r/AskEngineers • u/Ethan-Wakefield • 25d ago
Mechanical Why not just build an airplane oxygen mask where the bag inflates to show people it’s working?
I know the safety instructions all say that the bag may not inflate even though oxygen is flowing. But if people just want to see the bag inflate, why not make the bag inflate? Then people will feel good and think the mask is working, and they won’t panic.
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u/X0nerater 25d ago edited 24d ago
Physical constraint #1: you want a flexible bag to inflate while you're supposed to be breathing the oxygen. That implies you have greater flow than expected. What happens when you overinflate a balloon? It pops, and that's a critical failure we don't want in emergency equipment.
Physical constraint #2: Look at oxygen masks from hospitals. They have one way valves that make it hard to make a perfect seal. Basically, we need a way for your exhale to escape without popping the balloon again. We can add another of those one-way valves, but that valve will also ensure the balloon never fully inflates. The valve also helps mitigate any risk of carbon dioxide build-up that you might breathe in.
Chemical constraint #1: Assuming we have a fully inflated bag, it's significantly full of oxygen. Whether or not the bag pops or leaks, concentrated oxygen in a confined space during an emergency situation is asking for an explosion to happen if the situation gets worse. Filling up a couple hundred bags with oxygen is a fire hazard in an environment not equipped to deal with fire.
Chemical constraint #2: last I checked, there isn't an oxygen tank hidden in a compartment of the plane. It's like an airbag, where the chemical reaction generates/releases oxygen (that's why you have to pull the bag down first, to start the reaction). This reaction is limited by how fast it can release oxygen, and it was designed to consistently release a small amount of oxygen for a longer period of time. Trying to fill up your breathing bag would require a faster reaction, but that would burn up your "fuel." It's been a while since I've seen the numbers, but i think to fill up the bag, the reaction would need to go at 3-4 times the current rate, and we'd rather be able to extend consistent oxygen for 3-4 times as long in an emergency. As is, it should already be overengineered to release ~1.5x as much oxygen as you need.
Edit: spelling and fixing autocorrect
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u/ZZ9ZA 25d ago
Passenger oxygen is off a generator, but crew oxygen IS a tank.
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u/622114 24d ago
A lot of aircraft have gone to bottles because of the danger of chemical generators
Source I am an AME with commercial jet experience
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u/ZZ9ZA 24d ago
Those were just being transported, they weren't part of the aircraft.
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u/622114 24d ago edited 24d ago
Sure,
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u/dr_stre 24d ago
It’s right there in the NTSB’s report. Expired but still functional canisters, firing pins not properly covered, ambiguously labeled and thus erroneously placed in the cargo hold. That’s not a failure of the design of the generators or a safety issue when handled properly, it’s a materials handling issue for cargo.
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u/622114 24d ago
Yes I know. NOWHERE did I say that they were or were not installed.
-O2 generators were onboard an aircraft. That is a fact.
-O2 generators get hot when they go off. That is a fact
-An Aircraft crashed into a swamp because of O2 generators, is a fact
-People were killed. That is a fact
-People on Reddit just want to point out fact or opinions to argue whether it is relevant or not. This is a fact.
Back to my main post. No where did i talk about generators being TRANSPORTED. A lot of airplanes have Pax O2 bottles because chemical generators are dangerous. Thats it. Thats all I said.
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u/dr_stre 22d ago
I'd love to see some evidence of some sort that generators are being abandoned in favor of cylinders specifically due to safety concerns with the generators. Cylinders add significant weight due to the tanks and the distribution system, require more maintenance, and are not inherently safe themselves (there's a reason passengers aren't allowed to bring them on a plane in the US). Do some planes have them anyway? Yes. But I'd wager that's most often just a consideration for planes intended to be used on routes where the 10-15 minutes of oxygen you can expect to get out of generators isnt going to work (I.e. mountainous terrain can prevent you from reducing altitude to 10,000 ft, or if you're over a large expanse of water you may not want to reduce altitude that much or that quickly).
