What is the flasher control unit?

It is the unit that times and sequences the sequenced flashing lights in an approach lighting system. It generates the trigger signals that make each flasher head fire in turn, starting with the outermost and ending with the one nearest the threshold, and it repeats the sequence about twice a second. The result is a ball of light that appears to run down the centreline to the runway.

Three things: it sets the rate with a master timer, it fixes the order with a sequencer, and it sends a trigger to each flasher head. Each head normally has its own control box that charges a capacitor and fires the flash lamp when the trigger arrives, so the control unit does not carry the flash energy itself.

Any steady-burning approach pattern with sequenced flashers. ALSF-1 and ALSF-2 have 15 flashers at 2,400 ft and 21 at full 3,000 ft length, running from 1,000 ft outward. MALSR has 5 RAIL flashers between 1,600 and 2,400 ft at 200-ft spacing, and MALSF has 3 at the outer end. ICAO Annex 14 treats sequenced flashing lights as a supplement to the steady centreline, used where the characteristics of the system and local weather call for them.

A controller fault can stop the whole sequence, so the moving ball of light disappears and the steady white lights are all that is left. A fault in one head loses only that position, leaving a gap in the sequence. In both cases the steady-burning lights still show the pattern, because the flashers supplement them and do not replace them.

It takes its on/off command and brightness step from the approach lighting control, so the flash intensity can follow the intensity setting the pilot's visibility calls for, and it reports status back so a fault can be seen at the control point or on the ALCMS. How these signals are carried varies by manufacturer and installation, so follow the system documentation.

The control unit is the device that generates the timing and trigger signals. Synchronization is the matter of keeping every head in step with that timing, and with other flasher groups, so the sequence runs smoothly. The next entry covers it.

The flasher heads are capacitor-discharge units, so they hold stored energy after the supply is removed. Treat the circuit as live until it is isolated, locked out and proved dead under the permit-to-work, and discharge the capacitors as the manufacturer instructs before opening a head or cabinet. The structures are elevated and stand in the runway protection area, so access and aircraft-movement controls apply.