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Even though oxygen is flowing...
2

Even though oxygen is flowing...

Even though oxygen is flowing...

(OP)
The usual wording of the oxygen mask portion of the airline "safety briefing" is"

Quote:

Even though oxygen is flowing, the plastic bag may not inflate.

This has always struck me as not quite right.  Obviously this leaves open the question:

"If oxygen is not flowing, will the plastic bag still not inflate?  If so, how can I really know if oxygen is flowing or not?"

A somewhat better wording might be:

"When oxygen is flowing, the plastic bay may not inflate."

Although this leaves the same question open.

More truthful might be:

"Trust us, some gas will probably be flowing, and if you're lucky it might even be oxygen.  The plastic bag may or may not inflate, so you will have no way of telling.  Don't worry, if hypoxia sets in you really won't care anyway."

RE: Even though oxygen is flowing...

2
When I went throught the aeromed school about 20 years ago, they told us tests had been shown you have as little as 15 seconds' useful consciousness following a rapid depressurisation in an airliner (having previously been breathing air at well below 1 bara).  When they put us through the chamber, even though we'd been on 100% oxygen, I have to say I stopped being functional disturbingly quickly after taking my mask off.

With this in mind, the advice might as well be:

"Put the mask on and keep it on.  If it doesn't work, you won't have the time either to find out, or to do anything about it.  If you can't take a joke, you shouldn't have joined"

A.

RE: Even though oxygen is flowing...

Couincidentally, this is bordering on something I was thinking about the other day:
What is the general scenario when decompression of an airplane happens? Is there chaos and does everything get sucked out as in the films? Does the plane go out of control and plummet with the pilots wrestling the controls?
Is it survivable (if not then all the oxygen in MintJulep's bag won't matter a damn)?
Should I be worried?

Any offers?

RE: Even though oxygen is flowing...

and whats up with water landing?  Is it more like water crash?

RE: Even though oxygen is flowing...

I once heard a spoof safety briefing, one part of which went like this:

"We have supplied a lifejacket for each passenger. Don't bother looking for it, if you ever need it then a miracle has occurred, since in the entire history of civilian jet-liners nobody has ever used one successfully."

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Even though oxygen is flowing...

You're not in a position to do anything about it, anyhow.

RE: Even though oxygen is flowing...

The water landing's a tricky one.

Back in the eighties, I was cheerfully told (as I climbed aboard) that if we ditched, the latest type-specific modelling had shown there would be a massive snap-down that would subject everyone at the front of the jet to several tens of fast onset G, making a water landing unsurviveable for at least half the crew.

Some years later one of these jets did go into the water and the entire crew walked away from it.

Guess it all depends.

A.

RE: Even though oxygen is flowing...

Gaufridas,
Myth Busters (TV) explored precisely this aircraft explosive decompression scenario.
Using a fuselage on the ground they pressurised it to the equivalent of 30,000 ft or so and from inside fired a pistol through the window (remote control).
No damage to the dummy in the seat by the window. The window didn't fragment.
The most successful attempt to recreate this scenario was when they exploded a small charge right next to the fuselage. This ripped open the plane like a sardine can. The dummy was not pretty but was still in its seat.

They have similarly debiunked some of the other movie myths such as attaching a wire rope to the back axle of a police car ... when it took off, the rope broke.

Perhaps they will address the Al Gore film next.

JMW
www.ViscoAnalyser.com

RE: Even though oxygen is flowing...

gaufridus, the Aloha Airlines plane lost a large portion of the passenger cabin "roof", I believe one stew was lost, the plane otherwise landed safely. Several DC-10's lost the baggage door, collapsed the passenger cabin floor causing loss of many of the controls, at least one landed safely, the others crashed. With the floor went several rows of passenger seats, of course these passengers didn't survive.

To answer you question "It depends."

I pretty much always leave my seat belt on, both to possibly avoid to being sucked out if a decompression occurs, and more practically, to avoid banging the cabin overhead in case of turbulence.

Regards,

Mike

RE: Even though oxygen is flowing...

Zeusfaber refenced tests that showed only 15 seconds of consciousness after decompression, but I can hold my breath well longer than that.  (Just did 30 seconds no problem, and without a deep inhale first.)  So what happens physiologically that would result in unconsciouness after only 15? Maybe increased oxygen use due to higher metabolism in a sudden emergency, as opposed to sittingly calmy at my desk?

RE: Even though oxygen is flowing...

How can you kiss your fanny goodbye and still manage to get the mask on at the same time?

RE: Even though oxygen is flowing...

I suppose oxygen masks are very hygienic for mouth-to-mouth breath.

Luis

RE: Even though oxygen is flowing...

As Weird Al Yankovic notes in his song "Albuquerque", your odds of surviving a plane crash depends strongly on having your tray table up, and the seat back in the full upright position.

RE: Even though oxygen is flowing...

actaully greg, an airliner ditched near some islands in the indian ocean (it'd been hijacked out of ethopia, i think, and the hijackers insisted on heading for australia, refusing to believe that there wasn't enough fuel onboard).  anyways, some pax managed to make it out alive, even the pilot survived, cause of their lifevests.  mind you, many more died 'cause they didn't listen to directions and inflated them inside the plane (the bouyancy of the vests made it impossible to get out of the plane and many pax drowned in their seats) so the record is probably saved some, didn't save (ie killed) more, in countless millions (billions) of pax (or pax-flights, or pax-airmiles) so are they worth the investment ?  (personally i think not).  

