Definition
Slant range is the straight-line distance from the aircraft directly to a ground-based navigation station, measured along the line of sight rather than along the ground. Because this line is the hypotenuse of a triangle formed by the aircraft's altitude and its horizontal distance from the station, slant range is always greater than the actual ground distance, with the error increasing as the aircraft flies higher or closer to the station.
Plain English
It's the direct, diagonal distance from your aircraft to a station on the ground — not the distance across the ground beneath you. The higher you are or the closer you get, the more the diagonal stretches beyond the true ground distance.
Context Anchor
Seen in instrument flying when using distance measuring equipment, especially in discussions of why the indicated distance can be slightly wrong near a station.
Derivation
‘Slant’ comes from an old Scandinavian word meaning to lean or tilt, and ‘range’ means distance. Together they describe a distance measured along a tilted line — exactly what a DME signal travels from the aircraft down to the station.
Why Pilots Care
DME equipment reports slant range, which can overstate the actual ground distance when the aircraft is high and near the station, affecting navigation accuracy and position fixes.
Analogy
Like shining a flashlight from a tall building straight to a spot on the ground instead of measuring the shorter distance along the street.
Grounding Statement
If the airplane is nearly above the station, the indicated distance is mostly the airplane’s height above the station, not distance across the ground.
Intuition Check
Do not read slant range as map distance along the ground. It means the direct diagonal distance through the air from the aircraft to the station.
Example Sentence 1
At 12,000 feet directly above the VORTAC, the DME indicated 2 NM — a reminder that the unit reads slant range, not ground distance.
Example Sentence 2
Accounting for slant range error prevented the crew from misjudging their distance to the VOR during the approach.