Instrument Landing System (ILS)
The Instrument Landing System (ILS) is a precision approach and landing system that provides both lateral (localizer) and vertical (glide slope) guidance to pilots making an approach to a runway in reduced visibility. It is the most widely used precision approach system in the world.
ILS Components
- Localizer: Transmits on a VHF frequency (108–111.95 MHz); provides left-right guidance aligned with the runway centreline; usable to approximately 18 NM
- Glide slope: Transmits on UHF (paired with the localizer); provides vertical guidance at typically 3.0°; usable to approximately 10 NM
- Marker beacons (OM, MM, IM): Provide distance information at fixed points along the approach
- Approach lighting system (ALS): Provides visual guidance from DH to touchdown
ILS Categories
- CAT I: DH 200 ft, RVR 1,800 ft — standard precision approach
- CAT II: DH 100 ft, RVR 1,200 ft — requires enhanced crew training and aircraft equipment
- CAT IIIa/b/c: DH down to zero, RVR as low as 75 ft — autoland capability required
ILS Errors and Limitations
ILS is susceptible to signal interference from aircraft or vehicles on or near the runway, terrain reflections, and false glide slopes (above the true slope at multiples of approximately 9°). Always fly the localizer from the outer marker inbound and verify the glide slope from below, never capturing from above.
On the TC Written Exam
ILS is a core instrument exam topic. Know: localizer frequency range, glide slope pairing, marker beacon sequence, CAT minimums, and failure mode of each component.
Related Terms
See also: Localizer, Glide Path/Glide Slope, Decision Height, Marker Beacon.