FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsMedium
A swept-wing transport aircraft accelerates toward its Mmo. As airspeed increases, a shock wave forms farther aft on the wing, shifting the center of pressure aft and producing an uncommanded nose-down pitching moment. This tendency is best described as:
- AGround effect
- BMach tuck
- CAdverse yaw
- DSpiral instability
Show answer & explanationAnswer & explanation
Correct answer: B. Mach tuck
Mach tuck (tuck under) occurs when transonic shock waves move aft on the wing, shifting the center of pressure aft and creating a nose-down pitching moment; many transports use Mach trim systems to counter this tendency.
Why the other options are wrong
- A. Ground effect involves proximity to the runway surface, irrelevant at cruise altitude.
- C. Adverse yaw is a yaw effect from aileron deflection, unrelated to shock wave movement.
- D. Spiral instability is a slow roll-yaw divergence, not a pitch effect from compressibility.
Mach Tuck
A nose-down pitching tendency caused by aft movement of the center of pressure as shock waves form on the wing at high subsonic speeds.
- Occurs as aircraft approaches Mmo
- Center of pressure shifts aft due to shock wave location
- Mach trim systems automatically compensate for the tendency
Memory trick: Tuck under: nose dives as the shock slides aft