Date Released: 2017-09-22

Opinion: FAA Should Rescind Rule Dropping L-810 Intensity


BY ANDY RUDOLPH, P.E.

Under pressure from manufacturers of under-powered L-810 LED obstruction lights, the FAA has cut the minimum effective intensity requirement by 50 percent.  Unfortunately, FAA’s timing could not have been worse.  Recent advances in lighting technology have already resulted in large drops in L-810 intensity as manufacturers moved from incandescent to LED-based designs.  The 50-percent intensity reduction FAA added to AC 150/5345-43H means that today’s L-810 intensity may have 90 percent lower intensity than lights produced only a few years ago.

The story of how this has come to has come to pass has to do with migratory birds and LEDs (Light Emitting Diodes).  Two forces that are changing our world today, the environment and technology, have come together to create the unique situation in which we find ourselves.  First, environmental groups, concerned that steady-burning (non-flashing) obstruction lights may contribute to migratory bird deaths, sued the FCC to force action.  Technology is the second contributing force; in this case, the transition by manufacturers to LED-based obstruction light designs.

The FCC requires licensees such as broadcasters and wireless providers to comply with FAA Advisory Circulars regarding tower marking and lighting.  After being sued by environmental groups, the FCC partnered with the FAA to examine alternative lighting configurations that would eliminate steady-burning (non-flashing) lights on towers.  L-810 obstruction lights have historically been required to operate steady-burning at night and were located between levels of L-864 red flashing beacons on tower structures.  FAA has now published two new Advisory Circulars to effectively stop of the use of steady-burning lights on new tower structures, AC 70/7460-1L and AC 150/5345-43H.  Towers over 350 feet above ground level (AGL) no longer use L-810 lights.  Towers between 200 feet and 250 feet AGL are now required to flash the L-810 lights.

Just a few years ago, L-810 obstruction light designs were very similar.  Most used similar performing Fresnel lenses made of glass and 116 Watt traffic signal incandescent lamps.  These L-810 lights generally produced well above the 32.5 candela minimum intensity required by FAA, producing around 150 candela.  FAA has no maximum intensity requirement for L-810 lights.  Traffic signal lamps were widely available and their rugged design made them well suited for use on tower structures as well as in traffic signal lights.  Though the typical intensity of 150cd far exceeds the FAA minimum requirement of 32.5 it became a de-facto standard due to its continued use over decades.

The advent of high-powered LEDs made new low-power obstruction light designs possible and manufacturers wasted no time in using the new technology.  Additionally, switching power supply technology made the new designs almost immune to the effects of voltage drop, unlike incandescent lamps.  New plastic optics could focus the LED light into the precise beam patterns prescribed by the FAA Advisory Circulars with little overage; something impossible to do with glass lenses and incandescent lamps.  The result was that LED obstruction lights could be designed to output far lower intensity while still meeting FAA requirements over prescribed test conditions.  LED L-810 obstruction lights typically produced intensities around 50 candela.

Some manufacturers lowered the intensity of their LED L-810 designs very close to the 32.5 candela minimum intensity required by FAA.  These manufacturers would soon find themselves in a difficult position.  In order to prevent migratory bird deaths, the FAA revised the advisory circulars to require L-810 lights to flash.  FAA Advisory Circulars require the Blondel-Rey equation to be used to determine the effective intensity of a flashing light source.   The Blondel-Rey equation generally provides that a flashing light source will have an effective intensity lower than its steady-state intensity.  The equation can result in effective intensities much lower than the steady-state intensity as the on-time is reduced.  An on-time of 0.2 seconds results in a 50-percent reduction in effective intensity.  Manufacturers of L-810 lights very close to 32.5 candela minimum when steady-burning would not meet the minimum required intensity when flashing.

Manufacturers of such lights were not without choices.  One solution would be to increase the on-time of the flash, mitigating the effect of the Blondel-Rey equation.  Another solution would be to increase the intensity of their L-810 lights to meet the 32.5 effective candela (CD) minimum when flashing.  The last choice would be to pressure the FAA into dropping the minimum intensity by 50 percent resulting in a new 16.25 candela minimum.  Unfortunately, this last option is the choice that manufactures of under-powered L-810 lights have made.

Only a few years ago virtually all L-810 lights produced 150 candela.  The new generation of LED L-810 lights dropped that to 50 candela.  Now the FAA is allowing 16.25 candela L-810 lights.  L-810 intensity has dropped by 90 percent in few years and no one appears to be concerned.  The FAA, FCC and the obstruction lighting industry have all worked hard to preserve the lives of migratory birds; should we not work just as hard – if not harder – to preserve the lives of pilots and the flying public?  All L-810 obstruction lights should meet the 32.5 candela minimum whether operated steady-burning or flashing.  The FAA should remove the 50-percent intensity reduction immediately.

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