When are vehicle operators allowed to pass closely in front or behind aircraft with engines running?

Vehicle operators are allowed to pass closely in front or behind aircraft with engines running only if specific safety measures are observed. The correct answer highlights that permission must be granted by the marshaller, and the condition of the aircraft, such as chocking the wheels, plays a critical role in ensuring safety.

The marshaller is responsible for directing the movement of aircraft and ensuring that the area around them is secure. If the engines are running, there is always a risk of jet blast or suction that could endanger vehicles and their operators. Ensuring that the aircraft's wheels are chocked serves to stabilize it, and receiving the marshaller's permission confirms that it is safe to proceed. This protocol minimizes the potential for accidents and allows for safe movement in proximity to the aircraft.

During other scenarios mentioned in the other choices, such as when the aircraft is parked or when no passengers are boarding, the inherent risks associated with running engines persist. The presence of personnel on board or potential movement of the aircraft can still pose hazards. Thus, the requirement for explicit permission ensures that vehicle operators are always aware of the operational status of the aircraft before passing by.