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ILS Specifications and Components

Approach Lighting Systems


The MALSR (Medium Intensity Approach Lighting System With Runway Alignment Indicator Lights) is a medium approach intensity lighting system (ALS) installed in airport runway approach zones along the extended centerline of the runway. The MALSR, consisting of a combination of threshold lamps, steady burning light bars and flashers, provides visual information to pilots on runway alignment, height perception, roll guidance, and horizontal references for Category I Precision Approaches.

If the minimum published visibility for an ILS runway equipped with MALSR, is no lower than 3/4 SM, the pilot should be aware that there may be a penetration of the obstacle identification surfaces (OIS) and care should be taken in the visual segment to avoid any obstacles.


Runway end identifier lights (REIL) are installed for rapid and positive identification of the approach end of an instrument runway. The system consists of a pair of synchronized flashing lights placed laterally on each side of the runway threshold facing the approach area.

Marker Beacons

ILS courses may include two VHF marker beacon transmitters along the final approach course centerline. The outer marker (OM) is usually located 4-7 NM from the runway threshold. The middle marker (MM) is usually located 3,500 feet from the runway threshold. When the aircraft crosses the outer marker a flashing blue light will annunciate on the panel. When the aircraft crosses the middle marker, a flashing amber light will annunciate. An aircraft on the glide slope over the middle marker should be roughly 200 feet above the touchdown zone elevation.

Category II ILS approach systems may include a third marker beacon called the inner marker (IM), although these are becoming less common. The inner marker, where installed, is located on the front course between the MM and the landing threshold. It indicates the point at which an aircraft is at the decision height on the glidepath during a Category II ILS approach. The inner marker is annunciated by a flashing white light on the panel and a rapid series of short high-pitched tones (like Morse Code "dits").

Precision radar, or compass locators located at the Outer Marker (OM) or Middle Marker (MM), may be substituted for marker beacons. DME, when specified in the procedure, may be substituted for the OM.

Compass Locators

Compass locators are low-powered NDBs sometimes collocated with outer and/or inner marker beacons. The coding identification of the outer locator consists of the first two letters of the three-letter identifier of the associated LOC. The middle locator at DAL is identified by the last two letters.

ILS - Instrument Landing System