Approach Lighting Systems (ALS) are implemented at end of airport runways and is capable to provide visual guidance for pilots and help align the aircraft to the runway centerline to make safer landings, especially during night times and poor visibility conditions.
Approach guidance systems such as ILS installed in airports and aircraft are capable to provide primary instrument guidance to pilots and support air navigation from en route waypoint to the runway. However, during nighttime and poor visibility conditions runway lighting plays a vital role in approach guidance, especially for pilots to establish the visual reference of the runway.
There are different lighting configurations available in runways and it serves a different purpose. Especially Runway edge threshold & end lights are essential to all aerodromes and they are adequate only under good visibility conditions.
But in poor visibility conditions, runways must be supplemented by an Approach lighting system (ALS). ALS in general installed at end of the runway and indicates the runway centerline & approach direction when an aircraft approaches the runway for landing.
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 and align the aircraft with the runway upon arriving at a prescribed point on an approach.
ALS are a configuration of signal lights starting at the landing threshold and extending into the approach area at a distance of 2400-3000 feet for precision instrument runways and 1400-1500 feet for non-precision instrument runways.
In-flight operation perspective, the availability of runway lighting impacts the landing minimums. For example, In the U.S., a CAT I ILS approach without approach lights will have a minimum required visibility of 3/4 mile, or 4000-foot runway visual range.
With a 1400-foot or longer approach light system, the minimum potential visibility might be reduced to 1/2 mile (2400 runway visual range), and the presence of touchdown zone and centerline lights with a suitable approach light system might further reduce the visibility to 3/8 mile (1800 feet runway visual range).
Types of Approach Lighting System (ALS)
There are different Approach Lighting System configurations available,
ALSF I (Approach Lighting System with Sequenced Flashing Lights) is used on Category I runways during the instrument landing to align the aircraft with the centerline of the runway and to establish vertical orientation.
Up to twenty-one white lights create a sequential strobing flash pattern that rolls toward the runway threshold.
ALSF II (Approach Lighting System with Sequenced Flashing Lights) mode is used on Category II runways during the instrument landing to align the aircraft with the centerline of the runway and to establish vertical orientation.
Up to 174 steady burning white lights serve as a reference plane, and two rows of red side row barrettes increase the pilot's horizontal perception.
Up to twenty-one white lights create a sequential strobing flash pattern that rolls toward the runway threshold.
Three intensity settings allow the approach to be used under changing weather conditions.
The Dual Mode ALSF II/SSALR approach lighting system allows Category II runways to operate in either the full ALSF II mode or in the economical SSALR mode, depending on weather conditions.
SSALR (Simplified Short Approach Lighting System with Runway Alignment Indicator Lights) mode is used during instrument landing to align the aircraft with the centerline of the runway.
The white steady burning lights of the ALSF II are greatly reduced to conserve power under improved visibility and the side row barrettes are not powered.
Up to sixty-three steady-burning lights are used to create a reference plane,
Up to eight lights create a sequential strobing flash pattern that rolls toward the runway threshold.
SSAL (Simplified Short Approach Lighting System) is available.
Omnidirectional Approach Lighting System (ODALS) is a configuration of seven omnidirectional sequenced flashing lights located in the runway approach area. The ODALS provides circling, offset, and straight-in visual guidance for non-precision approach runways.
Medium Intensity Approach Lighting System with Sequenced Flashers (MALSF) system usually is a 1400-foot-long array of lights but can be longer or shorter depending on local terrain and requirements. A typical MALSF system consists of the following light fixture components:
9 light bars each with 5 steady burning white
5 sequence flashing white
Threshold bar of 18 steady burning green
The recent approach lighting systems are highly complex in their design and significantly enhance the safety of aircraft operations, particularly in conditions of reduced visibility.