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In North America, the transition altitude - the altitude below which everyone reports their altitude in feet above mean sea level (AMSL) and sets their altimeter to whatever the weather dictates that the local altimeter setting be, and above which everyone reports their altitude as a flight level (FL)1 and sets their altimeter to 29.93 inches of mercury (inHg) - is 18 kilofeet AMSL, this altitude having been chosen so as to put the transition altitude well above the elevation of every airport on the continent.

However, although the highest airports in North America are well below 18 kft, the highest land in the continent is not; Denali, the highest mountain in North America, reaches up all the way to 20.31 kft AMSL. It’s also not particularly inaccessible, and happens to sit in one of the most aviation-centric - indeed, in great measure, aviation-dependent - regions of North America, fairly close to said region’s major city and said city’s major airport.

Given that one’s ability to maintain terrain clearance (read “avoid flying into the side of the mountain”) depends on one’s height above the ground, which depends on one’s actual altitude, on the ground’s actual altitude, and on the difference between the two,2 how is the situation of a mountain poking its nose above transition altitude handled? Is there a higher-transition-altitude bubble surrounding Denali? Are pilots required to set one baroaltimeter to 29.93 inHg (for flight-level purposes) and a second baroaltimeter to the local altimeter setting (for terrain-clearance purposes)? Do they simply have to watch their radar altimeters like hawks anytime they’re flying near Denali’s peak? Something else?


1: One flight level is roughly equal to one hectofoot; it does not equal exactly one hectofoot unless the sea-level barometric pressure happens to be 29.93 inHg and the atmospheric profile happens to match the International Standard Atmosphere (ISA).

2: And given that, while terrain clearance is always a high priority, it’s even more so where even the gentlest crash would be unsurvivable for anyone but a well-acclimated mountaineer, due to Denali being inconsiderate enough to locate its peak in what is, for non-acclimated persons, the Death Zone.

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    $\begingroup$ The nearest airway is almost 30 mi away T242, and the minimum sector altitudes are 22700 in the area of Mckinley. But normally the minimum sector alts are ASL, and I would assume they are in this case, and looking at it from a strict technical standpoint where you were observing the sector minimum IFR alt, say wandering around with no comms, I would think you would observe the min IFR alt in the sector as ASL based on the Fairbanks altimeter setting, not a flight level. This is actually an interesting question. $\endgroup$ – John K Jun 15 at 4:53
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    $\begingroup$ @Sean I would look into how that is handled in Europe where the transition altitudes are very low and all sorts of terrain intrudes into the flight levels. $\endgroup$ – John K Jun 15 at 16:26
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    $\begingroup$ This is really a non-issue. Above FL180 you have to file IFR and be in contact with ATC, and if for some reason you are flying away from the airway and towards the mountain, you would be contacted to correct and/or vectored away from the rock. $\endgroup$ – Juan Jimenez Jun 16 at 9:04
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    $\begingroup$ @JohnK, in Europe the transition altitude varies by sector, and is low only in sectors that don't have any high ground. So I don't think that much high ground intrudes into the flight levels—though I didn't look up the sectors with high ground to check, so I might be wrong. $\endgroup$ – Jan Hudec Jun 16 at 19:15
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    $\begingroup$ @JuanJimenez Class A airspace does not exist below 1500 AGL, and therefore there is no requirement to be IFR and on a clearance when, say, sightseeing around Denali, even when above 18,000. $\endgroup$ – Dave-CFII Jun 17 at 14:01
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The US AIP (ENR 1.7 Altimeter Setting Procedures) is pretty clear about the question being asked, and provides an example.

Minimum altitude comes first.

The lowest usable FL is raised, and not just around Denali (Mount McKinley) in Alaska where the ground juts above 18,000 feet, i.e., the lowest FL can be higher than FL 180 in the contiguous United States.


Let's say the area is not marked yellow and is therefore not congested, and no natural reserves or special conditions exist. In this case the minimum safe altitude is 500 feet AGL.

Let's say ground is 20,300 feet as you present, it means the minimum safe altitude is now 20,800 feet (based on local altimeter setting and accounting for cold temperatures). This is what the AIP says in all caps:

1.2 EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN IN LOW TEMPERATURES AND PRESSURES.

Now that we've established 20,800 plus correction for temperature (assume none for the sake of explanation), the lowest FL now depends on that 20,800 plus a correction based on pressure via table ENR 1.7-2 (uploaded as image).

