14 CFR 91.211(b)(2) establishes a clear and frequently underappreciated requirement for flight operations above FL350. The regulation mandates that at least one pilot at the controls wear a secured and sealed oxygen mask at all times above that altitude — with one critical exception: if both pilots are simultaneously at their duty stations and the aircraft is equipped with quick-donning masks (donnable with one hand in five seconds), neither pilot is required to wear the mask continuously. The operational consequence flows directly from that exception. The moment one crewmember vacates the flight deck — whether for a lavatory break, a rest period, or any other reason — the remaining pilot loses the benefit of the exception and is legally required to don and seal their oxygen mask for the duration of that absence. On a 737 cruising at FL360, the answer to the Reddit poster's question is yes: the regulation, read plainly, requires it.
The parallel Part 121 provision at 14 CFR 121.333(c)(2) imposes essentially the same framework on air carrier operations, meaning scheduled airline crews are subject to an identical standard. The practical reality documented in airline culture and anecdotally confirmed across aviation forums is that mask-donning during brief lavatory absences is inconsistently observed. The physiological rationale behind the rule is not arbitrary: at FL360, a rapid decompression without a pressurization warning could incapacitate an unmasked pilot within seconds. Time of useful consciousness at that altitude without supplemental oxygen is roughly 20 to 30 seconds. A single pilot operating without a mask who suffers sudden incapacitation during a partner's absence would leave the aircraft with no functioning crewmember at the controls — precisely the catastrophic scenario the regulation is designed to prevent.
The question of actual compliance touches on a broader tension in aviation between regulatory text and line operational norms. Many pilots who are thoroughly conversant with their aircraft's systems and emergency procedures are genuinely unaware of the literal requirement, having absorbed an informal understanding that quick-donning masks satisfy the above-FL350 obligation without parsing the dependency on dual occupancy of the flight deck. Training programs and check airmen do not uniformly emphasize this specific scenario, and the mask-on requirement during solo-pilot moments above FL350 rarely appears on checkrides or recurrent training evaluations with the same prominence as, say, emergency descent profiles or pressurization failure memory items.
The broader operational relevance extends beyond the airline cockpit. Part 91K fractional operators and Part 135 charter operators flying pressurized turbine aircraft above FL350 — including large-cabin business jets — fall under the same regulatory framework. Crews in those environments, often operating with smaller teams and less formalized procedural infrastructure than major carriers, may be even less attuned to the specific crew-complement dependency embedded in the oxygen mask rule. For corporate flight departments operating Gulfstreams, Challengers, or similar aircraft routinely cruising above FL400, where time of useful consciousness without oxygen is measured in single-digit seconds, the stakes of non-compliance are correspondingly higher. The Reddit thread's observation that "I'd never heard anyone talk about this before" is itself a meaningful data point about the gap between regulatory intent and operational awareness in the professional pilot community.