Above FL180, what is the minimum climb speed in Mach or knots?

• A. 64M or 250KTS
• B. 200 FPM
• C. 250 KTS
• D. 60 KTS

Answer: (A)

Above high altitude, airspeed is governed by Mach rather than knots, because air density is low and Mach provides a consistent measure of performance. The minimum climb speed is set as the greater of Mach 0.64 or 250 knots. This rule keeps you from getting too slow for safe climb performance or stall margins while still respecting the aircraft’s performance limits at altitude. In practice, you’d climb at Mach 0.64, but if that Mach number would correspond to less than 250 knots indicated, you maintain at least 250 knots IAS; if Mach 0.64 yields more than 250 knots IAS, you follow Mach 0.64. That’s why the correct answer is 64M or 250KTS, whichever is higher. The other options don’t fit because one is a climb rate, another is only a speed in knots without the Mach relationship, and the last is far too slow to meet safe high-altitude climb requirements.