What are ILS critical area signs?
A pair of signs back to back at the edge of the area that protects the ILS signal. Red says stop short before you enter. Yellow says you are clear after you leave.
An aircraft or vehicle standing inside the ILS critical area can disturb the signal that a landing aircraft is flying. Two signs manage that edge. One tells a pilot where to stop, the other tells a pilot when the area has been left behind, so neither needs to guess from the pavement alone.
1The red sign: stop short
The ILS critical area holding position sign is white on red, like every mandatory instruction sign, and reads ILS. When air traffic control instructs, pilots and vehicle drivers stop short of it, keeping every part of the aircraft or vehicle outside the area.
2The yellow sign: you are clear
The ILS critical area boundary sign is black on yellow and carries a graphic of the ladder-style marking. It faces a pilot leaving the area, and is another visual cue that the aircraft is clear of it, as a runway boundary sign is for the runway.
3One line on the pavement
Both signs stand next to the ILS holding position marking, a ladder-style pair of lines across the taxiway. Sign and marking are read together, so the limit of the area is the same whether it is seen on the ground or on the sign.
| Element | Rule |
|---|---|
| Holding position sign | White inscription ILS on a red background: mandatory instruction |
| Boundary sign | Black inscription on yellow, with a graphic of the ILS holding position marking |
| Marking | Ladder-style ILS holding position marking across the pavement |
| Position | Both signs stand next to the marking, back to back, one facing each direction of travel |
| Holding position meaning | When instructed by ATC, stop short and keep every part of the aircraft or vehicle clear of the area |
| Boundary meaning | A visual cue that the aircraft or vehicle has left the ILS critical area |
| Works with | Runway holding position signs, CAT II/III holding position signs and stop bars |
Frequently Asked Questions
Q1What are ILS critical area signs?
Two signs at the edge of an ILS critical area. The ILS critical area holding position sign is white on red and reads ILS. The ILS critical area boundary sign is black on yellow with a ladder graphic.
Q2What is an ILS critical area?
The area around the localizer and glide slope antennas where an aircraft or vehicle can disturb the ILS signal. When the ILS is in use in low visibility, air traffic control may ask pilots to hold short of it.
Q3What is the difference between the two signs?
The red ILS sign faces aircraft approaching the area and means stop short when instructed. The yellow boundary sign faces aircraft leaving the area and tells them they are clear of it.
Q4What marking goes with these signs?
The ladder-style ILS holding position marking on the pavement. Both signs stand next to it, so the sign and the marking are read together.
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