When can DME be used instead of the OM during an ILS procedure?

DME can be used instead of the OM during an ILS procedure when it is specifically installed alongside the ILS and detailed within the approach procedures. This is crucial because the use of DME as a substitute for the Outer Marker (OM) is contingent upon the equipment being in place and properly functioning. When the approach procedure indicates that DME can replace the OM, pilots can use it to determine their position and distance from the runway, enhancing situational awareness and ensuring compliance with the necessary approach criteria.

In this context, having DME installed serves as a valuable backup for determining distance, especially in scenarios where the OM may not be operational. This substitution can provide additional flexibility and safety during the approach, ensuring that pilots have reliable information to guide their descent and final approach to the runway.