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An incident report from NASA’s Aviation Safety Reporting System (ASRS) gives insight into a pilot’s recurring, problematic observation. Through distraction and confusion, a Bonanza pilot misperceived the runway edge and centerline lights as they cycled off and on. Air Traffic Control (ATC) let him know that the centerline lights were constant, not blinking.

The pilot summarized his experience, “I was transiting the final approach path of . . . Runway 16R and observed the runway edge and centerline lights cycle on and off . . . at a rate of approximately 1 per second. It was very similar to the rate of a blinking traffic light at a 4-way vehicle stop. The [3-blade] propeller speed was 2,400 RPM. This was observed through the entire front windscreen and at least part of the pilot side window. I queried ATC about the reason for the runway lights blinking and was told that they were not blinking. It was not immediately obvious what was causing this, but I did later speculate that it may have been caused by looking through the propeller arc.

“The next day [during] IFR training while on the VOR/DME Runway 16R approach, we observed the runway edge and centerline lights cycle on and off . . . at a rate slightly faster than 1 per second. The propeller speed was 2,500 RPM. I then varied the propeller speed and found that, at 2,700 RPM, the lights were observed strobing at a fairly high rate and, at 2,000 RPM, the blinking rate slowed to less than once per second. This was observed through the entire approach that terminated at the Missed Approach Point (MAP). The flight instructor was also surprised and mentioned that he had not seen this before, but also he doesn’t spend much time behind a 3-blade propeller arc.

“I would speculate that the Pulse Width Modulation (PWM) dimming system of the LED runway lights was phasing with my propeller, causing the observed effect. I would also speculate that the effect would . . . significantly differ at other LED dimming settings . . . and behind a 2-blade propeller.

“I found the effect to be entirely confusing and distracting and would not want to make a landing in such conditions.”

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