Definition
The straight-line distance between an aircraft and a ground station, measured directly through the air rather than along the ground. Because the aircraft is above the station, this line forms the hypotenuse of a right triangle whose other sides are the aircraft's altitude and its horizontal distance from the station. DME (Distance Measuring Equipment) displays slant-range distance, not the horizontal distance over the ground.
Plain English
It is the actual line-of-sight distance from the aircraft to the station on the ground. Because the aircraft is up in the air, that line is always a little longer than the distance you would measure on a map.
Context Anchor
Seen when using DME, especially when comparing the DME readout with position on a chart.
Derivation
"Slant" means tilted or sloped — not level. The distance is called slant-range because it is measured along a slanted line from the aircraft down to the station, rather than along the level ground.
Why Pilots Care
At higher altitudes the difference between slant-range and true ground distance grows large enough to affect position awareness, arrival planning, and fuel calculations.
Analogy
Imagine standing at the top of a tall ladder and measuring the distance to a coin on the floor with a piece of string held tight. The string is longer than the distance from the foot of the ladder to the coin, because it slants down. DME measures the string, not the floor distance.
Grounding Statement
Picture the DME signal taking the shortest direct path from the airplane down to the station, like a straight string pulled tight between them.
Intuition Check
Do not assume slant-range distance means distance along the ground. In DME use, it means the direct straight-line distance between the aircraft and the station.
Example Sentence 1
At 6,000 feet directly over the VOR/DME, the DME still showed about 1.0 nautical mile because it was reading slant-range distance.
Example Sentence 2
Because we were at 10,000 feet the slant-range distance read noticeably longer than the actual distance over the ground.