O2 systems are probably in the same boat (plane?), a big investment for a safety system that just about is never used, and probably hasn't saved a significant number of pax.  in any case, why give the pax O2 ? the plane will descend quickly enough that the pax will be O2 deprived for only a minute and so (for the most part) suffer no harmfull side effects.  and consider the flip side, can you imagine being on O2 (as a pax) where for some horrible reason the pilots aren't ?

RE: Even though oxygen is flowing...

Back to the original question...I recently heard a version that made a lot more sense, along the lines of "depending on the air pressure in the plane, the bag may or may not inflate, but oxygen will be flowing."

Hg

Eng-Tips policies:  FAQ731-376

RE: Even though oxygen is flowing...

Quote JAE:
How can you kiss your fanny goodbye and still manage to get the mask on at the same time?

50% of the British population couldn't do this.
2 nations divided by a common language...

RE: Even though oxygen is flowing...

however, the advice they do give on the plane is.."sort yourself out first then the passenger sitting next to you.."

RE: Even though oxygen is flowing...

JNieuwsma,

The gotcha with airliner flight is this:  If you've been breathing air at a cabin altitude of 0 Ft (as in your breath-holding example - giving a PPO2 of 0.21 bar), or breathing an O2-enriched mix at a cabin altitude of around 10 000 Ft (as you might expect in a modern combat aircraft), then your bloodstream is carrying a significant reservoir of oxygen that can be drawn on - perhaps for up to three minutes - if you suddenly lose the opportunity to breathe in anything helpful.

If instead, you breathe air at a cabin altitude of 10 000 Ft, you're actually getting less than 0.15 bar PPO2, which is right on the border of hypoxia (and this is before anything accidental goes wrong).

Once the window goes and you're at a cabin altitude of 30 000 Ft, the PPO2 in the cabin drops to about 0.06 bar, and you're completely dependent on whatever is stored in your body.  

A.

RE: Even though oxygen is flowing...

Thanks for the explanation, Zeusfaber.  Too bad I can't use my points and upgrade to a modern combat aircraft for my next business trip.

RE: Even though oxygen is flowing...

(OP)
The cabin service is lousy in most modern combat aircraft.

RE: Even though oxygen is flowing...

Quote (MintJulep):

The cabin service is lousy in most modern combat aircraft.
And that distinguishes them from other aircraft how? winky smile

RE: Even though oxygen is flowing...

What... peanuts and crackers and 1/2 can of diet coke is my favorite dinner.

You'd think they could spare a whole can? Naaa.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Even though oxygen is flowing...

They give you a whole can and you'll need the toilet. Their primary objective is to make sure that you get DVT, ie, underhydrate you, and minimum exercise.

It's a plot I tell you.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Even though oxygen is flowing...

They don't want you going to the toilet because queues at the toilet are a security risk.

Hg

Eng-Tips policies:  FAQ731-376

RE: Even though oxygen is flowing...

Place the oxygen mask over your mouth and breathe normally. If travelling with small children, place your mask on first, then decide which of your children you like best........

Timing has an awful lot to do with the outcome of a rain dance.

RE: Even though oxygen is flowing...

A few years ago there was a remarkable story of a BA flight which lost a cockpit windshield. The pilot was sucked out of the aircraft and a crew member managed to grab him before he completely left the plane. The pilot survived this experience thanks to his colleague's intervention. The copilot must have giant brass ones.

This is the Air Accident Investigation Board's report:

http://www.aaib.dft.gov.uk/cms_resources/dft_avsafety_pdf_502702.pdf

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Even though oxygen is flowing...

I always love flying, not the cramped conditions painful security etc but the actual flying.

This despite the fact that from my degree (aero), limited Aero work experience, applying to the air force, flying scholarship etc I probably know far more of the things that can go wrong than most of the other passengers.

JNieuwsma I wouldn't recommend holding your breath during a rapid decompression, for the same reason you don't hold it when escaping from a submarine.  The air in your lungs is at the relatively higher pre decompression pressure.  This will cause your lungs to tend to expand.  Now I'm not sure the effective pressure difference would actually make you blow up but there could be some unpleasant effects, are you sure you want to risk it?

One thing worth mentioning, take the warning not to use cell phones seriously.  Despite rumors to the contrary there have been incidents where the cause was believed to be passengers using cell phones.  Now obviously if the FAA decides at some point in the near future it's OK then they must know what they are doing but till then...  

Also one of the incidents wasn't just navigation aids playing up (which except during instrument landing is fairly unlikely to be fatal).  An aircraft fitted with FADEC had trouble with fuel valves intermittently closing causing engine problems, last I heard their best guess was a passenger using a cell phone caused it.  (Maybe an aero person has more up to date info on this)

RE: Even though oxygen is flowing...

The serious side effect is called pulmonary odoema (sp?) and is a rather unpleasant way to die. Basically the alveoli in your lungs are torn by the overpressure, and you then drown in your own juices. Pulmonary barotrauma is caused by overpressure as small as 80 mm of mercury so 1/4 of an atmosphere, which is roughly what you would get in the example given, is way more than your body is designed to take. Hint, try getting a reading on a pressure meter by blowing into it. That is what your body is designed for.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Even though oxygen is flowing...

Thanks Greg, figured someone smarter than I would be able to do the math.

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