When do PAPI lights begin to flash?

PAPI (Precision Approach Path Indicator) lights are designed to provide visual guidance to pilots during the landing phase of flight, specifically to help maintain the proper glide slope. The lights begin to flash as a warning indication when the runway is not clear, which helps alert pilots to a situation that could affect their landing safety.

When the PAPI lights flash, it signifies that the aircraft is approaching an unsafe condition, and immediate action may be necessary. This could be due to an obstruction on the runway, such as another aircraft or vehicle that might pose a risk during landing. As such, understanding the function of PAPI lights and recognizing when they flash is essential for ensuring safety during approach and landing phases.

The other choices do not reflect the accurate context or timing for the flashing of PAPI lights. For instance, in a missed approach scenario, the lights would assist the pilot in maintaining glide slope as they execute the missed procedure, but they would not begin to flash. Similarly, runway occupancy doesn't trigger flashing lights; they are more focused on indicating an immediate hazard related to landing safety rather than general occupancy. Thus, recognizing that PAPI lights flash primarily when the runway isn’t clear emphasizes their critical role in preventing accidents during the landing phase.