Definition
The discrepancy between the distance shown by DME (Distance Measuring Equipment) and the actual horizontal distance to the station, caused by DME measuring the straight-line distance from the aircraft to the ground station rather than the distance along the ground. The error is greatest when the aircraft is high and close to the station, and smallest when the aircraft is far from the station relative to its altitude.
Plain English
DME tells you how far you are from a ground station in a straight line, but what you usually want to know is how far you are along the ground. When you are high up and near the station, those two distances are noticeably different, and that difference is the slant-range error.
Context Anchor
Seen when using DME distance close to a station, especially when flying at higher altitudes.
Derivation
‘Slant’ comes from the Middle English ‘slenten,’ meaning to slope or angle. ‘Range’ here means distance. Together, ‘slant range’ describes a distance measured along a sloped line — from the aircraft down to the station — rather than along the ground. Once you picture the slope, the error becomes obvious.
Why Pilots Care
Overestimates distance near the station or at higher altitudes, which can affect timing, position awareness, and instrument approach planning.
Analogy
Imagine standing at the top of a tall ladder directly above a friend on the ground. The ladder shows you are 20 feet from your friend, but the ground distance between you is zero. DME works the same way — it measures along the ladder, not across the ground.
Grounding Statement
At about 6,000 feet directly above a DME station, the DME can show about 1 nautical mile even though the aircraft’s horizontal distance from the station is zero.
Intuition Check
Do not assume a DME distance is always map distance. DME measures a straight diagonal line to the station, so altitude can make the displayed distance larger than the ground distance.
Example Sentence 1
Passing nearly overhead the VOR/DME at 6,000 feet, the pilot noted that the DME still read about one mile due to slant-range error.
Example Sentence 2
As the aircraft descended toward the DME station, the slant-range error decreased and the indicated distance approached the true ground distance.