Definition
An airport surface sign with a white inscription on a red background, displaying the runway designation followed by 'ILS' (for example, '15-33 ILS'), used to mark the holding position for the ILS critical area on a taxiway. When ILS approaches are in use in low-visibility conditions, aircraft and vehicles must hold short of this sign to keep clear of the protected area where their presence could distort the ILS localizer or glideslope signal being received by an arriving aircraft.
Plain English
A red-and-white sign on the taxiway that tells you to stop here when the instrument landing system is being used in poor weather, so your aircraft doesn't interfere with the radio signal guiding another aircraft down to the runway.
Context Anchor
Seen on taxiways near runways served by an Instrument Landing System, especially during low-visibility operations.
Derivation
Critical area' refers to the patch of ground near the ILS antennas where a metal object — like an aircraft or vehicle — can bend or reflect the radio beams enough to mislead an aircraft on approach. The sign marks where to stop to keep that area clear.
Why Pilots Care
Crossing this sign without clearance can distort the ILS localizer and glideslope signals, forcing arriving aircraft to execute missed approaches or lose precision guidance.
Grounding Statement
The sign marks a protected stopping point, not a parking spot or a general warning area.
Intuition Check
Do not read “critical area” as “dangerous area.” Here it means a protected signal area that must stay clear so the landing system works correctly.
Example Sentence 1
Tower instructed us to hold short at the ILS critical area holding position sign while the inbound aircraft completed its approach.
Example Sentence 2
During the taxi to runway 27, the crew stopped at the Ils Critical Area Holding Position Sign to protect the active ILS approach.