Are you folks trying to make me never fly again? Because I think you’re succeeding!
Maybe, but IIRC isn’t that line included in the safety briefing specifically because there was some incident decades ago where the masks were needed, and some passengers panicked because they thought the non-inflating bags meant they weren’t getting oxygen?
I like to watch Mentour Pilot’s videos, and a lot of his videos these days are about airline incidents. A friend watched one with me, and said, “I don’t know if I want to ever fly again after watching that.”
I replied, “I feel much safer after watching that. We saw what the problem was, the circumstances that caused it to occur, the things the flight crew did wrong, and we also saw what steps the FAA et al took to prevent it from ever happening again.”
Agreeing with LSLGuy says, and adding a depressurization calculation:
1. Flow into the Cabin : FAA regulations require Airplanes to be designed to provide the equivalent of at least 0.55 pounds of fresh air per minute per occupant. So for a typical airplane with 200 occupants, you will have 110 lb/min of air “pumped” into the cabin from the engines. Assume, the pressure and temperature inside the cabin is 14.7 psia and the temperature is 60 F. This gives 1440 CFM (actual cubic feet per minute flow into the cabin).
2. Flow out of the cabin due to leak : The pressure inside the cabin is 14.7 psia and outside is about 2.5 psia at 40,000 ft. You can use an orifice calculator to check the size of a hole before you will get rapid depressurization. For example a hole of 2 inch diameter, where the upstream pressure is 14.7 psia and downstream pressure is 2.5 psia gives a flow of about 700 CFM. A 3 inch hole gives a flow of 1600 cfm.
So even if someone fired a bullet, (I am not sure of bullet sizes) and made a hole in the cabin wall, it will not suffer catastrophic depressurization.
So my guess is that the average leak that results in depressurization is fairly small (less than an inch - maybe a gasket is not seated properly somewhere) and does not result in catastrophic depressurization. This was my experience as well, there was air in the cabin during the even I experienced.
Link for the 0.55 lb/min requirement https://www.faa.gov/newsroom/cabin-air-quality
Link to an orifice size calculator : Calculator: Air Flow Rate through an Orifice | TLV - A Steam Specialist Company (Worldwide)
My question is:
I thought airlines use chemical oxygen generators. Once on they just produce X-cubic feet of oxygen per minute. It does not matter if someone is breathing 100% of the oxygen produced or 50%. The oxygen generator just keeps churning out oxygen at a steady rate.
This may be where my confusion lies. Can they store the unused oxygen for later use? Does re-breathing some air actually extend how long the oxygen generator can produce oxygen?
Youre 100% right the generator pumps out oxygen continuously once triggered. But …
If there was no bag then the O2 generated while you inhale would go into your lungs and the O2 generated the other ~60% of your breathing cycle would be wasted, just escaping into the cabin.
The bag serves as an accumulator between breaths. During the time you’re not inhaling the generated O2 is going into the bag. When you next inhale you breath that too.
The generator can be made much smaller / lighter / cheaper with a bag since you’re no longer wasting 70% of its output.
The bit about “not inflating” is mostly that the flow rate isn’t great gouts; it’s an overgrown trickle. But O2 is potent stuff and that trickle is plenty. If folks are expecting the bag to promptly look like a full balloon that they’re sucking down with each breath, nope, that ain’t it.
In the end the only figure of merit that matters is the partial pressure of Oxygen. At altitude, since the absolute pressure is low, in order to make up the partial pressure, the concentration of oxygen needs to be higher. At lower altitudes the concentration can be lower.
We can survive with a lower partial pressure of oxygen that we breath on the ground, so there is some room to breath the atmospheric mix at higher altitudes. And a bit more wiggle room where we function in a degraded state. It mostly doesn’t matter what other gasses are mixed with the oxygen, just so long as the partial pressure of oxygen is high enough. Carbon dioxide in rebreathed gas isn’t too good as it tends to trigger heavy breathing and gasping for air when it gets too concentrated - but we can stand a pretty solid amount before it becomes an issue. Oxygen flow into the bags will tend to mean it reaches an equilibrium that is OK.
So overall, the oxygen masks will operate to deliver what is needed. They elevate the concentration of oxygen in the gas mix being breathed, which at altitude is required to get the partial pressure high enough to avoid serious problems. The bag avoids wasting oxygen in the exhaled breath that would otherwise still be useful. Just enough oxygen flows to replenish oxygen actually removed from the air by respiration, and flush carbon dioxide. Most of the oxygen goes in and out of the lungs untouched. But you must have a high enough partial pressure to push the equilibrium across into the haemoglobin. Too low and oxygen actually flows out of the blood, and you can quickly die.
Valid questions.
I do not have a background in medical science, but from what I have learned "Our blood has evolved to capture the oxygen we breathe in and bind it safely to the transport molecule called haemoglobin. If you breathe air with a much higher than normal O2 concentration, the oxygen in the lungs overwhelms the blood’s ability to carry it away. The result is that free oxygen binds to the surface proteins of the lungs, interferes with the operation of the central nervous system and also attacks the retina." - https://www.sciencefocus.com/the-human-body/why-does-breathing-pure-oxygen-kill-you/
So rebreathing the air is to keep the oxygen concentration below a threshold.
