The airspeed indicator shows erratic readings while the aircraft is in clean configuration.

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

Answer: (D)

The key idea is that the airspeed indicator can become unreliable when its sensing system is compromised. The ASI gets its information from the pitot-static system, with dynamic pressure from the pitot tube and ambient static pressure setting the gauge. If the pitot tube is blocked, or there’s a leak or blockage in the pitot or static pathways, the diaphragm inside the instrument can respond unpredictably to changes in speed, producing erratic readings even when the airplane is in clean configuration (no changes in wings, flaps, or gear that might otherwise affect the reading). This kind of fault is about the instrument’s sensor circuit itself, not the aircraft’s physical controls.

The other possibilities describe faults in control surfaces or trim mechanisms. An elevator system jam would affect pitch control, a rudder jam would affect yaw, and a stabilizer trim runaway would change attitude or trim tension. None of those issues would cause the airspeed indicator to flicker unpredictably on its own, so they’re not fitting explanations for erratic speed readings.