Tower Lights

     A commenter asked about this the other day: what's the deal with those red or white-strobe tower lights?  What do they have for backups?

     Obviously, the lights on tall structures are there to warn aircraft away.  Not just radio towers, but smokestacks, water towers, bridges, tall buildings, the taller power-line supports and sometimes even power lines.

     I'll stick to the ones I know.  The rules are similar for the rest, though you will find red (and occasionally yellow) spheres on power lines (and some guy wires) for daytime visibility, especially near airports and in crop-dusting regions.  But a typical red-and-white painted* radio tower over a hundred feet tall has a blinking beacon on top (a "code beacon," in that every aspect of it is determined by FAA rules), and maybe sets of steady-burning markers or "obstruction lights" down the side.  The marker lights are not always there -- research shows those are the lights that confuse migrating birds, and if the tower lighting is otherwise compliant with modern requirements, the owner can apply to turn them off.  There are rules for the arrangement of lights depending on height, rules which have changed over time.  Older towers are "grandfathered" until they replace their lighting system.  Currently, it is always beacon-obstruction-beacon-obstruction, etc, from the top down.  You'll see some older tall towers with a pair of marker levels below the top beacon.

     If the tower has white strobe lights, there will be a set on top and, depending on height, more lower down at regular intervals.  They are on all the time, at two different intensities (day/night) for "medium-power" strobe lights, and three (day/twilight/night) for the "high-intensity" strobes you're more likely to see on very tall towers.  Since they have bright lights all the time, towers with strobes don't have to have the red and white paint job.  There are also a few that split the difference, strobes by day and red lights at night, usually in areas where the bright white flash annoys the neighbors.

     Red lights or white strobes, the FAA defines what degree of "north sky illumination" calls for the lights to turn on and off.  You have to have a photocell with a clear view of the north sky (or, presumably, an operator on duty looking at a light meter who flips the switch).  You have to inspect it quarterly.  Most are sealed units these days, and they either work or they don't.  My "North Campus" site had a tube-type control until the late 1990s, with an adjustment, and yes, I would climb a ladder, holding a light meter, and sit out the sundown, seeing that the thing did its job correctly and adjusting the knob if it did not.  FAA-approved photoelectric sensors ran around $300 back then and when the tube unit finally conked out, I replaced it with a modern one and built an interface so the new unit could control the rest of the 1950s system.

     The lighting system must be monitored.  This is done either by having a person check it once a day after sunset by eyeball or remote telemetry (you monitor the current drawn, and the variation caused by flashing the code beacons) and logging the check (and if a beacon is out, calling the FAA reporting number so they can issue a NOTAM, a Notice To Air Missions, warning pilots of the failed light); or by an automatic system that calls, emails or texts a responsible party if there is a problem with the system.  Many tower owners do both.

     There is no general requirement to have backup power.  If the lights go out, you call the FAA, and they issue a NOTAM.  They always ask how long the problem will last; they won't issue "forever" NOTAMs.  They'll give you a week or two, but they're not happy about it.

     There are requirements for the lights themselves that result in redundancy.  All lights levels must be fully visible from 360 degrees around the tower.  Markers are often installed in pairs at each level in skinny, shorter towers, and tall ones usually have three or four, one per leg, mounted on the outside (where they can get whacked by falling ice, so sometimes there's a perforated "ice shield" a little way above them).  Each marker is a rugged, traffic light type 100 W+, 130V  bulb in a small red Fresnel-lens dome or a LED fixtures of the same shape and light output, including infrared.  Strobe lights are installed the same way, pairs, triplets or quads.

     The flashing code beacons are a whole other ball game.  The classic incandescent version is over a yard tall, a foot and a half wide, a metal framework supporting clear glass Fresnel lenses with red filters inside.  It hinges open in the middle, allowing access to a pair of 620 W bulbs almost as big as your head, one base down in the bottom section, one base up in the top.  They're wired in parallel, and as long as at least one is burning, it's in compliance with FAA requirements.  Old ones have asbestos-insulated wiring and asbestos in the gaskets between the sections of the Fresnel lens, so don't play with them if you find one by the side of the road.  (You probably won't.)
     The modern LED replacement code beacon -- about $5000.00, the last time I bought one -- looks nothing at all like this.  It's a short, white-and-clear cylinder loaded with super-bright, heavy-duty light-emitting diodes, and it will leave you literally dazzled if you wire it up in a workshop and turn it on with your back to it.  Ask me how I know!  They put out the same amount of light and IR as the older ones.
     Light-bulb versions are flashed from ground level, either with a motorized gadget based on a traffic-light flasher, or modern solid-state flashers, and all the lights on a tower have to flash in sync.  The old motorized flashers often used cams and sealed mercury switches.  When the flash-at-the-same-time rule was made, I synced up the set on our tower by carefully aligning the cams: you can't see the tower at all from the room where the thing was mounted on the wall.
     Code beacons have the same requirement for 360-degree visibility.  Below the top level, they are usually installed in pairs to ensure that at least one isn't blocked by the tower, no matter where you are.  And they often have ice protection, since the cost of replacement is high no matter if it is LED or incandescent.

     The big tower I am responsible for has a mixture of LED beacons and markers (the highest ones, where you have to shut important stuff off to send a tower guy) and incandescent.  We try to replace the incandescent bulbs once a year, in the fall.

     (P.S. Another commenter wonders if this was posted in response to the helicopter crash/tower collapse in Houston and posts a link to the video.  Nope, it was not; and that video has been widely shared elsewhere.  Video of people's deaths ought not be used idly and I will not post the link.  Many years ago, I was first on the scene of a fatal helicopter crash.  It's terrible to see the results and realize there is nothing you can do for the victims.)
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* The FAA defines the paint job, too, right down to the colors.  You can read about it and the lights here.