Much like the airbags in a car, the oxygen bags are triggered either by a low cabin pressure or the pilot can activate it too, and it results in a small explosion inside the system to get the reaction started.
No. Once the reaction is triggered, it will make oxygen for a specified amount of time (usually tens of minutes) and then be exhausted.
However, you can compare them to fireworks, If you light 100 fireworks at the same time, maybe 97 of them will behave alike but 3 will have maldistribution of reactants and if oriented differently later will reignite or burn longer or work differently.
Chemical reactions have lots of different failure mechanisms - some of them have to do with orientation, some with leaks, etc, etc.
Here is a picture of the internals of an oxygen generator
The oxygen generator makes oxygen for a fixed amount of time. Rebreathing some air is to bring the concentration of oxygen in the inhaled breath lower. I have read that some oxygen generators have CO2 absorbers (usually Lithium compounds) to remove CO2 from our exhaled breaths.
Did the plane land, or simply descend and level off?
Not all of the oxygen is used when a person inhales.
In fact, it’s less than half. In a typical breath, only about a quarter to a third of the oxygen is absorbed in the lungs. Which in a way is good, since if all was absorbed, CPR wouldn’t work.
The plane made an emergency landing. The flight was a delta flight from Cleveland, OH to Atlanta, GA. The emergency landing was in Knoxville, TN. This was in 2000
IIRC I read about an incident where the oxygen bottle for the pilots made an unplanned exit from the side of the aircraft (Australia?).
This fuels my theory about the Malaysian airliner, that the pilots had a leak from their oxygen bottle, cleverly stored in the same compartment as the radios and other electrical equipment. They turned off much of the electrics breakers and did a 180 to return to land before passing out realizing their oxygen supply was gone - while the oxygen bottle in a fire gracefully exited the side of the aircraft. So the passengers sat breathing emergency oxygen for 10 to 20 minutes until they passed out as the plane travelled back the way it came, on autopilot, over the Malay peninsula and out to sea until it ran out of fuel. And none of the military involved wanted to admit their radar was inadequate to track this.
I assume the pilots have no backup supply if there’s a problem with the oxygen bottle. Do they at least have separate bottles, or do they “sink or swim” together?
I was under the impression that the O2 bags can be made to drop from their ceiling compartments due to low cabin pressure or pilot action, but that the actual production/flow of O2 doesn’t begin until the passenger grabs the mask and pulls it toward them. Am I wrong about that?
Here’s what you get when a pilot is hypoxic and unaware:
Kalitta flight KFS-66 departed Manassas normally and was en-route flying at FL320: 32,000 feet over the sea. The flight crew had just been handed off to Cleveland’s Air Route Traffic Control Center when air traffic controller Jay McCombs noticed that the aircraft had a “stuck mike” – that is, the Push-to-talk microphone was being pressed , keeping the transmission open.
What was actually happening was that the First Officer was already unconscious and his arm was flailing violently and uncontrollably, disengaging the autopilot and forcing the Captain was trying to hand-fly the aircraft. The air traffic controller can’t understand the Captain and a second pilot in a different plane helps to get the message across.
Don’t worry, everything ended fine. Here’s the audio from the incident:
There was another incident I recall where some university football coach and his entourage were flying in a private aircraft, when somehow the pilot was no longer responding and eventually the aircraft crashed at sea.
I’m impressed that a prop plane flies at 41,000 feet. That seems unusually high.
The “service ceiling” – the level at which cabin pressurization can break down – is 37,000 feet for the Cessna, according to one Air Force officer who declined to let his name be used. If an airplane goes above the service ceiling, hypoxia – a form of oxygen starvation – can set in.
The Cessna climbed as high as 41,000 feet.
If hypoxia set in, the officer said, the men could have been light-headed before going unconscious. The plane could have continued to run on automatic pilot until it ran out of fuel.
The interesting thing is that the pressurization breaks down at 37,000 feet. Not clear what this means, but it does indicate that the problem may have been gradual.
I guess the other question is -what happens in that situation - are the occupants simply disoriented and then unconscious? Do they regain consciousness at lower altitudes, or is death from hypoxia a result in a matter of tens fo minutes, or in minutes, like drowning?
You are absolutely correct. I stated incorrectly.
Yes, it makes no sense to trigger the oxygen generators in the empty seats (back when airlines had empty seats). Or to waste oxygen while the passenger has not yet put on the mask (but do it quickly!!)
Yep.
With the slight detail that there is one generator for each group of masks over one row of seats. So when triggered by the pilots or the automatic loss-of-cabin pressure sensor, the door opens, and the masks drop via gravity: 1 mask per seat plus 1 extra. So on the typical narrow-body aircraft that’s 3 seats & 4 masks.
Each mask has a separate lanyard connected to the firing pin. When the first passenger of that set of seats yanks a mask towards them, the pin is pulled, the generator fires, and all e.g. 4 masks are receiving oxygen whether anyone is wearing them (yet).
the threat wasn’t limited to the cargo hold:
that problem was fixed, but as batteries proliferate more such incidents should be expected.
Yep, noted upthread: