What makes a beacon flash as it turns?
A motor turns the lenses round a lamp that stays lit, and each beam that sweeps past a pilot shows as one flash.
A rotating beacon makes its flash by moving its optics. A fixed lamp stays lit at the centre while a motor and gearbox turn a lens carrier around it. A clear lens gives a white beam and a green-filtered lens gives a green one (at a land aerodrome the coloured flash is green; at a water aerodrome it is yellow). The beams sweep the horizon, and a pilot sees one flash each time a beam passes, so white and green alternate.
The flash rate is the number of beams times the turns per minute. A beacon with one white and one green beam turning at 12 turns a minute gives 24 flashes a minute, inside the 20 to 30 that ICAO sets for an aerodrome beacon. Because the pattern depends on the motor, a drive fault stops the sweep and the flashing, so the beacon needs regular checks of the motor, gearbox, lens and lamp.
Frequently Asked Questions
Q1What is a rotating beacon?
A beacon in which a motor turns a lens assembly around a lamp. The lens gathers the light into beams that sweep round the horizon, so a pilot sees a flash each time a beam points their way. It is the classic way to build an aerodrome beacon.
Q2How does a rotating beacon make flashes?
The lamp stays lit. The lenses turn, and each beam sweeps past the pilot in a short time. A clear lens gives a white flash and a green-filtered lens gives a green one. The pilot sees them alternate.
Q3How is the flash rate worked out?
Flashes seen per minute equal the number of beams times the turns per minute. A beacon with one white and one green beam turning at 12 turns a minute gives 24 flashes a minute, inside the 20 to 30 range that ICAO sets.
Q4What happens if the motor stops?
The beams stop sweeping and the pilot sees a steady light, or nothing, depending on where the beams point. The beacon then no longer shows its flash pattern, so a motor or drive fault needs fixing quickly. Many installations monitor the beacon and raise an alarm.
Q5How is a rotating beacon different from an LED beacon?
A rotating beacon moves its optics to make the flash. An LED beacon can make the flash by switching its light sources on and off in sequence, with no moving parts. Both must meet the same flash colour, rate and intensity rules.
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