What provision must be present in aircraft with retractable landing gear?

In aircraft with retractable landing gear, the presence of an emergency landing gear extension system is essential for safety. This system allows the landing gear to be deployed manually in the event that the normal extension mechanism fails. The emergency extension system typically engages a separate mechanism that can be operated by the pilot, ensuring that the aircraft can land safely even if the gear does not extend automatically.

Having an emergency extension system is critical for maintaining operational safety, as it provides a fallback during critical phases of flight like landing, where the landing gear must be deployed for a safe landing. This system is commonly designed to function without reliance on electric power or the primary hydraulic system, thus ensuring that the landing gear can be extended under a variety of failure scenarios.

The other options, while they may serve important roles in aircraft systems, do not specifically address the critical need for a guaranteed method of deploying landing gear during emergencies. Backup power systems are helpful but don’t directly ensure the landing gear can be extended. Auto-retraction systems are not necessary for all aircraft and are usually implemented in specific designs. Weight sensors are important for load management and safety but do not directly relate to the function and reliability of landing gear extension.