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An O2 Question

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Well-known member
Dec 30, 2001
First of all, I am embarassed to ask this, but I must defend myself (haha). I have become complacent with my pressurized airline aircraft and supplemental O2 always being there.

Now...back to part 91 flying, I am confusing myself. In a pressurized aircraft, do you still have to meet the supplemental O2 requirements of 91.213a1, 2, and 3? Part 91.213bi is throwing me off. So if my plane tops out at 250 and my built-in supp system is at ZERO because of a leak we can't track down yet, am I limited to altitudes of 12,500 and below??

Persoanlly, I have been staying low in protest of the leak because I want it fixed (anything broke needs to be fixed), but I am curious about what is required....

Thanks in advance!!
Strictly under Part 91, you can fly as much as you want below 12,500' without oxygen. Above 12,500, you can fly for 30 minutes without oxygen. Above 14,000 cabin pressure altitude, you need oxygen all the time, and above 15,000, it must be available to passengers (but they need not use it).

The reference is 14 CFR 91.211.

You cited 91.213, which deals with inoperative instruments and equipment.
ooops, sorry about the 213. I meant 211.

For some reason, I was thinking "flight altitude" and "cabin pressure altitude" were the same thing.

So, if the aircraft I am talking about has a max cabin pressure altitude of 10,000, that pretty much means that I don't need supplemental O2 (unless I am above FL 250).

So I am right to protest the O2 by staying low (the owner HATES that I do it but he knows why and is going to get it fixed now), but my only real legal FAA grounds are that since there is no MEL, it needs to be fixed. Also, I think it is necessary in case of a smoke emergency.

Does that sound accurate? (thanks Avbug, by the way)
avbug said:
Strictly under Part 91, you can fly as much as you want below 12,500' without oxygen. Above 12,500, you can fly for 30 minutes without oxygen. Above 14,000 cabin pressure altitude, you need oxygen all the time, and above 15,000, it must be available to passengers (but they need not use it).

The reference is 14 CFR 91.211.

You cited 91.213, which deals with inoperative instruments and equipment.

he said in a pressurized a/c.
In an unpressurized airplane, the issue of cabin pressure altitude and your aircraft altitude is mu(oo)te...it's the same (cabin pressure altitude generally a tad higher than actual, due to slightly lower cabin pressure, but not enough to worry about).

In a pressurized airplane, the aircraft altitude is irrelevant, as we're only concerned about cabin pressure altitude. Under Part 91, it's all really the same...we're only concerned about how high our bodies think we are inside the airplane.

If your aircraft is pressurized and can maintain a cabin pressure altitude of 10,000', then you're good to go legally, from a 91.211 point of view. You may have other issues, such as inoperative equipment (comes back to 91.213 again)...but so long as you remain below 12,500' cabin pressure altitude, you can stay there all day long.

As a kid I thought the oxygen requirements were ridiculous. I grew up a severe chronic asthmatic, and lived right about 5,000' above sea level. On warm days, in the summer, sitting at home put me at a density altitude of 8,000 or more. Typical flying was often at a density altitude of ten thousand or more. The FAA's official recommendation that oxygen be used above 5,000' at night would have meant we'd have all been on oxygen all the time...not very practical. I was accustomed to far less oxygen due to frequent asthma...the idea of needing it all the time seemed silly.

While I use it when I need it, I wouldn't have any issues flying the airplane with cabin pressure altitudes of ten thousand. Pressurized, or not.

I'm not certain what smoke elimination has to do with the matter. If your pressurization isn't working at all, then eliminating smoke is the same at low altitude as high...open a window, descend, get a fresh air vent going. If you're pressurized, then you want the cabin pressure set high to open the outflow valve as much as possible to vent smoke...your mission is already accomplished.

Personally, there's nothing better than the smell of smoke in the cockpit.

Using oxygen in an emerency is another matter...sometimes it's wise, sometimes it isn't. Most light airplanes have nasal cannulas available, and these don't do much for you, anyway. If you're talking something like a Citation with a real oxygen system, yes, the 02 is better than nothing...far better if you have smoke goggles so you can see, too, but better than nothing.

If your ceiling is maxing out at twenty five grand, then getting down to useable air isn't much of an issue. If you can maintain a ten thousand cabin pressure altitude and won't ever be going above 25,000, you're not even required to have the oxygen on board or available. Certainly you should...and oxygen presents it's own hazards and the leak should therefore be fixed...but legally you can leave the system unfilled and go fly.

