When should you assume lost communications when being radar vectored on final for a PAR?

In the context of Precision Approach Radar (PAR) operations, assuming lost communications is critical for maintaining safe and orderly flight. The correct timeframe to assume lost communications when being radar vectored on final is five seconds. This short time frame allows for a rapid decision-making process in the event that the aircraft does not receive any further instructions from air traffic control (ATC).

The rationale behind this five-second rule is based on the understanding that near the airport, particularly during final approach when the aircraft is close to the ground and in a critical phase of flight, timely responses to lost communications are essential. By assuming lost communications quickly, pilots can initiate appropriate contingency procedures such as following the standard missed approach or going to an alternate plan, ensuring safety and reducing the risk of an accident.

Longer time frames, such as those indicated by other choices, could delay necessary actions and potentially lead to dangerous situations if communication truly has been lost. Hence, the five-second guideline provides a balance between responsiveness and safety during critical phases of flight, making it the correct answer in this scenario.