Definition
A precision approach navigation system that provides pilots with electronic guidance both horizontally (left/right of the runway centerline) and vertically (above/below the correct descent path) to a specific runway, allowing landings in low-visibility conditions. An ILS typically consists of a localizer for lateral guidance, a glideslope for vertical guidance, and approach lighting and marker beacons or DME for distance information.
Plain English
A ground-based radio system that gives the pilot two electronic 'beams' to follow during landing — one to keep the aircraft lined up with the runway, and another to keep it on the correct descending path to the touchdown point. It lets pilots land safely when they can't see the runway until the last moments.
Context Anchor
You will see this term on instrument approach charts, in air traffic control clearances, and during training for approaches in clouds, low visibility, or at night.
Derivation
Instrument' here refers to flying by reference to cockpit instruments rather than by looking outside. The name reflects that the system is designed for landings made on instruments, in conditions where outside visual cues aren't reliable until very close to the runway.
Why Pilots Care
It permits safe, accurate landings when the runway cannot be seen until very close to the ground, greatly reducing weather-related accidents.
Grounding Statement
Picture flying toward a runway inside a cloud: the Instrument Landing System gives you cockpit guidance for both “am I lined up?” and “am I descending correctly?”
Intuition Check
Do not think of an Instrument Landing System as one cockpit instrument or an automatic landing device. It is a complete guidance system, using ground signals and aircraft equipment, that helps the pilot fly the correct path to the runway.
Example Sentence 1
Cleared for the ILS approach to runway 27, the pilot intercepted the localizer first, then captured the glideslope and began the descent.
Example Sentence 2
In heavy fog the crew relied on the Instrument Landing System to maintain the correct glide path until the runway lights became visible.