If you need to make flights in the mean time, from a safety perspective, a small B cylinder from a local home health service filled with oxygen, a regulator capable of outputting at least six litres per minute (LPM), and a nasal cannula will meet your needs. If you want extra flow, carry a nonrebreather mask and ensure you can flow at least 15 lpm. Be sure you secure it properly and that the regulator and valve assembly is protected from damage. Such a unit is really only a backup in the event you do need some oxygen during a descent following a cabin pressure loss...most of the time it's going to sit unused.
Thanks. That is exactly what I was looking for.

As far as the supplemental O2 goes, I just figure that if I am going to fly at higher altitudes (up to 250), I want to have it in case I have any smoke emergencies where I can't get it out for whatever reason (or maybe if I have to discharge the extinguisher), and I need to breath as I make my descent. Maybe that is being too picky?

The owner mentioned a portable bottle for me if we can't fix the leak. Sounds like a good alternative.

Also, I just made another realization (I love you guys)... if my Cabin Alt light comes on at 10,000, all it means is that I need to be aware that I will require supplemental O2 as prescribed by 91.211 (right??) It doesn't mean that I have to do an emergency descent to 10,000. I believe in this case I was thinking of a cabin overpressure emergency.
You didn't say what type aircraft you're flying. As far as cabin overpressure, you'll have to refer to the aircraft flight manual and manufacturer data for specifics.

I don't think you're being picky. You're making a legitimate call as the Pilot in Command, for which nobody should question your judgement.

If smoke is truly the issue, remember that most supplemental oxygen systems are weak systems using a nasal cannula...even if you close your mouth and breathe through your nose, you're still going to get smoke. Even if you get a full medical mask such as a non-rebreather mask, you're still not eclipsing the smoke or providing any seal on the mask. It's better than nothing, but in a truly smoky environment, your eyes and lungs will still burn as the smoke contacts your mucus membranes. This still causes spasming, and still puts the toxins into your system rapidly. Yes, you're getting oxygen, but you're also getting the toxins, acids, and other elements of the smoke that kill you.

In most cabin overpressure situations the action is to shut off the bleeds and dump the cabin. The reason you're going on oxygen is to account for the upcoming cabin pressure loss. If you get an altitude light, normally it's just a warning that your cabin is exceeding a particular altitude. This usually takes place several thousand feet before emergency pressurization is activated, or before the masks drop, in systems where such things occur.

The altitude warning light may be tied to various functions...these are aircraft specific. If your aircraft is only warning of a ten thousand foot cabin and nothing more, then you're faced with two issues...one is an equipment issue (91.213, again), the other is a safety of flight issue which may or may not include donning oxygen masks. If the light activates, whatever it's purpose in your aircraft, certainly there's a prescribed proceedure to deal with it.

I think your last question was more specifically directed at the legal requirements of exceeding 10,000', however. There really aren't any, other than you can go normal speed :p . As far as needing to return to 10,000 specifically, if you have an altitude emergency...you need to get down to whatever altitude is going to provide you a level of safety. I fly in a lot of country that won't permit going down to ten thousand...big rocks. You may need to get lower...see how you're feeling, see how you're doing. You may be okay getting down to 14,000...see what it's going to take with what you have and where you are, at the time.

For typical skydives, we often go up to 18,000. It's just a quick hop in most turbine equipment these days, and it provides a decent freefall. Above that oxygen can be provided with a little tygon tubing, but it's never available on the equipment I've been jumping. Quickly up to FL180, then out. I've never seen anyone on a jump get sick or become impaired or pass out due to the altitude. If one stayed up there for an extended period, perhaps. It's a little like a fire on board, however...folks have this mindset that they need to get down and throw caution to the wind to do it, when usually what's needed is to sit on one's hands for ten seconds and think about it, before acting. At lower altitudes, having a pressure problem generally isn't that big a deal. It should be dealt with according to the manufacturers recommendations, but a pressure loss at 18,000 just isn't the kind of emergency for which one needs to come screaming earthward in a desperate bid for terra firma and dense atmosphere.

You're doing the right thing by refusing to fly, or climb high until the leak is located. For all you know, the leak could be a cracked cylinder or valve assy...filling it might be more than a leak hazard. I've seen that happen (I replaced one such valve last year, in fact). I grounded the airplane, and the shop to whom I forwarded the valve assy said it was the first case they'd seen in something like 25 years. Better safe than sorry, and in that case, it was both safe, and not sorry. You too...nothing at all wrong with making a safe decision. Those that will fret about it should only be reminded that thanks to your safe decision, they still have the option to sit around and fret about it.

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