An approach lighting system, or ALS, is a lighting system installed on the approach end of an airport runway and consists of a series of lightbars, strobe lights, or a combination of the two that extends outward from the runway end. ALS usually serves a runway that has an instrument approach procedure (IAP) associated with it and allows the pilot to visually identify the runway environment once he or she has arrived at a prescribed point on an approach.
The runway lighting is controlled by the air traffic control tower. At uncontrolled airports, Pilot Controlled Lighting may be installed which can be switched on by the pilot via radio. In both cases, the brightness of the lights can be adjusted for day and night operations.
Approach Light Systems are designed to allow the pilot to quickly and positively identify visibility distances in Instrument meteorological conditions. For example, if the aircraft is at the Middle Marker, and the Middle Marker is located 3600' from the threshold, the Decision Bar is 2600' ahead. If the procedure calls for at least 1/2 statute mile flight visibility (roughly 2600'), spotting the Decision Bar at the marker would indicate enough flight visibility to continue the procedure. In addition, the shorter bars before and after the Decision Bar are spaced either 100' or 200' apart depending on the ALS type. The number of short bars the pilot can see can be used to determine flight visibility. Approaches with lower minimums use the more precise 100' spacing systems for more accurate identification of visibility.
Approach Light Secrets: Like Rodney Dangerfield, Approach Lighting Gets No Respect. until It's Used to Descend a Bit Further, That Is. Fly Safer and Avoid Bending the Regs by Knowing Your Lights
Dec 01, 2012; As much as we drill instrument students on the 10 items from FAR 91.175 (c) you could see to go below DA/MDA, in the real world...