What constitutes a runway incursion?

A runway incursion is defined as any occurrence at an airport involving the unauthorized presence of an aircraft, vehicle, or person on the runway. Unauthorized entry onto the runway is a clear violation of the established air traffic control protocols and safety regulations, making it the correct answer.

This definition encompasses situations where an individual or vehicle enters a runway without proper clearance or coordination with air traffic control, which poses a significant risk of collision with aircraft that may be taking off or landing. Such incidents can have serious repercussions for aircraft safety and airport operations.

In contrast, exiting the runway without authorization is an action that, while potentially unsafe, does not necessarily fit the strict definition of a runway incursion in the same way that entry does. Crossing the VFR hold lines indicates a potential breach of separation but does not directly correspond to being on the runway itself. Parking in restricted areas is related to violations of airfield protocols but does not pertain directly to the runway context. Hence, the essence of a runway incursion lies specifically in the act of unauthorized entry onto the runway, which aligns perfectly with the chosen answer.