DME slant range

DME (distance measuring equipment): UHF navaid, usually paired with a VOR (VOR/DME) or VORTAC. Shows NM to station as slant range, straight-line aircraft-to-station distance, not ground distance (TC AIM COM 4.7).

Slant range geometry:

• Hypotenuse of right triangle; legs: ground distance, altitude above station in NM (feet AGL / 6,076).
• Overhead station at 6,076 ft AGL: reads about 1.0 NM, not 0.0 (PHAK ch.16). Station passage never reads zero.
• Error largest close to station and at high altitude; negligible far out.

Accuracy and readouts:

• Accuracy: ±0.5 NM or 3% of distance, whichever is greater (TC AIM COM 4.7).
• Groundspeed and time-to-station: accurate only when tracking directly to or from the station; allow a minute or two to stabilize after a turn (PHAK ch.16).

Trap: expecting 0.0 NM at station passage, or trusting GS/time-to-station while off a direct track to/from the station.

Practice