Which statement best explains why control surfaces should be locked when an aircraft is parked?

• A. To prevent corrosion
• B. To protect control surfaces from wind damage
• C. To prevent unauthorized use
• D. To prevent movement due to thermal expansion

Answer: (D)

Control surfaces can move even when the aircraft is parked, especially as temperatures change. Metal and linked components expand when heated and contract when cooled, so if surfaces are left unlocked, this thermal movement can cause slight shifts, misalignment, or stress in hinges and linkages over time. Locking devices prevent any unintended movement by securing the surfaces in place, protecting the control system from wear or damage due to temperature-driven motion. While wind can also push surfaces, the question focuses on the effect of temperature changes, which is exactly what the lock is designed to counter. Corrosion, security against unauthorized use, and other concerns are addressed by different measures, not by the primary purpose of locking for parked storage.