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So, 91.211 says I'm not allowed to fly above 12,500 ft without supplemental oxygen. I'm less clear if all planes with a service ceiling above that line are required to have the oxygen aboard in the first place.

For reasons of handling equipment failures, I presume that, regardless of the requirement to have it aboard, if I don't have it and ATC says "Climb and maintain one-tree thousand," I respond "Unable, no oxygen aboard."

But what happens if I'm only asked to climb to 10,000 ft - or even 8,000 ft (altitudes where hypoxic effects, though minor, have been observed) and while I recognize the legality of the operation I'm not - as PIC - comfortable with the risk?

Am I allowed to respond with "Unable" or do I have to request a lower altitude and hope ATC is feeling cooperative?

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    $\begingroup$ Related: aviation.stackexchange.com/q/11909/54557, aviation.stackexchange.com/q/7915/54557 $\endgroup$
    – randomhead
    Apr 5 at 19:21
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    $\begingroup$ If you are healthy 8-10k' shouldn't be an issue. $\endgroup$
    – Michael Hall
    Apr 6 at 1:29
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    $\begingroup$ @MichaelHall at night, however, the FAA Civil Aerospace Medical Institute (known as CAMI) recommends supplement oxygen at night above 6000 msl. $\endgroup$
    – 757toga
    Apr 6 at 1:57
  • $\begingroup$ @757toga, interesting, I didn't know that. It definitely improves vision though! $\endgroup$
    – Michael Hall
    Apr 6 at 2:01
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    $\begingroup$ Well, for someone in very ill health, or with one lung, even 4K' unpressurized might stress them medically. My point is, if asked to climb to 8'K and you respond "unable due to no O2", the controllers probably won't challenge you, but they may exchange quizzical looks between themselves. (not that you should care...) Certainly don't file for any higher than you are comfortable flying. $\endgroup$
    – Michael Hall
    Apr 7 at 1:31

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FAR 91.211 only requires supplemental oxygen be used by the pilot/minimum required flight crew for that part of the flight at altitudes above 12,500 MSL to 14,000 MSL that is of more than 30 minutes duration.

There is no regulatory requirement that all aircraft with a service ceiling above 12,500 MSL are required to have oxygen aboard (although it is an important safety consideration and should always be available if needed even at lower altitudes).

If ATC assigns (by way of a clearance or instruction) an altitude that you believe may be unacceptable/unsafe for any reason, you can tell them that you are "unable" and can ask for a different altitude assignment. If a different altitude is not available for assignment and you are VMC you can always cancel IFR and continue the flight under VFR (with advisories). If you are IMC and need to continue IFR you can ask for an alternative route or other option that may be available in order to maintain an altitude that meets your safety needs.

However, as the Pilot-in-Command (PIC), you will never be required by regulation to operate contrary to your assessment of what is safe. FAR 91.3 - Responsibility and authority of the pilot in command states (in pertinent part):

(a) The pilot in command of an aircraft is directly responsible for, and is the final authority as to, the operation of that aircraft.

Also, it may be helpful to review (in concert with FAR 91.3), FAR 91.123- Compliance with ATC clearances and instructions. This regulation specifies your obligation as a PIC to comply with an ATC clearance or instruction as well as the circumstances that allow for deviations from, or operations contrary to, an ATC clearance or instruction.

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    $\begingroup$ Thank you for the citations about my authority as PIC. Some questions remain for me: 1) What do I do in the meantime, with an assigned altitude above my determination of what I can safely fly, while ATC and I sort it out, and 2) since you seem to suggest I'm not telling ATC I'm unable, what does my request to ATC look like? $\endgroup$
    – William Walker III
    Apr 5 at 16:54
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    $\begingroup$ @WilliamWalkerIII Say “unable” and either suggest or request an alternative. Until you get an instruction you can accept, keep doing what you previously accepted. $\endgroup$
    – StephenS
    Apr 5 at 17:32
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    $\begingroup$ @WilliamWalkerIII I wasn't implying that you should not use the word "unable." I've added that to my answer for clarification. Also, based on your question, I assume terrain avoidance would not be an issue or you probably would not be on the route you are on in the first place. There are many variables that can be associated with your question so I imagine telling ATC what the problem is in plain language would allow for the issue to be quickly sorted out. $\endgroup$
    – 757toga
    Apr 5 at 18:11
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    $\begingroup$ @757toga Saw the edit. It's helpful, thank you! $\endgroup$
    – William Walker III
    Apr 5 at 18:16

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