What is the main purpose of a parking brake in an aircraft?

The main purpose of a parking brake in an aircraft is to keep the aircraft in place when it is parked. This function is crucial, particularly for larger aircraft, as they can be affected by strong winds or sloping terrain, which can cause them to roll unintentionally. The parking brake locks the wheels in place, ensuring that the aircraft remains stationary until the crew decides to move it again. This is especially important during pre-flight checks where the aircraft must be secured, and during ground operations where other activities may take place around a stationary aircraft.

The other options do not accurately reflect the primary function of the parking brake. For instance, collision avoidance involves the operation of various radar and sensor systems, rather than a mechanical brake. Engine efficiency is related to the aircraft's propulsion systems and operations, not its parking mechanisms. Lastly, the anti-skid system is designed to prevent wheel locks during braking on landing or when slowing down on the runway, which is separate and distinct from the purpose of a parking brake.