When parked, what is necessary for vehicles in motion on the airfield?

Turning engines off when a vehicle is parked on the airfield is crucial for several reasons. First, it helps to reduce the risk of accidents or incidents that can occur from unintended vehicle movement. An engine left running can potentially lead to the vehicle rolling away or causing harm to nearby personnel or equipment. Additionally, turning off engines significantly lowers the risk of fire hazards, which is particularly important in an environment like an airfield where flammable materials are present.

Furthermore, it contributes to noise reduction, making the airfield environment more efficient, particularly with regards to air traffic communications and operations that require a quieter setting. In doing so, it also promotes a culture of safety and compliance with regulations that govern airfield driving protocols.

In contrast, the other options might factor into operational safety, but do not directly address the immediate risks associated with a parked vehicle. For example, keeping the doors locked and ensuring no passengers are left inside are valuable for security and safety but do not relate directly to the operational status or safety of a vehicle that is stationary. Setting an alarm system also enhances security but does not mitigate the risks presented by the vehicle's engine being left running.