What is the primary purpose of an aircraft's parking brake?

The primary purpose of an aircraft's parking brake is to hold the aircraft stationary while it is parked. This feature is crucial for ensuring that the aircraft does not roll or move unintentionally when it is stationary, particularly on inclines or when subjected to wind forces. Proper engagement of the parking brake after the aircraft has come to a stop is essential for safety during servicing, boarding, and other ground operations.

While the other options may pertain to various aircraft functions, they do not relate directly to the specific role of the parking brake. For instance, taxiing requires different mechanisms and controls, and aerodynamic efficiency pertains to the design of the aircraft rather than how it is secured on the ground. The lighting systems, while integral to operation, are not linked to the function of the parking brake. Thus, the role of the parking brake is distinctly about securing the aircraft in place during parking conditions.