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.
A key ingredient of all Approach Lighting Systems in the United States is the Decision Bar
. The Decision Bar is always located 1000' from the threshold, and it serves as a visible horizon to ease the transition from instrument flight to visual flight. It also is situated such that at Decision Altitude, the Decision Bar is just appearing to pass under the cowling of the plane.
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.
Several ALS configurations are recognized by the International Civil Aviation Organization
(ICAO); however, non-standard ALS configurations are installed at some airports. Typically, approach lighting systems are of high-intensity. Many approach lighting systems are also complemented by various on-runway light systems, such as Runway End Identification Lights
(REIL), Touchdown Zone Lighting
(TDZL), and High-Intensity Runway Edge Lights
(HIRL). The most common approach light system configurations include:
- MALSR: Medium-intensity approach light system with Runway Alignment Indicator Lights
- MALSF: Medium-intensity approach light system with Sequenced Flashers
- SALS: Short approach light system
- SSALS: Simplified Short approach light system
- SSALR: Simplified Short approach light system with Runway Alignment Indicator Lights
- SSALF: Simplified Short approach light system with Sequenced Flashers
- ODALS: Omnidirectional approach light system
- ALSF-1: Approach light system configuration 1
- ALSF-2: Approach light system configuration 2
- CALVERT I/ICAO-1: ICAO-compliant configuration 1
- CALVERT II/ICAO-2: ICAO-compliant configuration 2
- LDIN: Lead-in lighting