Critical Thinking: The ILS Critical Area

Question: You’ve been cleared to “taxi to the runway” when you come up to a red “ILS” sign and double-stripe, yellow taxiway marking — can you taxi further?

Question: You’re taxiing out at a nontowered airport served by an ILS, and come upon the same type of ILS Critical Area hold line and sign. Can you cross it?

Answer: To quote a retired airline captain friend of mine, “it depends.” It depends on the weather conditions, and it depends on who else is flying around the airport at the time.

ILS: A Review
The Instrument Landing System (ILS) is an approach system that provides pilots very precise directional and descent guidance for arrival in poor weather conditions. Two independent transmitters emit signals defining the instrument approach. The LOCALIZER is aligned with the runway centerline, the GLIDESLOPE delineates an obstacle-free descent path. Receivers and instruments in the airplane translate the signals into a visual presentation for the pilot to follow to the runway.

Limits: Most instrument approaches allow pilots to fly the ILS down to 200 feet above ground level (AGL) when visibility is as little as one-half mile at the surface, and (if the runway environment is in sight at that point) still land. Airplanes with advanced equipment flown by specially certified pilots can sometimes fly the ILS to as low as 100 feet (Category II certification), or even all the way to the surface still obscured in cloud — Category IIIa, IIIb or IIIc. Visibility required with advanced certification can be as little as 1200 feet (Cat II), 700 feet (Cat IIIa), 150 feet (Cat IIIc), or a truly “blind” (zero visibility) landing if the plane and the crew are certified to Category IIIc standards.

ILS Critical Areas
Red “ILS” signs and yellow segmented taxiway signs identify the boundary of the ILS Critical Area. It’s vital (“critical,” even) for the airplane to receive localizer and glideslope signals without interruption when flying any category of ILS. Taxiing airplanes and other vehicles on the ground can get between transmitters and approaching airplanes, “blanking out” localizer and glideslope signals. To prevent that, runways equipped with an ILS are often marked with what’s called an “ILS Critical Area.

It Depends — TOWERED Airports
Taxiing aircraft must remain clear of the ILS Critical Area whenever the cloud ceiling is less than 800 feet and/or the visibility is less than 2 miles — until cleared for takeoff or to “taxi into position.

Important: Ground control will not necessarily remind taxiing pilots to remain clear of the ILS Critical Area when weather conditions dictate (we’re expected to already know this stuff).

How it works: If the weather is lower than 800 and/or 2, stop short of the ILS Critical Area sign and pavement markings as if it was the “usual” runway hold short line. Call the tower for clearance onto the runway from the ILS Critical Area hold line, and don’t cross the line until cleared onto the runway.

More ATC Help Air Traffic Control (ATC) isn’t supposed to let anything past the line if an airplane is between the Final Approach Fix and the airport unless the landing pilot has already reported the runway in sight and has begun a side-step maneuver or a circling approach to another runway. Controllers also will protect signal integrity by not assigning holds between the outer marker and the airport below 5000 feet AGL when the weather is below 800 and/or 2 — airplanes in such holds “may cause localizer signal variations for (other) aircraft conducting the ILS approach,” according to the Aeronautical Information Manual (AIM). Again, these rules apply only when the weather is below 800 and/or 2. If the weather’s that bad, don’t cross the ILS Critical Area line until cleared onto the runway.

It Depends — NONTOWERED Airports
Do… If you’re departing under Instrument Flight Rules (IFR) and getting your departure clearance on the ground (with a Clearance Void Time and/or on a Hold for Release), then proceed across the ILS Critical Area line only after you have your “void time” or have been “released” to depart. If you’re cleared to go IFR from a nontowered airport, by definition there are no airplanes inbound on the instrument approach.

Don’t… When departing under Visual Flight Rules (VFR) — even if you intend to “pick up” your instrument clearance in the air — don’t cross the ILS Critical Area line if you have reason to believe an airplane under IFR is inbound on the ILS approach to that runway.

EXCEPTION — The Coupled Approach
Regardless of weather conditions, ATC is supposed to keep ground traffic out of the ILS Critical Area any time an aircrew reports it is inside the Final Approach Fix while flying a coupled approach.

Translation: The pilot-in-command is using the airplane’s autopilot to follow the ILS. The localizer and glideslope signals must be protected if an airplane is using automation to fly the approach.

Why: Autopilots can’t anticipate localizer and glideslope needle movements (they only respond to deviations that have already occurred), and may react adversely to momentary deflections that result from ground traffic blocking a signal. Consequently, the AIM recommends flight crews advise the control tower (or presumably, the Common Traffic Advisory Frequency [CTAF] at nontowered airports) any time they will conduct a coupled or “autoland” approach when the weather is better than 800 and/or 2. This prompts ground controllers to advise taxiing pilots to remain behind the ILS Critical Area sign and line (in this case, we’re not expected know) to protect the signal for the inbound airplane.


  • Anytime the ceiling is below 800 feet and/or the visibility is less than 2 miles, do not pass the ILS Critical Area line until cleared for takeoff or “into position and hold” at a towered airport.
  • In the same weather conditions at nontowered airports, do not pass the ILS Critical Area markings if you have reason to believe an airplane is inbound on the ILS approach.
  • If the weather is 800 and 2 or better, ignore the ILS Critical Area markings and taxi to the “usual” hold line unless directed otherwise by Ground Control, or at a nontowered airport, an inbound airplane reports flying a coupled ILS approach.
  • If you’re flying a coupled or autoland approach in conditions of 800 and 2 or better, tell the tower or report a coupled approach on CTAF to protect the ILS signal.
  • Be watchful for unusual needle movements or localizer/glideslope “flagging” any time you’re flying an ILS approach, and be ready to make manual corrections if something enters the ILS Critical Area and interrupts the signal.

BOTTOM LINE: When reviewing (or teaching) techniques to avoid runway incursions, taxiing out during low weather conditions, or flying an ILS approach, don’t forget the ILS Critical Area.