Definition
The straight-line distance between an aircraft in flight and a ground-based station, measured along the line of sight rather than along the ground. Because the aircraft is above the station, this distance is always greater than the actual horizontal (ground) distance, with the difference becoming significant at high altitudes or when close to the station.
Plain English
The direct, diagonal distance from your aircraft to a station on the ground — not the distance you would measure on a map. Since you are flying above the station, the straight line from you to it is longer than the distance across the ground.
Context Anchor
Seen in DME error discussions, especially when the aircraft is high and close to the DME station.
Derivation
Slant comes from Middle English meaning to slope or lie at an angle. Range here means distance. Together: the distance measured along a sloping line — which is exactly what the signal travels between an aircraft above and a station below.
Why Pilots Care
DME equipment displays slant range rather than horizontal ground distance, which can lead to position errors especially at high altitudes near the station.
Analogy
If you hold a string from your hand down to a point on the floor, the string is longer than the floor distance to that point. DME measures the string-like diagonal distance.
Grounding Statement
Picture a string stretched from your aircraft straight down at an angle to the station on the ground — that string's length is the slant range.
Intuition Check
Do not assume DME distance always means flat map distance. Slant range distance is the diagonal aircraft-to-station distance, so altitude is part of it.
Example Sentence 1
The DME showed 2.0 NM as we passed nearly overhead the VORTAC at 12,000 feet, a reminder that slant range distance is not the same as ground distance.
Example Sentence 2
Near the VOR station the slant range distance reading exceeded the actual map distance due to the aircraft's altitude.