I was reading an article recently on the effects of rapid depressurization when I came across this tidbit: According to FAR 135.89.2, aircrews of part charter craft flying pressurized aircraft above 35,000 MSL are required to have one member of the aircrew on oxygen at all times.

I've seen a few videos of charter pilots flying planes around FL410 and I don't recall any where they had masks on (admittedly, there are not a lot of small business jet charter flight videos out there.) In general, though, it seems the rule is ignored.

So, why isn't the rule followed? Why don't pilots seems to wear oxygen masks in situations the regulations say they are supposed to?


In the original, unedited, version I made the mistake of thinking FAR 135 applied to airliners, when that's clearly 121 that applies to airliners...

Still, in FAR 121.333 it states that the rules around airmasks for commercial airline aircraft (over 30 passengers) require that pilots always have oxygen when they are either a) over FL410 or b) over FL250 and one of the flight crew leaves the flight deck.

Of these conditions, it seems condition A is followed by default (flying under FL410), but condition B is often ignored. So I'd be curious why that part of the rule isn't followed as well.

Basically, I'm asking why pilots generally don't wear oxygen masks when regulations require that they are supposed to.

A big thanks to Zach Lipton for pointing out my error to me.

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    $\begingroup$ Did you mean 35,000 feet? $\endgroup$
    – Simon
    Commented Aug 10, 2016 at 5:36
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    $\begingroup$ Sorry Jay, VTC. Your question states "aircrews flying pressurized aircraft above 25,000 MSL are required to have one member of the aircrew on oxygen at all times". The FAR does not state that. It's therefore unclear what you are asking. I assume that the title and the body both mean to reference FL350. $\endgroup$
    – Simon
    Commented Aug 10, 2016 at 6:17
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    $\begingroup$ @Federico Decided to change the title to just be more inclusive overall. $\endgroup$
    – Jae Carr
    Commented Aug 10, 2016 at 15:04
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    $\begingroup$ Just an anecdote, I heard that there was an FAA inspector in the NE USA who regularly would hop on board an airplane which just landed after flying from Europe above the altitude where oxygen was required and looked at the oxygen gauge. If it was full, he had some questions along with a violation for the pilots. This led to some pilots simply depressing the oxygen test button during the flight in order to bleed off some of the "excess" oxygen. It's very hard to make people do something when they don't feel that it's needed.... $\endgroup$
    – Lnafziger
    Commented Aug 10, 2016 at 20:44
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    $\begingroup$ @Lnafziger The last sentence of your comment sums up the consensus of the culture during the 1980s and 1990s. We did always don the mask and check it for operation during the preflight. We never donned it enroute unless we had an FAA inspector jumpseating. $\endgroup$
    – Terry
    Commented Aug 10, 2016 at 23:27

1 Answer 1


The rule you've found is under part 135 of the FARs, which generally governs commuter and on-demand operations like many charter flights. Most commercial airlines, such as those flying large passenger jets on regularly scheduled routes, are part 121 operators, which follow different rules.

For part 135 operators, there are a few caveats. First, above 25,000 MSL, there is an exception to the oxygen requirement if "each pilot has an approved quick-donning type oxygen mask." If the aircraft is so equipped, masks are only required above 35,000 MSL or when one pilot has left the cockpit.

The rules for part 121 operators are a bit different and can be found in 14 CFR 121.333. Under these rules, if quick-donning masks are available and the aircraft has more than 30 seats or a payload capacity of more than 7,500 lbs, oxygen is not required at or below FL410 unless one pilot has temporarily left the controls.

Since the majority of videos you may see online probably involve two pilots flying a large part 121 airliner (or one under a foreign regulatory regime, where different rules apply), oxygen masks would not be required in most of the situations where cockpit films are released.

That said, I found this article, which may be of some interest: Study: Pilots Ignore Oxygen Regulations. It cites the results of a survey conducted as part of a student's master's thesis which concludes that the rules are ignored quite often and that there really aren't effective ways to enforce them either:

The compliance numbers for Part 121 respondents were higher, with 39 percent reporting always using oxygen when required above 25,000 feet and 48 percent above 41,000 feet.

Part 135 respondents were nearer the Part 91 results, with 21 percent saying they always use supplemental oxygen when only one crewmember is at the controls above 25,000 feet and 18 percent continuously using oxygen when flying above 35,000 feet.

So for part 121 operators, the primary time when the regulation is relevant is when one pilot has left the cockpit above FL250. The article goes on to discuss some of the reasons pilots may not follow the regulation and whether the regulation makes good sense. Some pilots on the PPRuNe Forums discussed the rules here (before they were changed to FL410) and also here. One author notes, with regard to donning masks during bathroom breaks, that "there is probably no more violated FAR in US operations than this one." Several reasons are posited in those threads, among them:

  • Some non-military pilots do not receive decompression chamber training, and so do not have an appreciation for the effects of severe hypoxia and its dangers
  • This practice would use up much of the cockpit oxygen supply, especially on long-haul flights, leaving less available in the event of an emergency (some information on how long oxygen supplies tend to last)
  • The expense and time involved in topping off oxygen tanks all the time if they were routinely used.
  • A desire to reduce the complexity of single-pilot operations during breaks, not wanting the remaining pilot to complicate things by donning the mask and adjusting audio settings accordingly instead of focusing on the aircraft
  • In some cases, restowing the masks may require maintenance personnel after the flight, which is impractical
  • The same rules do not apply under many other regulatory regimes, which leads people to conclude they are unimportant

Note that some pilots discuss using the masks routinely, both to ensure that their operations are as safe and legal as possible, and to give the crew practice and experience in donning and using the masks so they will be comfortable with them in the event of an emergency.

  • $\begingroup$ Yes, you're right, I meant charters and I meant over FL350... Anyway, I've edited the question to fit with what you brought up, you might consider editing your answer to refelct that (not that you have to, just thought I'd point it out to you.) Thanks for straightening me out :) $\endgroup$
    – Jae Carr
    Commented Aug 10, 2016 at 12:30
  • $\begingroup$ There's a lot of good information here, but very little of it answers the question of why pilots ignore this specific regulation. Could you perhaps summarize the main 'theories' from the PPRuNe forum discussions? $\endgroup$
    – Pondlife
    Commented Aug 10, 2016 at 19:33
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    $\begingroup$ I remember once when jumpseating on freighters back in the 90's seeing the FO put on his mask as the captain went to the bathroom. I must have had a panicky look on my face because the flight engineer quickly explained the rule to me. Out of numerous jumpseat trips that was the ONLY time I ever observed anyone put on a mask except during pre-flight. $\endgroup$
    – TomMcW
    Commented Aug 10, 2016 at 19:34
  • $\begingroup$ @Pondlife Was thinking the same thing, now that it's been done, I'm going to mark this the answer. $\endgroup$
    – Jae Carr
    Commented Aug 11, 2016 at 1:25

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