ILS critical area boundary sign

What an ILS critical area boundary sign is

An ILS critical area boundary sign is located on a taxiway to tell a pilot leaving the runway when the aircraft is clear of the ILS critical area — the protected zone around the localizer and glide slope antennas that must be kept free of aircraft when an approach is being flown in low visibility. Like the runway boundary sign it carries no text: its face reproduces the ILS critical area holding-position (boundary) marking in black on a yellow background. It is grouped with the information signs and uses the yellow-and-black scheme.

An FAA sign — and the red ILS critical area holding position sign

The ILS critical area boundary sign is an FAA sign type, defined in the FAA's advisory circulars (AC 150/5340-18). It is the "exit" companion to the red mandatory-instruction ILS critical area holding position sign: the red holding-position sign tells a pilot approaching where they must stop to stay out of the critical area, while this yellow boundary sign tells a pilot who has crossed it when they are clear of that area on the way out. There is no direct single-sign equivalent under ICAO Annex 14 Volume I or EASA CS-ADR-DSN, which convey the "clear of the protected area" idea through the named runway vacated sign.

How the ILS boundary marking pattern reads

The face is a pictorial repeat of the ILS critical area holding-position marking painted on the taxiway, rendered in black on yellow. A pilot exiting the runway crosses that marking on the pavement and sees the matching pattern on the sign, so the on-pavement and on-sign cues agree. The rule mirrors the runway boundary sign: once the whole aircraft is beyond the sign it is beyond the marking and clear of the ILS critical area, at which point air traffic control can release the critical area for an aircraft on approach.

Why the critical-area boundary matters

An aircraft sitting inside the ILS critical area can distort the localizer or glide-slope signal for an aeroplane on final, so during low-visibility operations that area must be protected. The boundary sign gives the exiting crew an unambiguous "you are now clear" cue for exactly that zone, distinct from the runway boundary sign that marks clearance of the runway itself. Keeping the two boundary signs — and the red holding-position sign — visually distinct is why the FAA holds each to its own marking pattern and colour.

Draw them in Wingframe

Wingframe draws ILS critical area boundary signs to FAA geometry — the black critical-area holding-position pattern on yellow — so the sign face matches the ILS critical area marking it mirrors, alongside its red holding-position counterpart. See what Wingframe can do.