Scott Dyer [HPN/NY]
(Top Gun)
12/15/17 04:25 PM
Strobing Runway Lights

No, I'm not referring to the rabbit. Here's a report of how LED runway/centerline lights may cause some problems for arriving aircraft. (CALLBACK is great, and this month's issue had some especially interesting reports...including this one).

Now You See it, Now You Don’t

A Bonanza Pilot became distracted and confused when he perceived the runway edge and centerline lights cycling on and off while ATC assured him that they were on steady.

■ I was transiting the final approach path of…Runway 16R and observed the runway edge and center line 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 Rwy 16R approach, we observed the runway edge and center line 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 he also 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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