Aircraft and vehicle access to the ILS critical area must be controlled to ensure ILS integrity whenever the ceiling is less than ________ or visibility less than __________.

• A. 800 ft; 2 SM
• B. 1000 ft; 3 SM
• C. 600 ft; 4 SM
• D. 750 ft; 1.5 SM

Answer: (D)

The main idea here is protecting the ILS critical area to maintain the integrity of the instrument approach signals. When weather tightens to a point where visibility or ceiling could make aircraft or vehicles more likely to approach or interfere with the localizer, glideslope, or associated equipment, access to that area must be controlled. This minimizes the chance of interference or disruption to the ILS signals during approach or departing operations.

The specified thresholds—750 ft the ceiling and 1.5 statute miles of visibility—define the point at which access must be restricted to protect the ILS infrastructure. Those conditions signal enough risk to warrant controlling movements near the critical area, so controllers ensure the signals aren’t compromised by nearby traffic.

Compared with the other options, those set higher weather values that would require protecting the ILS critical area more often, which is more conservative than necessary for standard procedures. The chosen values align with the guidance used in practice for triggering protection at the level where ILS integrity could realistically be affected, balancing safety with normal operations.