Approach Lighting System and Landing Category
Some installations include medium or high intensity approach lighting system (ALS) can be seen . Most often, there are at larger airport but many small general aviation airports have approach lights to support their ILS*1 installations and obtain low-visibility minimum. ALS assist the pilot in transitioning from instrument to visual flight, and to align the aircraft visually with the runway centerline. Pilot observation of the approach lighting system at the DH allows the pilot to continue descending towards the runway, even if the runway or runway lights can not be seen, since the ALS counts as runway end environment. Such ( ILS installed ) airports define the landing categories which provide pilots with DH and RVR for each runway respectively for securing safety landings and efficiency of air traffic.
*1 Instrument Landing System (ILS):
A radio navigation system which provides the pilot with horizontal and vertical guidance just before and during landing and, at certain fixed points, indicates distance to the reference point of landing.
*2 Decision Height(DH):
With respect to the operation of aircraft, the height at which a decision must be made ,during an ILS instrument approach, to either continue the approach or to execute a missed approach (go-around).
*3 Runway Visual Range(RVR):
With respect to the operation of aircraft the distance over which a pilot of an aircraft on the centerline of the runway can see the runway surface markings delineating the runway or identifying centerline.