Strategic Technical Planning for Broadcasters


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Save some birds, save a little money - but only if your tower is at least 450' tall.
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Side Light Extinguishment

May 6, 2013
Save some birds, save a little money - but only if your tower is at least 450' tall.
The FAA, FCC and other agencies and NGOs have been studying the issue of bird mortality for many years. This work culminated in a report (DOT/FAA/TC-TN12/9) describing research into bird fatalities coupled with flight testing of various lighting schemes on tall towers in Michigan.

The report indicates that migratory birds, which often fly at night because that is the safest time for them to travel, become attracted to steady-burning side lights (obstruction lights) in times of poor visibility. They become confused and fly into the tower itself, or more commonly, into guy wires, sometimes with fatal results.

The birds are not as strongly attracted to flashing beacon lights, so one way to help the birds would be to eliminate the side lights entirely. Another way would be to flash the side lights along with the beacons. It appears from the report that both methods are equally effective with respect to bird mortality.

In order to recognize a tower at night, a pilot needs at least two lights stacked vertically. The side lights help enormously because they provide a reference for the pilot while the beacons are off.  According to the report, however, if the beacons flash quickly enough, the pilot doesn't need the side lights to keep track of the tower.

The traditional standard for beacon flashing is 30 flashes per minute (fpm), +/- 10 fpm, so your beacons are legal if they flash at rates from 20 to 40 fpm.

According to the report, flight testing determined that 20 fpm is far too slow for pilots to track the tower in the absence of side lights. The report concludes that 27 fpm is the minimum acceptable rate.

At 40 fpm, the filament in a traditional incandescent beacon light doesn't cool down sufficiently to go dark before it is turned on again, and the birds are attracted despite the varying intensity.  In order for the filament to go dark long enough to break the birds' attention span, the maximum rate is about 33 fpm.

These factors combine to indicate a requirement that the beacon flasher be set to 27-33 fpm.

Since a pilot needs at least two stacked lights to recognize a tower, only towers with two beacon lights are eligible for side light extinguishment. Towers of 450' or more overall height are required to have two beacon light levels, and such towers are eligible.

So if you have a tower that is 450' or higher and is equipped with at least two beacon light levels, and you can set your beacon flash rate to 27-33 flashes per minute, you may be permitted to extinguish, and ultimately to remove, the side lights from your tower. 

Prior permission in the form of an approved FAA aeronautical study is required.

Contact Skywaves for more information.
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