FAR part 91 Sec 91.117 for example states that "unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots". All of the major jurisdictions I can find reference to have the same rule.

I've highlighted MSL (mean sea level). Why is MSL used? What's the point of such a regulation when aircraft would be able to manoeuvre at high speed close to the surface of high elevation fields?

How, if at all, is this varied when operating to or from an airfield with a high elevation, for example Quito, Ecuador (SEQM) at 7,910 feet?

  • 2
    $\begingroup$ If it was to vary then the regulation would have said AGL wouldn't it? $\endgroup$
    – DeepSpace
    Dec 28 '16 at 10:34
  • $\begingroup$ @DeepSpace I wasn't asking about that specific rule and this forum is usually polite and free of sarcasm which is neither wanted nor appreciated. What is the point of having a 10,000 foot rule when it would only apply for a couple of thousand feet above high elevation fields such as SEQM. Is the intent of my question not clear? Do you have a proposed edit? $\endgroup$
    – Simon
    Dec 28 '16 at 11:17
  • 1
    $\begingroup$ Related, but not the answer: Origin of the 250 knots below 10,000 ft rule. Also, for EU: Did this aircraft illegally exceed 250kts below 10,000ft? $\endgroup$
    – mins
    Dec 28 '16 at 12:27
  • $\begingroup$ @mins Hi. I think I haven't been clear. The rule is established to aid sequencing and separation. These regs mean that an aircraft could be manouvering in class A at high speed only 3000 feet about the surface. Surely there must be something to deal with this? $\endgroup$
    – Simon
    Dec 28 '16 at 13:32
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    $\begingroup$ I'm a bit confused by your question and comments. First, you quoted 117(a) but 117(b) has speed limits that apply above class C/D airspace based on AGL, not MSL; that may answer part of your question. Second, which jurisdiction are you asking about? Part 91 is irrelevant in Ecuador, and class A airspace in the US starts at 18,000MSL so to be "3000 above the surface" would require an airport elevation of 15,000MSL, but the highest one in the US is at 9,927. $\endgroup$
    – Pondlife
    Dec 28 '16 at 17:16

No, the 250 KIAS below 10000 MSL rule of 91.117(a) is not varied within FAA jurisdiction, including near airports with high field elevation.

The 200 KIAS near Class C and D airfields rule of 91.117(b) mitigates some of the risk you might perceive surrounding high elevation airports.

For example, Aspen (KASE) has a surveyed field elevation of 7838 ft and is within Class D airspace. Aircraft operating within 4 NM of Aspen would be speed limited to 200 KIAS up to 2500 AGL, which would equate to over 10000 MSL.

Other high elevation airfields in Class E or G airspace would presumably have less traffic, with the expected result that higher indicated airspeeds would pose less risk in the uncongested airspace..


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