High Altitude Operations
The coffin corner
As altitude increases, the speed band between low-speed stall buffet and high-speed (Mach) buffet narrows. Where they meet, the aircraft has essentially no margin - the 'coffin corner.' Questions probe whether you understand that at high altitude the limiting factor is Mach, not indicated airspeed, and that turbulence penetration speed and buffet margins shrink with altitude.
Mach, altimetry, and RVSM
Above the transition altitude (18,000 ft in the US) set 29.92 inHg; below it, local altimeter. RVSM airspace is FL290–FL410 inclusive with 1,000-ft vertical separation, and requires specific equipment and training. Altimeter errors, static source errors, and temperature extremes matter: colder than ISA means the aircraft is lower than the altimeter indicates.
High-altitude stall and upset
High-altitude stall recovery begins with reducing angle of attack - pushing - because thrust is limited and pulling only deepens the stall. Do not add power first. Beware Mach tuck at high Mach: the nose pitches down as shock waves form, and the recovery is to reduce speed, not to pull hard.
Oxygen and time of useful consciousness
Time of useful consciousness falls sharply with altitude: roughly 30 minutes at FL250, 1–2 minutes at FL350, and 15–20 seconds at FL400. For a rapid decompression at high altitude, the mask goes on before anything else.
Worked example: At FL350 with an explosive decompression, expect about 30–60 seconds of useful consciousness - enough to don the mask and start the descent, but not enough to troubleshoot.
Rule of thumb: at high altitude, unload first, power second; cold temperatures mean true altitude is lower than indicated.