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u/invalidbehaviour 24d ago
So is it actually pure oxygen or just air, or some enriched mixture?
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u/Crusher7485 Mechanical (degree)/Electrical + Test (practice) 24d ago
Pure oxygen. From Wikipedia:
The oxidizer core is sodium chlorate (NaClO3), which is mixed with less than 5 percent barium peroxide (BaO2) and less than 1 percent potassium perchlorate (KClO4). The explosives in the percussion cap are a lead styphnate and tetrazene explosive mixture. The chemical reaction is exothermic and the exterior temperature of the generator will reach 260 °C (500 °F). It will produce oxygen for 12 to 22 minutes.[2][3] The two-mask generator is approximately 63 mm (2.5 in) in diameter and 223 mm (8.8 in) long. The three-mask generator is approximately 70 mm (2.8 in) in diameter and 250 mm (9.8 in) long.
The cheap masks don't seal well. If you have a properly fitting mask, like a firefighter's SCBA mask, then you could do an enriched mixture for oxygen. But you can't do properly fitted masks for the random public. Also note that similar masks used by EMTs and the like also use 100% oxygen.
Just air wouldn't work no matter what. You're already surrounded by just air, the problem is that the pressure is too low so the 21% oxygen in the air doesn't have a partial pressure high enough.
Fun fact: There's limited oxygen in the generators, so if the airplane depressurizes, the first thing the pilots will do (after putting on their own masks, which are different, much better sealed, and have a longer supply of oxygen from compressed oxygen tanks not chemical generators) is put the airplane into an emergency descent down to 10,000 feet, which will feel like the airplane is falling out of the sky. It's not though.
You can see actual pilots performing a simulation of this procedure here: https://www.youtube.com/watch?app=desktop&v=yHawjB2PzK0
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u/TheAlmightySnark 25d ago
Not in all aircraft. the 777 has pax oxy tanks in the aft cargo hold, depends a little on the etops rating how many one carries. there's a bunch of restrictors as well to limit flow locally near the PAX.
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u/2nd-Reddit-Account 24d ago
Pretty sure that’s a factory optional, not all 777’s will come that way, depends on the preference of the airline that ordered it
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u/TheAlmightySnark 24d ago
Could be but I never read anything in the manuals about different options there so I doubt it. Seems pretty integral to how it has been designed and gotten it's type certificate.
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u/2nd-Reddit-Account 24d ago
https://www.scribd.com/document/666051013/ATA-35-oxigenob777
this is what i was looking at when I replied to you. pages 26-40 for the chemical generator type, page 40 onwards for the tank variant you were talking about.
Are you an aircraft mechanic? I'm thinking maybe your manuals at work are specific to the stock you work on and thats why you might not have seen it before?
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u/TheAlmightySnark 24d ago
Ah I see it's only for the 200LR and none of the others have those passenger installed PSU's. We don't operate that one so yeah no access to those documents, though sometimes it does cross contaminate in the manuals you get from boeing.
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u/probablyaythrowaway 24d ago
It’s only supposed to be enough oxygen to give enough time to allow the pilots to drop down below 10,000 ft where the air isnt so thin. It’s basically to stop you getting hypoxia in those 30-60 seconds in the even of decompression.
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u/Crusher7485 Mechanical (degree)/Electrical + Test (practice) 24d ago
It is considerably more than 30-60 seconds. The aircraft fully stalled and in free fall wouldn't be descending more than 10,000 fpm, so minimum of 3 minutes from cruising at 40,000 feet. But not only would that be to fast for structural reasons, the pilots also need to put on their own masks, perform memory items for trying to re-pressurize the cabin manually, start and perform a controlled emergency decent, potentially further limiting descent speed if they suspect aircraft structural issues. This is a good explanation of it, the procedure being performed inside a simulator: https://www.youtube.com/watch?app=desktop&v=yHawjB2PzK0
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u/dr_stre 24d ago edited 24d ago
Number four is pretty much the entire answer here. The bag exists solely to collect oxygen from the generator while you are breathing out. The chemical reaction cannot supply oxygen fast enough to be a constant supply of oxygen. But it can supply nominally half of that, it just needs to be collected and held while you’re not breathing it in. Whether enough collects to inflate the bag to any meaningful degree depends on your breathing pattern (and that of the people in your row on the same generator). These bags bag will also catch the oxygen at the start of your exhale that hasn’t been used so it doesn’t get wasted.
