What navigation equipment is required for an NDB approach with a step-down fix based on DME?

For an NDB approach with a step-down fix based on DME, the essential navigation equipment required is both an ADF (Automatic Direction Finder) and DME (Distance Measuring Equipment). The ADF is necessary to receive signals from the Non-Directional Beacon (NDB) to determine the aircraft's bearing to the NDB, which is crucial for the initial approach segment. Meanwhile, DME provides the pilot with distance information to the DME station, which is particularly important for determining the altitude at the step-down fix.

In this context, while the ADF alone might enable the aircraft to navigate to the NDB, the inclusion of DME is what allows for the precision needed to safely transition to the step-down fix, thus ensuring a proper vertical profile is maintained during the approach.

This combination allows pilots to accurately track their position both in distance and direction, making it essential to have both instruments on board for such an approach. Relying solely on the ADF without DME would not fulfill the requirements for an approach that involves step-down fixes based on distance.