What is a condition under which the outer marker can be substituted during an ILS approach?

The correct answer highlights a crucial aspect of Instrument Landing System (ILS) approaches. The outer marker serves as a key navigational aid during an instrument approach, indicating a specific point along the glide path.

Substitution of the outer marker can occur under conditions that are explicitly depicted on the approach plate or identified by a Notice to Airmen (NOTAM). These official sources provide pilots with the necessary information about any changes to the approach procedure, including the substitution of markers or other guidance points. Using officially published information ensures that pilots have accurate and current data regarding their approach, which is essential for maintaining safety and compliance with aviation regulations.

The other options may not necessarily provide the right conditions for substituting the outer marker. For instance, while radar can enhance situational awareness, it does not inherently validate the substitution of navigation aids like the outer marker. Similar situations apply to the availability of DME or the direction from which the approach is made; these factors, while important, do not govern the official protocol for changing designated markers as outlined in approach procedures. Thus, reliance on the approach plate or NOTAM is essential for ensuring adherence to established aviation standards.