Definition
An airport surface marking sign that identifies the edge of the ILS critical area on a taxiway. The sign has a yellow background with a black design that matches the runway holding position marking for the ILS critical area. It tells pilots and vehicle drivers that, once past this sign while exiting the ILS critical area, they are clear of the protected zone where their presence could distort ILS signals being used by an aircraft on approach.
Plain English
A taxiway sign that marks where you leave the area that must stay clear to protect the instrument landing system signal. Once you taxi past it on your way out, you are no longer in the protected zone.
Context Anchor
Seen on taxiways near runways served by an Instrument Landing System, especially during taxi, runway exit, and low-visibility operations.
Derivation
ILS stands for Instrument Landing System. 'Critical area' refers to the zone near the runway where vehicles or aircraft can disturb the ILS radio signal. 'Boundary' marks the edge of that zone. Together the sign names exactly what it does: it shows the boundary of the ILS critical area.
Why Pilots Care
Vehicles or aircraft inside the critical area can distort the localizer and glideslope signals, causing false guidance during an approach.
Grounding Statement
Picture a marked space around sensitive landing equipment where aircraft and vehicles stay out when the signal must be protected.
Intuition Check
Do not read critical area as a general danger area. Here it means a protected signal area for the Instrument Landing System.
Example Sentence 1
After landing, the pilot continued taxiing until the ILS critical area boundary sign was behind the aircraft, then reported clear of the ILS critical area to ground control.
Example Sentence 2
During low-visibility operations, all ground traffic remained behind the ILS critical area boundary sign to protect the signal.