Definition
A precision approach system that uses paired radio signals to guide an aircraft down to a runway in low-visibility conditions. It provides both lateral guidance (left/right alignment with the runway centerline) from a localizer transmitter, and vertical guidance (the correct descent angle) from a glide slope transmitter. Marker beacons or distance measuring equipment provide distance information along the approach. Together these signals let a pilot follow a precise three-dimensional path to the runway threshold using cockpit instruments alone.
Plain English
A radio-based approach aid that shows the pilot, on cockpit instruments, exactly where the runway is and the correct path down to it, even when the runway can't be seen until the last moment.
Context Anchor
Pilots encounter ILS during instrument approach procedures, avionics checks, and discussions of whether approach equipment is working or inoperative.
Derivation
The name is descriptive: a system used to land an aircraft by reference to instruments rather than by looking outside. The term dates from the 1930s–40s when the technology was first standardized for civil aviation.
Why Pilots Care
It allows safe landings at equipped airports when weather prevents visual approaches, reducing diversions and maintaining schedule reliability.
Grounding Statement
Picture the ILS as two guidance cues leading to the runway: one keeps you lined up left and right, and the other helps you descend at the correct angle.
Intuition Check
An ILS does not land the airplane by itself. It provides guidance; the pilot or approved autopilot still flies the airplane.
Example Sentence 1
Approach cleared us for the ILS to Runway 27, so we set the localizer frequency and briefed the glide slope intercept altitude.
Example Sentence 2
On the ILS approach the glideslope needle centered, confirming the correct descent path.