2.1.3 (...) the lowest usable flight level must be the flight level equivalent of the minimum altitude plus the number of feet specified in TBL ENR 1.7-2.

(Bold emphasis mine.)

If it's a rather low-pressure day, then the correction can be upwards of 2,500 feet. Assume that for emphasis. Our total is now 23,300 feet, which makes the lowest usable FL, FL 235 (or as directed by ATC as stated in 91.159 for VFR, which the AIP mentions).

The example the AIP gives:

The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and 29.42 inches of mercury. The lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude plus 500 feet).


This answer is based on the USA AIP at the time of writing this, it is generic, and it is not about a specific location, so do not use for flight.

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    $\begingroup$ I disagree. The question is "how to set VFR altimeter when the terrain is above the transition altitude." An answer about usable flight levels doesn't address that. The transition altitude is 18,000 feet, FAR 91.121.a.2. There is no exception in there for terrain (like I think there should be, and probably actually is by local custom near Denali). "Lowest Usable Flight Level" is something different; it's usually just the first mod 5 flight level above 18,000. It's there so that no one gets assigned a FL below 18,000 feet. Or that will hit a mountain. It's not an altimeter setting rule. $\endgroup$ – Dave-CFII Jun 19 at 10:14
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Sean I guess you could actually break the answer into two parts; how is it handled in a normal operational way, and how it is handled from the standpoint of an extreme theoretical emergency situation, where find yourself, somehow, in that airspace with nothing but the minimum sector altitude on the chart as your guarantee of obstacle clearance (IFR rules are all about separation of aircraft and obstacle clearance, in the end).

Say you were IFR, in cloud, and had a total comm failure and were lost and thought you might be near McKinley and therefore needed to observe the sector altitude of the airspace around it, 22700, ft as your last gasp guarantee you won't hit the mountain (that's really what the sector altitude is for). Even though you are in the flight levels, you would need to set your altimeter to the closest local altimeter setting you could obtain. In the IFR world, the rules are the rules, but still, when it comes down to survival, ya do what ya gotta do, applying common sense.

That's the way-out-there theoretical worst case, where the fact that the min IFR ASL altitude is above the transition level has to be allowed for by the crew to save their skins.

Another, maybe slightly more likely, theoretical case: Say ATC has you on a heading that will take you across Denali at FL280. You have a comm failure before that. Well, no big deal, carry on as cleared following IFR comm failure protocol. Then right when you are in the 22700 ft sector, you have a depressurization and have to initiate a descent to the minimum IFR altitude right now.

That 22700 ASL sector alt is as low as you can go until you are clear of that sector and can get down to 10000ft. You'd just have to use common sense based on the circumstances in the moment, and you'd set the best alt setting data you had, plus temperature correction, and maybe some extra fudge factor for comfort, to come up with an interim level off altitude until you were clear of that sector.

In the normal operational world, you just go where ATC tells you and how high ATC tells you. If you are on the nearest airway, that's nearly 30 miles east, so no big deal. If you being routed by ATC over the mountain, ATC takes care of the altitude assignment and makes allowance in the flight level assigned. In any case, I would bet that ATC generally does not route aircraft over those sectors to avoid emergency descent issues as described above.

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14 CFR 71.33(b) defines class A airspace as...

(b) That airspace of the State of Alaska, including that airspace overlying the waters within 12 nautical miles of the coast, from 18,000 feet MSL to and including FL600 but not including the airspace less than 1,500 feet above the surface of the earth and the Alaska Peninsula west of longitude 160°00′00″ West.

So for Denali you've got a 1,500 ft buffer zone where it's not class A, but you'd better have oxygen and hope you don't run into mountain waves or other strange things that could make you go against the mountain side, or climb like a lovesick ghost into altitudes you don't want to be. :)

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    $\begingroup$ That does not say anything about the transition, and Class A and transition altitude, while coincident in most USA, are unrelated concepts. $\endgroup$ – Jan Hudec Jun 17 at 18:49

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