A pilot flying a manual (non-fly-by-wire) transport notices that as airspeed increases above the trimmed speed, increasing back pressure is required to hold altitude, and as airspeed decreases below trim, forward pressure is required. What characteristic does this describe, and why is it operationally important?
- APositive static longitudinal (speed) stability, which cues the pilot to speed deviations through changing control force and tends to return the aircraft toward trim speed
- BDutch roll damping, which suppresses lateral-directional oscillations
- CMach tuck compensation, which offsets nose-down pitching moments at high Mach number
- DAdverse yaw, which couples aileron deflection with an opposing yaw moment
Show answer & explanationAnswer & explanation
Correct answer: A. Positive static longitudinal (speed) stability, which cues the pilot to speed deviations through changing control force and tends to return the aircraft toward trim speed
An increase in control force (stick force) with speed deviation from trim, in the correct sense to oppose the deviation, is the hallmark of positive static longitudinal stability, often measured as stick force per knot. This gives the pilot tactile feedback about speed changes and a natural tendency to return to trim speed if the controls are released.
Why the other options are wrong
- B. Dutch roll is a lateral-directional oscillation unrelated to pitch/speed force feedback.
- C. Mach tuck is a nose-down pitching tendency at high Mach, addressed by Mach trim, not this force gradient.
- D. Adverse yaw relates to roll-yaw coupling from aileron use, not longitudinal speed stability.
Static Longitudinal (Speed) Stability
The tendency of an aircraft to require increasing back pressure as speed increases above trim and increasing forward pressure as speed decreases below trim, providing tactile cues and a self-correcting tendency toward trim speed.
- Measured as stick force per knot
- Positive stability means force increases with speed deviation from trim
- Important for pilot workload and handling qualities certification
Memory trick: Push away, pull back — the stick fights to bring you home to trim.