The four DME channels not dedicated to RTUs are used for what purpose?

• A. FMS position accuracy
• B. Redundant communications
• C. Timekeeping
• D. Weather data

Answer: (A)

The remaining DME channels are used to feed the Flight Management System with distance measurements so it can determine the aircraft’s position more accurately. DME provides slant-range to ground stations, and with multiple DME ranges from known station locations, the FMS can triangulate (triangulate via trilateration) the aircraft’s position. Having these channels available to the FMS improves navigation accuracy, provides cross-checks, and supports RNAV integrity, especially when GPS or other sources are degraded.

Other options don’t fit because weather data, timekeeping, or redundant communications aren’t supplied by DME channels.