FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsEasy
A transport aircraft has a Vmo of 340 KIAS and an Mmo of 0.82. During a climb at a constant 340 KIAS, at approximately what point does the applicable maximum operating limit speed change from being defined by Vmo to being defined by Mmo?
- AAt FL180, the standard transition altitude used by all transport aircraft
- BAt the tropopause, regardless of the specific Vmo/Mmo values
- CAt the crossover altitude where 340 KIAS corresponds to Mach 0.82
- DAt the altitude where true airspeed first equals Mach 0.82
Show answer & explanationAnswer & explanation
Correct answer: C. At the crossover altitude where 340 KIAS corresponds to Mach 0.82
As altitude increases at a constant IAS, TAS increases while the local speed of sound decreases (until the tropopause), so the Mach number corresponding to that IAS rises. The crossover altitude is where 340 KIAS equals Mach 0.82; above it, Mmo becomes the limiting speed, below it Vmo governs.
Why the other options are wrong
- A. FL180 is a common transition altitude for altimeter setting, unrelated to Vmo/Mmo crossover.
- B. The tropopause affects temperature lapse but is not itself the crossover point.
- D. TAS equaling a Mach-based value numerically doesn't define the crossover; IAS vs Mach does.
Vmo/Mmo Crossover Altitude
The altitude at which a constant Vmo (in KIAS) and the aircraft's Mmo (Mach limit) produce the same speed; above it Mmo limits, below it Vmo limits.
- Below crossover altitude, Vmo (KIAS) is the limiting speed
- Above crossover altitude, Mmo (Mach) is the limiting speed
- Crossover altitude depends on the specific Vmo and Mmo values for the aircraft
Memory trick: Low and IAS rules, high and Mach rules — they swap at crossover.