Which sign marks the exit boundary of the ILS critical area?

Which sign marks the exit boundary of the ILS critical area?

• A. ILS Critical Area Boundary
• B. Runway/Runway Hold Position
• C. Taxiway Location
• D. Runway Distance Remaining

Answer: (A)

Understanding how taxiway signs protect ILS signals helps explain this. The ILS critical area is a protected zone around the localizer and glideslope to prevent ground movement from distorting the ILS signals. To mark where you exit that protected zone, there’s a dedicated sign that reads ILS Critical Area Boundary. It is placed on the far side of the ILS hold line along the taxiway, so as you taxi past it you know you are leaving the protected area and the ILS signal protection no longer applies. The other signs serve different purposes—one marks where to hold short of a runway, another shows which taxiway you’re on, and another indicates how much runway remains—none of which identify the ILS boundary.

Understanding how taxiway signs protect ILS signals helps explain this. The ILS critical area is a protected zone around the localizer and glideslope to prevent ground movement from distorting the ILS signals. To mark where you exit that protected zone, there’s a dedicated sign that reads ILS Critical Area Boundary. It is placed on the far side of the ILS hold line along the taxiway, so as you taxi past it you know you are leaving the protected area and the ILS signal protection no longer applies. The other signs serve different purposes—one marks where to hold short of a runway, another shows which taxiway you’re on, and another indicates how much runway remains—none of which identify the ILS boundary.