The stabilizer trim is actively moving in a single direction during cruise, with no pilot input.

• A. Elevator System Jammed
• B. Rudder System Jammed
• C. Stabilizer Trim Runaway
• D. Unreliable Airspeed

Answer: (C)

Stabilizer trim moving by itself in one direction is a runaway trim condition. The stabilizer trim changes the aircraft’s pitch by driving the stabilizer surface with an actuator controlled by the trim system or autopilot. If that actuator or its control circuitry faults, it can command continuous motion in a single direction without any pilot input, which is dangerous because the airplane can pitch up or down unexpectedly.

That behavior is the clearest indicator of a trim runaway, not a jam or a simple sensor issue. A jam would typically leave the trim stuck or resistant, not actively driving in one direction. An unreliable airspeed can cause autopilot or stability systems to behave differently, but it doesn’t by itself produce uncommanded continuous trim movement. A rudder or elevator problem would affect those surfaces directly, whereas the observed symptom specifically points to the stabilizer trim system acting autonomously.