FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsMedium
A swept-wing transport is accelerating through Mach 0.80 in cruise. As Mach number increases further, the aerodynamic center shifts aft, producing a nose-down pitching tendency (Mach tuck). To counteract this and preserve positive longitudinal speed stability at high Mach, the airplane's automatic system commands:
- AAn automatic reduction in engine thrust to slow the airplane
- BExtension of ground spoilers to increase drag
- CRetraction of the wing leading-edge slats
- DA progressive nose-up trim input via the stabilizer or elevator as Mach increases
Show answer & explanationAnswer & explanation
Correct answer: D. A progressive nose-up trim input via the stabilizer or elevator as Mach increases
The Mach trim system automatically applies nose-up trim (via stabilizer or elevator feel adjustment) as Mach number increases beyond a threshold, compensating for the aft aerodynamic center shift that causes Mach tuck. This restores the normal 'stick force per knot' speed stability that would otherwise be lost or reversed at high Mach.
Why the other options are wrong
- A. Thrust reduction is a pilot/autothrottle function, not how Mach trim corrects pitch instability.
- B. Spoiler extension increases drag but does not address the pitching moment change.
- C. Slats are a low-speed high-lift device and are already retracted in high-Mach cruise.
Mach Trim System
An automatic system that commands stabilizer or elevator trim changes as Mach number increases, compensating for the aft aerodynamic center shift (Mach tuck) to preserve speed stability.
- Activates automatically above a set Mach threshold
- Commands nose-up trim to counter Mach tuck
- Restores normal stick-force-per-knot speed stability
- Distinct from yaw damper and autothrottle systems
Memory trick: Mach trim 'tugs the nose up' to fight the tuck