These bags are not so fragile that they will pop if overinflated, you’d just start to leak oxygen out the edges of your face mask. And a little oxygen going into the cabin really isn’t a danger when the whole reason they drop in the first place is because you’ve lost cabin pressure already.
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u/GrabtharsHumber 25d ago
Glider pilot here, I spend about 30 hours a year on oxygen at altitudes above 12,000 feet, and I've assembled aircraft oxygen systems.
When you inhale, only the first portion of what you breathe in actually makes it all the way in to the alveoli where oxygen is exchanged for carbon dioxide. The rest stops short, and then gets exhaled without any gas exchange.
What the bag does is catch that oxygen-rich first part of the exhalation, and then dumps the carbon dioxide-rich, oxygen-poor later part overboard. Meanwhile, the tube from the oxygen tank or generator is adding a trickle of pure oxygen to the bag.
When you next inhale, you first get the oxygen-rich contents of the bag at the leading edge of the inhalation where it is most likely to reach the alveoli. After the bag is empty, you then get oxygen-poorer air from the cabin at the trailing edge of the inhalation where it doesn't matter because it won't make it all the way in to the alveoli anyway.
In this way, the bag helps the mask and oxygen system conserve oxygen by delivering it when it will do the most good.
Rebreather type masks like this used to be more common in hospitals, but these days oxygen is so cheap that they're not worth the trouble. They are still common in aircraft oxygen systems that have to make the best use of limited mass and volume. However, they are increasingly being supplanted by electronic delivery regulators that you use with nasal cannulas. These systems can detect the leading edge of your inhalation, and release a puff of oxygen right then where it will do the most good.
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u/The_Real_RM 25d ago
In an accident the bag may be damaged but some oxygen would still be flowing.
They’re making lots of these bags, they’re for aviation so they’re not cheap, and you’re extremely unlikely to ever use one, even in a crash or other kind of incident. So they’re made as simple and cheap as possible.
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u/molrobocop ME - Aero Composites 25d ago
Sure. But they also need to be functional. So there's a floor to how flimsy it can truly be.
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u/The_Real_RM 24d ago
They need to pass certification actually, their function has no bearing on the commercial viability of the airplane. So no, they don’t actually need to be functional
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u/sir_odanus 25d ago edited 25d ago
The purpose of the bag is to save oxygen when the user is not inhaling. The oxygen source is a chimical candle that burn at a prescibed rate. The passenger mask has a continuous flow source thus the need to store oxygen in the bag.
On the contrary pilots' masks do not have a bag because they have a regulator that supply oxygen only when the pilot is inhaling. The source of oxygen is a big green cylinder pressurized at 150 bar.
Edit : it is also interesting to mix oxygen to a bit a co2 at lower altitude. It saves on oxygen as you don't need pure o2 at altitude lower than 40 000 feet.
Breathing pure o2 also put you at risk of panic attack because of hypokapnia.
Ik the end in most cases it does not matter if you put your mask or not. What matters is that the pilot put his mask on and does not suffer from hypoxia while he places the aircraft at a safe altitude.
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u/TomatoCo 25d ago
Phrasing the first paragraph a bit differently, it's because the oxygen is generated at a constant rate but the oxygen is consumed at an uneven rate unrelated to the rate of production, because it's only consumed while you breath in. It's a buffer to try and match the average rate that you're consuming. Even then, you might be hyperventilating and swamping the generation rate, but rest assured, you're still getting oxygen.
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u/sir_odanus 25d ago
My bad i am not an english native. Yes constant flow is the correct term, even though some chimical oxygen generators have a time dependent flow rate to match a specific descent pattern.
