When is the ILS critical area hold short requirement in effect?

The ILS (Instrument Landing System) critical area hold short requirement is specifically in effect when visibility is less than 800 meters or 2 miles (800-2). This requirement is crucial for maintaining the integrity and effectiveness of the ILS, as the system relies on precise signals and measurements to guide approaching aircraft. Under reduced visibility conditions, the risk of interference or disturbances that could affect these signals increases significantly.

Establishing hold short requirements in such situations helps prevent other aircraft from entering the critical area, thus ensuring that the ILS operates without any obstruction and maintains not only safety but also efficiency in the landing process. This is especially important when instrument approaches are being used, as pilots depend heavily on these systems for safe landings in poor weather conditions.

The other options do not influence the activation of the hold short requirement for the ILS critical area; normal weather conditions and heavy traffic periods do not inherently require these restrictions unless visibility also falls below the specified threshold. A closed runway situation would generally prevent any landing operations altogether, making the ILS critical area hold short requirement irrelevant in that context.