O2 is probably cheaper than the fuel from having to stay low.

Most are using the third option: fly high with no O2.

The vast majority of hypoxia events are of the slow insidious kind. This usually happens on initial climb out. Payne Stewart, Bo Rein, etc. Just about the mid 20s, the plane goes silent and then the plane crashes if off autopilot (like the Citation V in Oregon) or flies until fuel is gone on autopilot (like the 551 in the Baltic sea recently).

A prototypical explosive decompression event is so rare that I have no examples I am aware of. Not saying it doesn't happen, but must be quite rare.

The worst decompression accident I am aware of is N804ST, a Citation 501 at FL430 that had a clamp fail on the cabin pressure duct, *and* had a check valve fail to prevent cabin back flow. The cabin depressurized in about 20 seconds. The single pilot, not wearing an oxygen mask, didn't put their mask on in those 20 seconds, passed out (apparently after turning off the autopilot and pulling back power), the plane fell from the sky, he regained consciousness before ground impact and bent the wings in a pull out. All were safe, but that was close.

On my plane, the clamp design is much better, I have redundant check valves, and they are made of metal and not plastic. So I am better in 3 ways than N804ST was.

I've adopted a procedure that if there is ANY emergency or abnormality at high altitude, I always don my oxygen mask first if I am not already wearing it. In many cases, this will be unnecessary, but sometimes emergencies start with misleading first indications. It is never wrong to be wearing the mask. At worst, it will provide you with more oxygen to think clearly during the emergency. I do practice putting it on regularly.

Mike C.

The 551 that went into the Baltic recently is a real head-scratcher. Maybe the 10,000' cabin alt warning wasn't working, but the cabin masks should have dropped at 13,000 unless they also malfunctioned or were inhibited. The pilot's wife was apparently also a rated pilot (though not typed on the 551), so maybe she was in the right seat.

Is the 10,000' warning system checked during one of the regular inspections?

Here's a Piaggio P180 depressurisation incident (a clamp connecting a hose to the Environmental Control System had become loose): https://www.gov.uk/aaib-reports/aaib-in ... -ii-d-ippy

All the insidious decompression accidents are like that. It boggles my mind because they all would have the cabin pressure warning when passing 10,000 ft. Why are pilots ignoring it? Or maybe the lights don't work.


Correct. You can inhibit the masks (on my plane at least) with the valve in the "crew only" position. N804ST had the valve in that position, for example.


It is checked in my phase 5 inspections tasks.

I also know mine works due to an unintended instance where my cabin went above 10,000 ft and the light came on.


They noticed, dealt with it, non issue. I'm sure most cases are like this, but then you get the others and can't figure out what is going wrong with the pilots.

Mike C.

Just remembered that the Baltic 551 pilot reportedly notified air traffic control early in the flight, after takeoff, about a cabin pressure malfunction. That makes it even more mind-boggling. Maybe the pilot put on his mask but wasn't receiving oxygen for some reason. Or perhaps the pilot delayed putting on the mask thinking he would be ok "for just a minute until I sort things out", but got hypoxic enough to lose his judgment and never put it on.

So, good advice to put the mask on at the first sign of any pressurisation issue.

What about a flameout event? I was in a LR25 that had one flameout at FL390. (and successfully re-lit) Just curious if most twin jets can maintain a reasonable cabin at that high altitude on one engine?

That was an interesting few minutes, we were on TOP of a T-storm, at night, lots of lightning below us. Some ice before re-start too.

My plane will hold full cabin pressure on either engine.

Personally tested, at FL430, at 8.9 PSI differential.

If it can't, needs to be fixed. That can be hard to find sometimes, though.

Mike C.

Most are using the third option: fly high with no O2.

The vast majority of hypoxia events are of the slow insidious kind. This usually happens on initial climb out. Payne Stewart, Bo Rein, etc. Just about the mid 20s, the plane goes silent and then the plane crashes if off autopilot (like the Citation V in Oregon) or flies until fuel is gone on autopilot (like the 551 in the Baltic sea recently).

A prototypical explosive decompression event is so rare that I have no examples I am aware of. Not saying it doesn't happen, but must be quite rare.

