An instrument landing system (ILS) uses two separate ground-based radio transmitters to provide both lateral and vertical guidance to pilots during an instrument approach procedure that normally includes the lowest landing minimums. A Category I ILS approach, for example, typically offers a 200-foot decision height above the ground. ILS approach visibility minimums are measured either in statute miles or by using runway visual ranges down to zero-zero for a Category IIIc ILS approach. (Approaches of Cat 2 and greater require special flight crew certification.)
How an Instrument Landing System Works
Key Takeaways:
- An Instrument Landing System (ILS) uses ground-based radio transmitters, the localizer for lateral guidance and the glideslope for vertical guidance, to provide precise navigation during instrument approaches, allowing for very low landing minimums.
- The ILS system's sensitivity significantly increases as an aircraft approaches the runway, requiring pilots to make progressively smaller and more precise corrections to stay on course and glideslope.
- While some ILS approaches include distance-indicating marker beacons and localizers can be used for "back-course" approaches, a glideslope failure transforms an ILS into a higher-minimum "localizer-only" non-precision approach.
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