A discrepancy between indicated airspeed and actual airspeed arises during stable flight conditions.

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

Answer: (D)

The key idea is instrument reliability in steady flight. Indicated airspeed comes from the air data system (pitot and static pressures) and is used to compute true airspeed along with altitude and temperature. If the air data system is unreliable—due to a blocked pitot tube, blocked or leaking static ports, or a faulty airspeed indicator—the indicated speed can read incorrectly even though the aircraft is flown at a constant, stable attitude. That creates a mismatch between indicated airspeed and the actual airspeed, pointing to unreliable airspeed as the cause. The other faults would cause changes in flight attitude or control rather than producing a persistent IAS vs TAS discrepancy in stable flight.