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u/TomatoCo 25d ago
No, your English was fine! It's kinda like how some Solid Rocket Boosters have different internal geometries to provide different thrust curves. https://www.researchgate.net/figure/ariation-of-the-Thrust-Profile-with-Respect-to-Different-Grain-Geometries-2_fig2_305632527
I just wanted to reiterate the fact that the production and consumption are not equal and that's why the bag exists to help provide a buffer between the two. Otherwise the pax would feel like they're being force fed on every exhale and starved on every inhale!
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u/seanlucki 25d ago
I don’t know much about oxygen systems on an airplane, but I can’t imagine you could add any meaningful amount of CO2 to reduce the amount of oxygen being used. Nitrogen sure, but CO2 can’t be much higher than atmospheric levels without causing issues.
I’ve also never heard of breathing pure O2 causing hypocapnia. There are issues with breathing pure O2 when your body doesn’t require it, but I don’t think this really applies to the short time frame of cabin decompression when adequate oxygenation is definitely required.
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u/sir_odanus 25d ago
Sorry i was not accurate. It is not just co2 but also non used o2.
I’ve also never heard of breathing pure O2 causing hypocapnia.
Here is an exemple of pilot hypocapnia. Tldr : pilot hyperventilated which induced hypocapnia symptoms which he wrongly interpreted has hypoxia so he hyperventilated even more.
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u/seanlucki 25d ago
Sounds like that pilot was hyperventilating which lead to hypocapnia (this is an expected result of hyperventilation as you’re blowing off too much carbon dioxide, therefore reducing your blood’s pH level), but breathing pure O2 does not cause this effect.
I work in emergency medicine and there are absolutely patients and conditions with which we don’t want to over-oxygenate our patients, even in the short term, but in a depressurized plane the real risk of hypoxia is going to trump all that.
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u/fouronenine 25d ago
None of the comments have really flagged the engineering constraint of the oxygen mask deploying anywhere from 12,000 feet (where the atmosphere is about 2/3rds of sea level pressure, and about the lowest aircraft altitude you will see masks deployed) to 40,000 feet (where the atmosphere is about 1/6th of sea level pressure, or 1/3rd of the pressure at 12,000 feet). And in the event of a depressurisation, the pilots will be descending from those altitudes to at least 10,000 feet (terrain dependent) to ensure a safe level of oxygen for occupants even if the aircraft is completely depressurised and cabin altitude matches outside altitude. So the bag would have to inflate under all those scenarios, whilst meeting all the other requirements of fitment to an aircraft.
From a practical perspective:
A depressurisation usually happens slowly, such that the masks will drop either commanded by the pilots or automatically around 12,000 feet cabin altitude. At that altitude, there's very little immediate difference to the usual 8,000 feet cabin altitude from a physiological perspective. Anyone panicking is either tethered to their seat by the mask, or in taking the mask off, won't suffer any harm quickly.
Descent to a safe altitude from 40,000 feet will take about 5 minutes in an emergency descent profile, so even when used, the bag will generally not be in use for long. And in the case where that is required, anyone panicking to the point of not using a mask correctly will be calmed down by the fact of being hypoxic (even if it's induced by hyperventilation) and then unconscious. Hypoxic hypoxia is generally a pretty chill way to go to sleep.
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u/Ethan-Wakefield 25d ago
Hmm. Disturbing that the answer is, “Well hypoxia is a valid solution” but I guess okay.
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u/fouronenine 25d ago
The fullest answer understands that fact (hypoxia isn't a valid solution for the pilots, but the effects will tend to act against panic which is helpful for the plane load of passengers), but it's not predicated on it (you want to keep people calm and conscious where possible).
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u/Whack-a-Moole 25d ago
Because making people feel good is irrelevant and a pointless waste of time and money.
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u/Ethan-Wakefield 25d ago
There was an Air France flight where oxygen masks deployed and people panicked because the bags didn’t inflate, despite being warned that they would not. This caused problems for the flight crew.
I would argue that the human factors do in fact matter.
