On final approach, the stabilizer trim keeps moving nose-down without pilot command.

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

Answer: (C)

Stabilizer trim runaway is when the horizontal stabilizer keeps moving on its own, changing the pitch without any pilot input. On final approach, seeing the nose move down because the trim is continually driving toward nose-down means the trim system is the culprit, not you manually commanding it. The elevator system being jammed would lock movement of the elevator itself, not drive the stabilizer toward a new angle by itself. A rudder jam affects yaw, which doesn’t explain the constant nose-down trim motion. Unreliable airspeed can cause various control issues, but it doesn’t by itself cause the stabilizer trim to move autonomously toward nose-down without input. Because the symptom is a continuous, self-initiated nose-down trim change, that points to a stabilizer trim runaway. In that case, cut out the stabilizer trim (and disconnect any autopilot) and maintain control manually, following the applicable emergency procedures.