If you leave the mask in the wrong mode, then it has "failed". This is far more likely to occur by inadvertent manipulation during donning, or forgotten on a checklist.

BTW, you can't see the mode switch when wearing the mask.

Mike C.

Not with O2 masks, but with respirators for occupational exposure control.

With a beard, you cannot get a tight seal. So with a normal inhalation, a significant amount of the air inhaled is leakage. A short stubble it worse than a full beard, but it is incremental.

Under pressure, the poor seal leaks a LOT of O2. And prevents pressure build up in the mask.

The military ones supply no O2 under 10,000 feet cabin altitude. Then they supply up to 100% based on altitude.

We were limited to 25,000 feet without pressurization.

There was a 100% setting, if you needed it. And an Emergency, that supplied above ambient pressure to increase O2 partial pressure.

The regulator I used looked like this:

Not sure why most of that functionality in our planes lives on our masks where you can’t see it.

Chip-

I practice mask donning 1-2x per year in the various jets we fly. My big takeaway is I need to rip everything off my head first (headset and sunglasses), otherwise I fumble too long.

-Citation Jet Exchange

Just did hypoxia training again. The fella teaching taught in the military chambers a couple of decades before doing it in the civilian arena. He said if you aren’t wearing your mask with an explosive decompression in the high 30’s and 40’s you probably aren’t making it, and just fooling yourself. The startle factor, fogging, sound, pain of expanding gasses everywhere, as well as gases escaping your body from multiple orifices, the fumbling to get to the mask and then you are pretty much done. I have rapid decompressed to 25,000 fet in the chamber, and even that is unpleasant. Everyone is different though. I got through 25,000 feet playing Tetris pretty well. At 30,000 I was feeling pretty rough after 1 minute, and he made me go back on O2 when my sats dropped below 60%. My hypoxia symptoms had changed from when I was flight status as a youngun in the AF. I got euphoria then. Not a good first symptom, because I am always happy when flying. This time I had dysphoria, and felt like something bad was happening. The instructor said over time peoples symptoms can change.

Explosive decompression probably means the plane isn't making it either. Those events are extremely rare, like wings falling off. Structures are inspected, windows are checked, so the odds are extremely small.

I believe there has never been an expulsive decompression of a Citation in its entire recorded history. This is true of many types.

At FL450, 85% of the atmosphere is below you, so it is pretty much like walking out an airlock on a space station if the cabin empties. Water would boil at 116 F, 47 C.

Mike C.

The CJ fleet is getting older.

That's definitely a picture, but that plane had close to NINETY THOUSAND cycles at over 8.0 psi. Most planes in the CJ fleet (and probably the Citations) haven't made it to nine thousand yet.

That 737 had 90,000 cycles when it blew its top due to extremely short flights among the Hawaiian islands.

No CJ is even remotely close to that.

Fatigue is mostly about cycles, not calendar age.

Mike C.

There's no chance that is correct.

A hole the size of a window with 6 PSI drop across it (14,000 ft to FL400) would require abut 25,000 SCFM of air, or about one ton of air per minute. Just no way the engine can deliver that much air, maybe not even out the tailpipe.

Mike C.

Video or it didn't happen!

Mike C.

Had a door seal fail on the Aerostar. While a much smaller air source, not even close to pressurize the cabin. The door gap may be small, but the total area is larger than one might think.

Video or it didn't happen!

Mike C.

TJ in my mind you are always right