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.