Under what condition cannot 20 miles be used for separating DME aircraft?

The condition under which 20 miles cannot be used for separating DME aircraft relates to the limitations of the Distance Measuring Equipment (DME) system based on altitude and proximity to the navigational aid (NAVAID). Specifically, 20 miles cannot be utilized for separation above 10,000 feet when aircraft are closer than 10 miles from the NAVAID.

At altitudes above 10,000 feet, the effectiveness of DME for ensuring sufficient separation may be compromised when aircraft are within 10 miles of the DME station. This limitation stems from the performance characteristics of DME, where the accuracy and reliability of distance information can diminish at closer ranges, alongside the factors of terrain and potential signal interference. Thus, adherence to the 20-mile separation requirement in such situations is not feasible, reflecting the need for adequate safety margins.

The other options do not accurately encapsulate the relevant conditions or present situations that do not align with the rules surrounding DME distance separation. The focus on altitude and proximity to the NAVAID is essential in maintaining safe and effective air traffic management involving DME-equipped aircraft.