During the safety briefing on every flight I’ve been on, the emergency oxygen masks are demonstrated. I know that pulling sharply on the hose begins a chemical reaction that generates oxygen. In other words, the oxygen is not in pressurized tanks, but is instead created when needed. The oxygen should last around 20 minutes or so while the pilots work to descend to a breathable altitude.
The flight attendant points to a bag below the facemask and says that the bag may or may not inflate. What is this bag for?
For one thing, the oxygen is going to be produced at a constant rate, but it’s not going to be consumed at a constant rate: If nothing else, there will be separate breaths. So the bag is to accumulate the oxygen until the wearer is next ready to inhale.
It might not inflate completely, depending on a variety of circumstances. But it’ll at least sometimes inflate fully, or they would have made them with smaller bags.
Plus the rebreather aspect. When breathing an oxygen-heavy atmosphere, not all of it will get absorbed on a breath. So giving a chance for the exhaled unusually-oxygen-rich air to be breathed a second time helps get the full value of what oxygen is being generated.
The thing they really ought to include in the briefing is that a) the oxygen generation process is noisy; b) it gives off a LOT of heat; c) it stinks; and d) the heat will set a LOT of dust to smoking & stinking, just like when you first start your home furnace in the fall.
That is IMO far more alarming than any BS about a partly inflating rebreather bag.
As I have posted before, I’ve had the “opportunity” of using an aircraft oxygen system. This was more than 20 years back, so here is my first hand experience :
It happened at cruising altitude. The pilot announced that there was a leak and within seconds the oxygen masks fell out.
I have a background in chemistry and I remember smelling initially the hot oxygen (from peroxide or chlorate and dusty iron oxide maybe). It never inflated for me - and in fact the flow was not a whoosh but a trickle. I may have breathed in panic initially and may have tried to “suck in more air” than normal.
There is a silent panic all around you - you can see the fear of imminent death in people’s eyes all around - people holding hands. Panic makes it worse.
The airplane went into what felt like a free fall - and I lost sense of time. It felt like it was minutes till we got to a safe altitude.
Some people passed out - the bag helps in letting the oxygen flow to your nose even when you have passed out.
I had severe ear pain for many many days and many others had their ear drums rupture. This adds to the pain / confusion. In fact my main memory of the incident is ear pain to the point that you cant think anything else.
I had a sort of PTSD thing for many years after that and panicked every time my flight would hit rough weather.
Thank you for sharing. That probably wasn’t all that easy to write.
Your experience is pretty much exactly what they teach us to expect and to do.
Different airplanes have different descent rates, but 5-7 minutes from cruise to “masks not required” is pretty typical. Might be as much as 10 if the airplane was damaged and we need to be genteel lest we break something even worse. The typical oxygen generator systems are designed to produce oxygen for 12 minutes from when the canister is fired. Which happens when you “pull on the mask to start the flow of oxygen”.
During the descent there’s not much for passengers to do. Which means there’s lots of time to contemplate your recent change of circumstances with few distractions. That’s not always a comfortable topic for ruminations.
Severe ear pain, screaming infants & pre-verbal toddlers, a few bloody ears, and a couple of unconscious passengers are to be expected. Your ears just went from a pressure of about 8,000 feet up to maybe 35,000 feet in the space of a few seconds to a couple of minutes, then back down to 10,000 feet in a few minutes more. That’s a real barometric rollercoaster. And once near an airport, the descent from low altitude cruise to landing will also be uncomfortable for the ears compared to what you’re used to.
A simple pressurization failure is real low-risk, assuming you put on your mask timely & properly and the pilots do the same. IOW, if everybody does the right thing in the first 20 seconds, the remaining 20-40 minutes to landing will be boring. It’s nerve wracking, but not especially dangerous.
