What is ODALS?

ODALS is the Omnidirectional Approach Lighting System. It is a simple approach lighting aid of seven omnidirectional sequenced flashing lights placed in the approach area of a non-precision runway. It has no steady bars: it is a short line of flashes that leads the pilot towards the threshold.

Five flashers sit on the extended runway centreline, the first 300 ft from the threshold and the rest at equal 300-ft intervals out to 1,500 ft. The other two sit one on each side of the threshold, 40 ft from the runway edge, or 75 ft where the runway has a VASI. Some metric specifications use 90-m spacing out to 450 m with the threshold pair about 12 m from the edge, so check the applicable standard.

The flash runs from the outermost light inward along the centreline. When it reaches the threshold, the two threshold lights flash together, and the cycle repeats about once per second. The sequence acts as a rabbit that points to the runway and ends at the threshold.

The flashheads give 360-degree horizontal coverage with a narrow vertical beam, so the lights can be seen from any side of the approach. That is why ODALS supports circling and offset approaches as well as a straight-in one, which a directional centreline system does not do as well.

It is used on non-precision runways where a full steady-burning system is not provided. Unlike MALS, MALSR or the ALSF systems it has no steady white bars, no crossbar and no red bars: only seven flashers. It identifies the approach and runway end rather than giving the full rows-and-bars picture, and the visibility benefit comes from the approved procedure, not from the system name.

Flashheads are normally variable-intensity with high, medium and low steps. They are fed either from a voltage supply with separate control wiring or, for the current-driven type, from an airfield series circuit through an isolating transformer. Nominal intensities and timing come from the manufacturer and the standard, so use the data sheet for the unit installed.

Yes. ODALS flashers are capacitor-discharge units, so stored charge can remain after isolation; isolate, lock out and prove dead under the permit-to-work, then discharge as the manufacturer instructs before opening a head or cabinet. The units stand in the runway protection area, so access and aircraft-movement controls also apply.