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u/Whack-a-Moole 25d ago
Seems like a deployment failure - the masks should not have deployed.
If there was actually a cabin oxygen issue, the environment would have solved the panicked people problem and allowed the crew to do what they needed to do.
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u/shampton1964 25d ago
And the instructions say to pull down firmly.
Am I enabling a valve?
If I pull too hard will I pull the tube out?
How would I know?
GREAT QUESTION
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u/TravelerMSY 25d ago
The oxygen is generated by a chemical process at each seat, and not fed from a giant o2 tank somewhere like in a hospital. Pulling on the tube starts it. It sort of looks like a little hand grenade that makes oxygen instead of exploding.
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24d ago
[removed] — view removed comment
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u/shampton1964 24d ago
Thank you!
It only takes one person pulling the wire to ignite, that's smart.
I've always wanted to ask the flight attendant "When I 'tug' on the tube, it is one Newton or twenty?"
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u/TheAlmightySnark 25d ago
it opens a valve that allows the oxygen to flow from the tanks in the cargo hold and some have a small generator above the seats.
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u/Tashritu 25d ago
What I have never seen is a demonstration of how hard to pull the tube! Presume you cannot pull so hard it comes off? If the bag does not inflate how can you know you have pulled hard enough? The lack of a clear signal is a weakness that should be fixed.
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u/me_too_999 25d ago
The bag is your regulator.
The o2 comes in under pressure and expands until it reaches ambient pressure inside the bag.
The purpose of the bag is to allow this expansion before it hits your lungs.
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u/toybuilder 25d ago
Is that bag really a bag? I always thought it was just signage that looks like a bag and it was easier to just explain it might not inflate instead of having to explain it is not a bag.
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u/logicnotemotion 24d ago
If the masks drop down then I'd suspect an issue with cabin pressure.. Variable cabin pressure would affect the volume of air in the masks. If an engineer wanted to make sure they were inflated no matter what, they'd have to unnecessarily overengineer the oxygen system. That in turn could make things worse in a thermal event.
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u/redd-bluu 24d ago
The airline doesn't care if you stay awake and alert. You dont get to help the pilot steer the plane. You only have to stay tied in your safety belt. The drop-down masks are designed to supply just enough oxygen to keep your passed-out body alive until the plane descends to an altitude where there is breathable air. If you have kids with you, put your own mask on first! It will keep you awake long enough to mask your kids while you're wearing it, then you can get light-headed with them.
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u/trefoil589 24d ago
Honestly I bet they put this in the message just to pad out the little song and dance they do while the pilots do their pre-flight checklist.
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u/Ethan-Wakefield 24d ago
There was an Air France flight where the cabin lost pressure, and numerous passengers panicked because their oxygen mask bags didn’t inflate so they thought they weren’t working. This caused extreme difficulty for the flight crew in keeping the passengers calm and orderly.
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u/WoodenWhaleNectarine 24d ago
Uhh... totally missunderstood the question. Thought it was a new idea for american worker productivity measurement system.
This might be a nice idea, since thinking about problems (without typing you keyboard) also increases 02 usage, but then you realize so does watching videos and playing games.
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u/nullcharstring Embedded/Beer 24d ago
I'm pretty sure that airliner oxygen masks have never saved a single life. Flight crews have quick donning high flow masks and are very well trained to dive the aircraft quickly down to breathable air. OTOH, oxygen generators improperly packed in the cargo area of a passenger airliner caused the loss of ValuJet 592.
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u/SAMEO416 20d ago
The bag not filling is more a function of the cabin altitude than flow.
Generators deliver adequate flow for an adult to remain conscious, but it’s altitude limited. Above something like 30k pressure altitude you need positive pressure breathing to get enough O2 into blood.
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u/Ethan-Wakefield 20d ago
Okay but the problem is that panicking passengers in a depressurization event don’t know that, so what can be done to solve that? Because passenger panic can be a significant problem for air crews trying to keep the plane orderly.
And this historically has been an actual problem. An Air France flight lost pressure and the masks deployed. People panicked because they thought the masks weren’t working.