That can’t be understated. They get so hot that special measures are required when canisters are being transported. Failure to do so is what started a cargo hold fire that brought down Valujet 592:
If the oxygen generator is running at a fixed rate and spewing out oxygen whether it is used or not why do they need to try to get more oxygen out of exhaled air? If you re-breathe your exhaled breath the oxygen generator is just dumping what it produces to no effect.
Its not temperature but the pure oxygen. I work with Air Separation Units and pure oxygen demands respect. Spill liquid oxygen on asphalt, and asphalt will explode. Depending on pressure and temperature, oxygen will eat through (read burn) carbon steel and even stainless steel pipes. If you look at the rockets that use liquid oxygen, the pipes are usually incoloy. And even the grade of incoloy depends on the pressure, temperature and the velocity of oxygen flow.
And oxygen cleanliness is another big issue. There can be not a trace of grease in oxygen piping, and gaskets and flanges are different designs. Normal graphite gaskets would burn off. There are no commercial oxygen compressors. High pressure Oxygen is obtained by pumping liquid oxygen to high pressures and then vaporizing it through heat exchangers.
Many space flight related accidents are actually oxygen related.
Even the home oxygen concentrators people use at home can significantly expose them to higher risk s :
“Even though oxygen users only represent 0.5% of the overall population, this figure represents between 4% and 6% of the total fire deaths in the US. This means you are between 8 and 12 times more likely to die in a residential fire if you are a home oxygen user in the States.”
Normally, inhaled air contains 21% Oxygen and exhaled air contains about 16% Oxygen. When Oxygen masks are in use, the air in the cabin is escaping the cabin due to a leak in the cabin (obviously). So the exhaled air is not let out of the mask: it has 16% oxygen and the oxygen generator boosts it up to 21% (rinse and repeat)…
This will not work for long durations because CO2 will accumulate in the system - but for the short few minutes, this works fine.
I agree that an atmosphere with elevated O2 concentration can make some materials spontaneously oxidize or even burst into flame. The investigation into Valujet 592 claimed both were factors in the crash:
Chemical oxygen generators, when activated, produce oxygen for passengers if the plane suffers a decompression. However, they also produce a great quantity of heat due to the exothermic nature of the chemical reaction involved. Therefore, not only could the heat and generated oxygen start a fire, but the oxygen could also keep the fire burning. The fire was worsened by the presence of two main aircraft tires (one of them mounted on a main wheel) and a nose tire and wheel that were also included in the list of materials shipped as COMAT. Investigators determined that one of the oxygen generators was likely triggered when the plane experienced a slight jolt while taxiing. As the aircraft taxied and took off, the generator began accumulating heat, soon setting fire to its surroundings.
Laboratory testing showed that canisters of the same type could heat nearby materials up to 500 °F (260 °C). The oxygen from the generators fed the resulting fire in the cargo hold without any need for outside air, defeating the cargo hold’s airtight design.
Which is why we can no longer transport live chemical oxygen generators in the hold. “Makes monster heat AND makes pure oxygen” is not a great idea in a small space full of other ordinary flammable materials like cloth and plastic.
The next big ugly event like this will be from modern lithium ion or lithium metal batteries. We don’t carry bulk packages of them as cargo, but we do carry whatever’s in people’s luggage, plus cargo which consists of some device with an installed Li-ion battery. So a pallet of laptops or phones or e-cigs is fine, but a pallet of batteries is not.
I don’t know whether this battery limitation is company policy or federal / ICAO regulation.
Somebody somewhere is going to get unlucky & we’ll have another below-decks inferno. There are only a few potential emergencies that keep me up at night. This is number 1 on that sHit Parade.
I’m surprised we aren’t having more of these. People tend to bring their cell phones/laptops/tablets in the cabin with them, but there are so many suitcases below deck with battery-powered shavers, toothbrushes, vibrators, kids’ toys, and who-knows-what-else, all of which may have dropped multiple times over their life and have probably been fully charged before departure with a charger of unknown quality. Cheap hoverboards and vape pens have already been outed as fire hazards, but if even big-name reputable companies like Samsung and Boeing have trouble assuring good quality on the batteries they’re procuring for their products, then every battery-powered consumer electronic device in the cargo hold seems like one more roll of the dice.
