Definition
A precision approach navigation system that uses ground-based radio transmitters to provide an aircraft with both lateral guidance (alignment with the runway centerline) and vertical guidance (descent angle to the runway threshold), enabling pilots to descend to a low altitude in poor visibility before either seeing the runway and landing or executing a missed approach.
Plain English
A radio-based system that beams two invisible guide paths up from the runway -- one telling the pilot if they are left or right of centerline, and one telling them if they are too high or too low -- so they can fly down to the runway in cloud or low visibility.
Context Anchor
Seen in instrument approach, HUD, and landing guidance discussions, especially when a display shows guidance for lining up with a runway in low visibility.
Derivation
"Instrument" because the pilot flies by reference to cockpit instruments rather than looking outside. "Landing system" because, unlike navigation aids that just point to an airport, this one guides the aircraft all the way down to the runway.
Why Pilots Care
Enables safe landings when weather prevents visual approaches, reducing the need for missed approaches and improving schedule reliability.
Intuition Check
Do not read this as a general phrase for any instruments used during landing. In aviation, instrument landing systems are specific approach systems that provide runway alignment and descent guidance.
Example Sentence 1
With the ceiling at 300 feet and visibility at one mile, the crew briefed and flew the instrument landing system approach to runway 27.
Example Sentence 2
With the HUD showing the guidance cues from the instrument landing systems, the pilot continued the approach through the cloud layer.