Ask a CFI: What is an ILS critical area and when should I hold short?
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When taxiing towards the starting point of a runway, you may see a red sign on the left with the letters ILS. There will also be a corresponding yellow marking on the taxiway, which looks like a ladder. This identifies the critical area for the instrument landing system, located near the ILS antenna array, where an airplane on the ground (or vehicle) in that space could interfere with the radio signals for airplanes flying an ILS instrument approach.
An ILS consists of two separate facilities that operate independently but come together in the cockpit to enable both lateral and vertical precision guidance. The localizer transmits VHF signals to provide lateral guidance, while the glideslope facility transmits UHF signals to provide vertifcal guidance.
You are only expected to hold short of this area when instructed to by ATC; however, at non-towered (pilot-controlled) airports, pilots should remain clear of ILS critical areas when an aircraft is within two miles of the runway threshold and the ceiling is below 800 feet OR the visibility is less than two miles.
ILS critical area
The ILS Critical Area Boundary Sign has a yellow background with a black inscription with a graphic depicting the ILS pavement holding position marking. This sign is located adjacent to the ILS holding position marking on the pavement and can be seen by pilots leaving the critical area. The sign is intended to provide pilots with another visual cue which they can use as a guide in deciding when they are clear of the ILS critical area.
ILS critical area boundary sign
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