FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsHard
During cruise, a pilot momentarily releases the flight controls while the aircraft is in a shallow bank. Over the next minute, without any pilot input, the bank angle and turn rate slowly and continuously increase. This tendency indicates the aircraft has:
- ANeutral static longitudinal stability
- BStrong positive dihedral effect combined with weak directional stability
- CDutch roll tendency caused by yaw damper failure
- DSpiral instability, where directional (weathercock) stability dominates over dihedral effect
Show answer & explanationAnswer & explanation
Correct answer: D. Spiral instability, where directional (weathercock) stability dominates over dihedral effect
Spiral instability occurs when directional (weathercock) stability is strong relative to dihedral effect. In a bank, the aircraft yaws into the turn due to strong directional stability, which increases lift on the outer wing and steepens the bank, creating a slow, continuously tightening spiral if hands-off.
Why the other options are wrong
- A. This describes pitch stability, unrelated to the lateral-directional behavior described.
- B. Strong dihedral relative to directional stability would tend to roll the aircraft back to wings-level, not increase the bank.
- C. Dutch roll is an oscillatory yaw-roll coupling, not a slow continuous divergence in bank angle.
Spiral Instability
A lateral-directional stability characteristic where strong directional (weathercock) stability relative to dihedral effect causes a bank to slowly and continuously steepen without pilot input, unlike the oscillatory nature of Dutch roll.
- Caused by directional stability exceeding dihedral effect
- Produces a slow, non-oscillatory divergence in bank angle
- Opposite imbalance (too much dihedral, too little directional stability) causes Dutch roll tendency
Memory trick: Weathercock wins, wings slowly spiral down