So, I know the regulation says that the pilot needs to have oxygen if flying above 12,500 ft PA for more than 30 minutes. I never fly that high, but was wondering if you fly at 13000 ft for 25 minutes and then descend to 12500 or lower: how long to you have to wait before flying back up to 13000 again? (I'm not talking about what is physiological or what is smart. I'm asking whether there is a regulation or an FAA guidance letter.)
The regulations do not explicitly prohibit what you suggest doing, and I can find no legal interpretations regarding 91.211. However, I think any aviation physiologist would tell you it's a recipe for serious hypoxia, so the FAA might call it “careless/reckless” if something bad happened as a result. As for FAA “guidance” rather than regulations, try these:
(And my post should have said MSL, not PA.)
The regulation says, “At cabin pressure altitudes above 12,500 feet (MSL),” which is a bit ambiguous, but physiologically speaking, it really is the pressure altitude which counts.
Expanding on Ron's answer, there really is no specific wait time required before venturing back up to above 12,500 feet MSL. However, let me add this thought (or read of the regulation) - the time is cumulative; not reset each time you ascend back up above 12,500 feet. The regulation addresses “that part of the flight at those altitudes that is of more than 30 minutes duration". That statement can be read a few different ways. In my mind, each climb above 12,500 feet adds to the previous time spent there in computing the 30 minutes.
The regulation could be worded better, I agree, but I believe there is merit in this read, and, a good safety precaution as previously noted, as excessive time above 12,500 feet without supplemental O2 can seriously degrade pilot judgment and decision making, and is not a wise choice.
Daddis At AOPA:
However, let me add this thought (or read of the regulation) - the time is cumulative; not reset each time you ascend back up above 12,500 feet. The regulation addresses “that part of the flight at those altitudes that is of more than 30 minutes duration". That statement can be read a few different ways. In my mind, each climb above 12,500 feet adds to the previous time spent there in computing the 30 minutes.
I think it should be noted that in the absence of any prior interpretation or case law on point, the FAA may advance its own original interpretation of a regulation as part of an enforcement action, and the ALJ and NTSB are bound to accept it. It would take a successful appeal to the US Court of Appeals to defeat that interpretation, and that is a very rare event. I also think it likely that any FAA interpretation of any ambiguity in this regulation would be based on the science, which pretty well tells us that, physiologically speaking, this would be a really bad idea. So I think that if they FAA ever caught someone doing this, they'd come up with an interpretation to support a charge of violating 91.211(a)(1).
Of course, that's just speculation on my part, and we could debate the interpretation of that rule from now ‘til Kingdom come, but as with many other discussions on the finer distinctions of interpretation of regulations regarding some improbable or downright dumb things pilots might do, I think it would be largely pointless and, more importantly, with no means of resolution. I suppose one could ask the FAA for such an interpretation, but “only those requests that present a novel or legally significant issue, as determined by the Chief Counsel, will be considered as potentially warranting a legal interpretation”, and I don’t think this one would fit that definition.