When are groundspeed values given through DME accurate?

• A. On arcs
• B. Always
• C. When tracking directly to or from the station
• D. Never

Answer: (C)

The main idea is that DME-based groundspeed relies on how fast the distance to the station changes, which is the radial component of your velocity. This radial change matches your actual groundspeed only when you are flying directly toward or away from the station. If you’re on an arc or any track that isn’t aligned with the radial line, part of your speed is perpendicular to the line to the station, so the distance changes at a rate not equal to your true groundspeed. Therefore, DME-derived groundspeed is accurate only when tracking directly to or from the station.