A battery in a single e-cig or laptop going thermal runaway will cause a fire in the hold. We can handle most sorts of baggage fires. If the right pile of the wrong stuff starts to go and we get a chain reaction we’re probably screwed.
We can sooner tell people to travel with no electronics than we can tell them to travel with no clothes. It’s a risk that cannot be managed away at that level.
Thank you for the informative replies. The rebreather bag makes sense.
So, why is a system used to create pure O2? If the cabin is depressurized (-ing), then something significant has already gone wrong. It doesn’t make much sense to me to introduce a bunch of O2 into what may already be a chaotic situation. Also, why is a chemical reaction used instead of simply compressed gas? As I understand it, the flight crew gets its emergency oxygen from a bottle rather than a generator. Is maintaining bottles less reliable, heavier, or more expensive than chemical generators? Also, since it is an exothermic reaction, how is the heat handled? Is the O2 sent to the passenger uncomfortably hot? A hundred heat generators in the overhead doesn’t seem like a great idea.
If you don’t get enough oxygen in your blood you become unconscious, brain damaged, and dead in turn. At the reduced atmospheric pressure at altitude, there are only two ways to get enough oxygen into your blood: a) have a tightfitting mask that forces air or oxygen into your lungs under high pressure. b) increase the percentage of O2 in the gas you’re breathing to well above the normal 21%.
Choice B makes a lot more sense for untrained passengers. You get ambient air supplemented with 100% O2 at a flow rate that ensures the partial pressure of oxygen is close enough to sea-level normal that you won’t pass out for the few minutes it takes us to get to a survivable altitude. At least assuming you’re just sitting there more or less quietly.
Choice A is what pilots get. Which, trust me, is a miserable experience. Especially while trying to handle the rest of the flail for whatever went / is going wrong.
If the cabin is depressurizing you’ll get oxygen; regardless of any chaos in back, it can’t hurt & will help. Conversely if there’s smoke or fire, you won’t. For the obvious reason that more O2 is the last thing you want inside a mostly sealed tank with a fire in it.
If there’s both smoke/fire AND a depressurization, you also won’t get O2. At least not immediately. There is awareness of the tradeoffs here. But if that’s the situation, everybody is having a very unlucky day.
All of the above. Bottles need to be replaced regularly as they leak down. There are many single points of failure that would deny oxygen to everyone. They’re bulky & heavy. Most aircraft will go their entire career never deploying the pax oxygen. Generators make more sense in every measure of merit.
Conversely, up until COVID the oxygen masks were used briefly by the pilots on nearly every flight. They still get tested before every flight. Which testing consumes some of the bottle pressure and so those bottles are replaced with full bottles when the pressure drops to a certain point. A system that carries enough for 2 people is very different from a system for 100, 300, or 450 people.
Each generator is a canister about like a quart milk bottle. It’s got a heat sink and heat shielding around it. The immediate surroundings are able to take the heat for the few minutes it’s being generated. I have never heard anything about the O2 temp being hot enough to matter. But that is an interesting question. There is the stink of the chemical reaction and some stink from dust having settled on the heat sinks being singed off. Neither are show-stoppers enough to offset the benefits.
Right, because at 40,000 feet (and anything above 26,000 feet), even 100% oxygen isn’t going to be enough to keep your blood oxygen levels up to maintain consciousness for long. For the few minutes it should take to get down to a safer altitude, it’s most likely enough to keep anyone from dying, especially if they are just sitting in their seat, but the pilots are going to be burning more oxygen, and they need to stay alert. An unconscious pilot, or even a pilot operating at less than peak alertness due to decreased oxygen levels, is a problem for everyone on the plane.