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ILS Critical Area: When Should You Hold Short?

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When do you need to hold short of the yellow ILS critical area markings on a taxiway?

ILS Critical Area Markings

Hold position markings for ILS critical areas consist of two yellow solid lines spaced two feet apart, connected by pairs of solid lines spaced ten feet apart extending across the width of a taxiway. While the markings look similar to runway hold-short markings, they signify completely different requirements.

A sign with the inscription "ILS" in white on a red background is located adjacent to these hold position markings.

Google Maps

When Do You Need To Hold Short?

Air Traffic Control protects the ILS critical areas when arriving aircraft are inside the outer marker/final approach fix (FAF) on an ILS approach, and the reported ceiling is less than 800 feet or visibility is less than 2 miles.

When instructed by ATC to hold short of the ILS critical area, pilots MUST STOP so that no part of the aircraft extends beyond the holding position marking.

Once the landing aircraft is on the ground, you'll get clearance to cross the ILS critical area and continue your taxi to the runway.


While all of this works well for towered airports, what about non-towered airports?

If you're at a non-towered airport, there's an aircraft on an approach inside the FAF, and the ceiling is less than 800 feet or visibility is less than 2 miles, you should follow the same guidlines. Hold short of the ILS critical area until the landing aircraft is on the ground, then continue your taxi to the runway.

Finally, you should follow the same exact rules at towered airports when the tower isn't in operation.

FAA Warning For Pilots Flying Instrument Approaches

The FAA warns pilots of reports that ILS signal integrity is affected by equipment operating within the ILS critical area. "In several cases, the aircraft autopilot followed ILS fluctuations causing the aircraft to pitch and roll excessively" (FAA).

Whether or not your autopilot is coupled to the ILS, you should be aware of these fluctuations to ensure you continue flying a stable approach. At all times, you should maintain positive control of the airplane. Disconnect the autopilot if ILS signal interference causes excessive roll or pitch.


ILS Critical Area Dimensions

The FAA conducts research using various sized aircraft around ILS antennas to determine how much room is needed before the interference is noticeable. In fact, they sometimes conduct flights using sensor-measuring aircraft to test the integrity of ILS signals.

In the FAA's words, "critical area dimensions have been determined for the different facility antenna configurations and categories of operation by mathematical modeling and empirical validation using various sized aircraft... For example, although a 747 aircraft may routinely taxi near a glide slope, which has been marked for an appropriately large critical area, a grass mower, which has a much smaller profile, can penetrate the outer portions of the large (aircraft) critical area without significant effect."


When was the last time you were instructed to hold short of an ILS critical area? Tell us in the comments below.

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