What determines the normal PNR points?

The correct answer is based on the concept of "safe endurance," which refers to the maximum amount of time an aircraft can safely operate under specific conditions before it needs to land or refuel. Normal PNR (Point of No Return) points are calculated in relation to this safe endurance, as they define the point during a flight beyond which the aircraft does not have enough fuel to return to the departure airport or reach a safe alternate airport.

Safe endurance encompasses various factors, including fuel consumption rates, weather conditions, and the aircraft's specific operational capabilities. Therefore, understanding how long an aircraft can sustain flight while considering these factors directly influences the determination of the normal PNR points. This concept is critical for flight planning and ensuring safety in aviation operations.