In the US, required components for an ILS include glide slope, localizer, and outer marker. If the outer marker is unavailable, what can replace it?

The correct choice indicates that substitutes depicted on the approach plate can replace the outer marker when it is unavailable during an ILS (Instrument Landing System) approach. The outer marker is traditionally used to indicate the aircraft's position as it approaches the glide slope and localizer. However, if it is out of service or not functional, pilots can refer to the approach plate for alternative guidance.

Approach plates may provide visual references or specify different navigational aids (like NDBs or other markers) that can assist in ensuring safe and accurate approach procedures. These alternatives help maintain the integrity of the approach in the absence of the outer marker, allowing pilots to safely land without relying solely on the outer marker's functionality.

The choice to use only another NAVAID, visual references only, or none of the above does not account for the detailed guidance provided in approach plates, which specifically outline substitutes and procedures to mitigate the loss of the outer marker. This comprehensive approach enhances safety and maintains operational standards during instrument approaches.