The worst decompression accident I am aware of is N804ST, a Citation 501 at FL430 that had a clamp fail on the cabin pressure duct, *and* had a check valve fail to prevent cabin back flow. The cabin depressurized in about 20 seconds. The single pilot, not wearing an oxygen mask, didn't put their mask on in those 20 seconds, passed out (apparently after turning off the autopilot and pulling back power), the plane fell from the sky, he regained consciousness before ground impact and bent the wings in a pull out. All were safe, but that was close.

On my plane, the clamp design is much better, I have redundant check valves, and they are made of metal and not plastic. So I am better in 3 ways than N804ST was.

I've adopted a procedure that if there is ANY emergency or abnormality at high altitude, I always don my oxygen mask first if I am not already wearing it. In many cases, this will be unnecessary, but sometimes emergencies start with misleading first indications. It is never wrong to be wearing the mask. At worst, it will provide you with more oxygen to think clearly during the emergency. I do practice putting it on regularly.

Mike C.

The way I understand it, the mask can be operated in 3 modes:

Diluter, sometimes called normal: supplements cabin atmosphere with additional oxygen.

100%: uses only 100% oxygen for inhalation

EMER: continuous flow of oxygen without regards to inhalation

My manuals direct that oxygen masks be kept in 100% mode above FL250, so I leave them there all the time, there is no automatic switch over from diluter to 100% mode. They use no oxygen in this mode in the holder. They only emit oxygen on inhalation demand, when you are breathing in, the Darth Vader sound.

If I was using them for a long time below FL250, which seems like a rare case, I could switch to diluter mode which saves oxygen. Then they meter out somewhat less oxygen per breath.

EMER mode is continuous flow of oxygen, not metered per breath. It is used in situations of smoke in the cabin to exclude external air. There is also a vent on top of the mask which can be opened to pressurize smoke goggles, which I have.

The oxygen masks on my MU2 were kind of a joke, like the plastic ones you get at the hospital. The ones with my Citation are serious business.

Mike C.

Most twin jet aircraft can’t maintain much above mid FL 250 on one engine. For cabin pressurization, a lot depends on how tight the cabin is to begin with (door seals, windows, ducting, etc.). Unless you can relight quickly, you will be drifting down at MCT on one.

The other difference between 100% and EMER is that EMER Is 100% O2 under slight pressure so that, when you breath in, O2 is “forced” in to the lungs.

Brad

Don, are you referring to the Baltic Sea 551 crash? That plane was climbing at 1000 FPM through FL 220 and was cruising at FL 360 and 360 KT ground speed for hours until it ran out of fuel. A 551 can't do that on one engine.

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My manuals agree.

With one engine, I can maintain FL250 at max weights. As I get lighter, I can go higher, for example 12,500 lbs to FL310.


It will be a slow drift down. As you near the limit, you are getting lighter, so things improve.

The specific range on one engine isn't as good as two, but not as good as one. Examples:

One engine: FL270, 13,500 lbs, LRC, 761 pph, 263 KTAS, 34.6 nm per 100 lbs.

Two engines: FL410, 13,500 lbs, MCT, 1077 pph, 403 KTAS, 37.4 nm per 100 lbs.
Two engines, FL410, 13,500 lbs, LRC, 798 pph, 346 KTAS, 43.4 nm per 100 lbs.

This is handy to know in case of a long over water leg and what to do if you have an engine failure half way. Your single engine range is reduced, but not horrifically. As you get lighter, you can go higher and it gets better.

One engine: FL310, 11,500 lbs, LRC, 636 pph, 259 KTAS, 40.7 nm per 100 lbs.

Mike C.

That pressure is quite small and probably negligible physiologically.

The EMER mode is stated to assure you do not take in any external air from the cabin, and thus used in cases of cabin smoke or contamination. It is not necessary for keeping you alive oxygen wise. My mask as a way to vent that into my smoke googles for the same reason, to exclude smoke.

Mike C.

No, just referring to my own experience. Didn’t really think about it at the time, lots of alarms going off. Just wondered if the cabin altitude would stay safe.