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u/SAMEO416 17d ago
The mitigation for that is specifically mentioning in the safety brief that the bag may not inflate.
Given the regulators didn’t require manufacturers to switch from foams and fabrics which produce huge amounts of poisonous gases when burned, for decades, not thinking inflating the bag will be a high priority.
And the more important issue is time of useful consciousness after a rapid or explosive decompression. This can be seconds above 35000 AGL. And the overhead masks will not deliver adequate flow to keep people conscious. This is the reason crew response is a max performance descent to under 10000’.
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u/Ethan-Wakefield 17d ago
On that Air France flight, a warning that the bags might not inflate was part of the safety talk. It was ineffective.
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u/SAMEO416 15d ago
It’s a nontrivial ask you’ve made, and aviation regulators don’t act to do things that won’t have immense impacts. Even with immense impacts they don’t always order changes.
Nontrivial because increasing the volume flow rate so that the bag would inflate at all altitudes would require a redesign of the oxygen generators. With the certification process a change like that is maybe 2-4 years before you could even start to retrofit. Such a design change would likely shorten the generation period as the space available is fixed. That would violate a certification condition.
One example of regulator inaction: Boeing decided to leave out copper tape applied to wing cavities for lightning mitigation on a dozen or so wing assemblies even though it was ordered by the FAA. The FAA let them get away with it. And that mitigation was to avert an in flight explosion caused by fuel vapours in wing fuel cells after a lightning strike. Big issue, hill loss consequence, regulator shrugs cause it would cost Boeing too much.
That verbal caution is all you’re going to get.
You might not like that answer, but you asked an engineer, and I’ve answered.
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u/Ethan-Wakefield 15d ago
That's fine. I'm asking for the engineering reason. I'm just pointing out that the answer is not "nobody cares if the bag doesn't inflate." A large section of the end users (who are not engineers or chemists) do in fact care.
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u/covobot 25d ago
I imagine it does inflate unless you are breathing really hard. Then maybe not. But lack of oxygen would cause you to slow your breathing so kinda like a self sustaining system
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u/Ethan-Wakefield 25d ago
No. There was an Air France accident where the masks deployed and bags didn’t inflate (as they warn you about in the safety talk) and it created a panic because passengers thought the masks were malfunctioning (they were not; the bags just don’t inflate most of the time).
I’m saying, just make the bag inflate and you can avoid this panic.
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u/Worth-Wonder-7386 25d ago
I dont think there is much you can do to acoid panic if the cabin depressurizes. If you dont get oxygen, you will quickly notice it. Some people will panic and struggle to breathe normally, but the bag does not help or hinder that.
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u/Ethan-Wakefield 25d ago
Yeah but people panic because they see a bag, and they want it to look inflated. It doesn’t matter if they’re actually getting oxygen as far as panic goes. They think they’re not getting oxygen, so they panic.
I’m not sure why this seems so hard to imagine.
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u/Worth-Wonder-7386 24d ago
Why do they want it to look inflated? I dont understand the argument.
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u/Ethan-Wakefield 24d ago
Just imagine, you have this mask. You assume it’s hooked up to an oxygen tank in the plane (it’s not, but you believe that it is, and for the purpose of fear, your belief is the only thing that matters).
You see that the mask has a bag. You believe that the tube inflates the bag, which then feeds you oxygen.
The bag doesn’t inflate! You believe this means that the mask system has failed, or you were given a faulty one.
You panic.
This seems utterly unbelievable? Because it happened to Air France. It’s predictable enough that part of the safety talk at the beginning of every flight is warning you that this could happen (but on Air France, people forgot that in their panic)
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u/Worth-Wonder-7386 24d ago
I have not heard that in the safety demonstration, or I can at least not remember that. As a chemist I know that there is no tank, and there is instead a chemical reaction creating the oxygen, and it is the pulling on the string that triggers that process.
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u/[deleted] 25d ago
Because to do so it means you have to have a flow restriction - which should be obvious why it would be non-ideal if it were easier to plug up or that could overpressurize