the situation.
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TimT From United States of America, joined Jun 2001, 168 posts, RR: 0 Reply 4, posted (8 years 2 months 2 days 15 hours ago) and read 3471 times:
Just about all systems (commercial aircraft) are NOT using bottled 02- they use solid chemical generators. I remember the early days they F/A's briefing included the words, "you may notice a slight burning smell". I don't think the generators can make enough volume to get the bag to inflate, but do cause enough flow to keep you going until the altitude gets to a level the atmosphere can sustain you. The cup has a small exhaust valve built into it, and the supply line. When you inhale, the valve closes and you get whatever is in the bag, and then on exhale the valve opens, letting out the C02.
Loggy From United Kingdom, joined Oct 2005, 15 posts, RR: 0 Reply 5, posted (8 years 2 months 2 days 13 hours ago) and read 3464 times:
So, if you breathed reeaaall slow the bag would inflate because you wouldn't be using all the ? Pure o2 ?, or just plain old high altitude, say 10000 Ft, air being supplied ? .
If it were pure o2 then the supply would be quite low ? . Enough to keep you conscious, panting, or enough to keep your vital systems going ? .
Obviously if the supply were low then unfit and smoker pax would suffer more, albeit temporarily, than fit healthy people.
Loggy From United Kingdom, joined Oct 2005, 15 posts, RR: 0 Reply 7, posted (8 years 2 months 2 days 12 hours ago) and read 3449 times:
Quoting HAWK21M (Reply 6): Unfit ok.But why would a Smoking Pax would suffer more.
Well depending how long they have smoked would determine their lung capability for extracting the necessary oxygen from any air supply into the blood stream. I would imagine a moderate smoker noticing the effects of a reduced oxygen supply moreso than a non smoker due to the damaging effects on the lungs through smoking.
Your'e right, that was the wrong word. What I was trying to describe is that the flow of oxygen goes into the mask, and the bag is filled by exhaled air, which still contains oxygen. Then, eventually, inhaled air will be a mix of the supplied pure oxygen plus the exhaled air from the bag. When we breathe normally, we don't inhale pure oxygen, so this approximates normal breathing.
Electech6299 From United States of America, joined Aug 2005, 616 posts, RR: 3 Reply 11, posted (8 years 2 months 20 hours ago) and read 3292 times:
Try this LINK from a report on the oxygen systems of a MD-11. It is supporting information for the TSB crash investigation of SR111.
It explains the masks in detail, and will answer most of the questions about the generators. It is a little thin on info about the reservoir bag, tho:
Quote: Each passenger oxygen mask is fitted with a reservoir bag that is attached to the oxygen generator with a flexible hose. When a generator is initiated, the reservoir bag unfolds and may inflate (depending on the altitude) until breathing begins.
I assume this means cabin altitude- which is obviously above 14,000 feet if the masks have deployed. But basically, it is as described by Slamclick- a reservoir bag, that will inflate to provide pressure relief and a reservoir of O2 until the passenger starts breathing, but the bag is not intended to supply the breathing air. The passenger breathes cabin air, supplemented by O2 from the generator supply tube (or reservoir bag if any has accumulated there). Exhaled air is vented through the exhalation valve in the mask directly to the cabin, not into the bag.
But to answer the question directly- based on altitude. That's what the report says. I don't know which way the altitude affects it- I would think high altitude, thinner atmosphere, the bag inflates more. But perhaps lower altitude, denser atmosphere, more efficient operation of the canisters which provide more O2, inflating the bag more. I dunno
[Edited 2005-10-21 01:34:17]
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Electech6299 From United States of America, joined Aug 2005, 616 posts, RR: 3 Reply 15, posted (8 years 1 month 4 weeks 1 day 20 hours ago) and read 3175 times:
Quoting Tod (Reply 14): Oxygen generator canisters are self contianed and effiency is not impacted by altitude.
Well, you can see that I didn't read all the info in my link Oops... I was more interested in the aneroid switches and door release, and glossed over the canisters. Glad somebody knows...
Quoting SlamClick (Reply 13): Well, my only hangup was the word "rebreather" which, to me, conjures images of underwater or firefighter breathing gear using CO2 scrubbers.
Most of the firefighting gear I have worked with is strictly compressed air SCBA, I haven't personally seen or heard of rebreathers outside of underwater/submersible vehicle or contained HAZMAT site use (Like chemical dump cleanup). Do airport firefighters use them? Or cockpit crew? Any info you have on this would be welcome.
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HAWK21M From India, joined Jan 2001, 31457 posts, RR: 57 Reply 18, posted (8 years 1 month 4 weeks 11 hours ago) and read 3072 times:
Quoting DC8FriendShip (Reply 17): For the flight crew, maybe. but i doubt that there are tanks onboard that hold enough O2 for over 250 people. They most likely use generators like most pax aircraft do.
Didn't the B742 have Piped O2 for the Pax system.
Tod From Denmark, joined Aug 2004, 1699 posts, RR: 3 Reply 19, posted (8 years 1 month 4 weeks 5 hours ago) and read 3070 times:
Quoting DC8FriendShip (Reply 17): but i doubt that there are tanks onboard that hold enough O2 for over 250 people. They most likely use generators like most pax aircraft do.
The pax oxygen tanks on 747 are located in the forward cargo compartment, right side just aft of the cargo door, next to the halon tanks. Additional oxygen tanks are sometimes installed between the main deck floor beams in the forward cargo compartment. I designed the installation of two extra tanks for the Corsair 744 fleet of ex-UA planes. Some of the asian operators such as SQ have as many as 14 extra bottles on their 744 to facilitate high pax loads and long range requirements.
On 777 gaseous oxygen is an option. About half the ones I've work on have gaseous systems.
Quoting HAWK21M (Reply 18): Didn't the B742 have Piped O2 for the Pax system.