Plain English Meaning

Your DME shows the diagonal distance from your aircraft to the station on the ground, not the flat distance across the earth. The higher you are and the closer you get, the more the diagonal stretches the number — so the reading is always a little too big, and directly overhead it shows your altitude rather than zero.

Full Definition

The error in DME (Distance Measuring Equipment) readings caused by the fact that DME measures the straight-line distance from the aircraft to the ground station, not the horizontal distance across the ground. Because the aircraft is above the station, the measured distance is always longer than the actual ground distance, and the error grows larger as the aircraft flies higher or closer to the station.

Context Anchor

Seen in instrument flying when using DME distances near a station, especially during approaches, holding, or position checks close to the DME antenna.

Derivation

Slant means leaning or tilted. A slant range is the distance measured along that tilted line from aircraft to station, rather than the flat distance across the ground. The word is used because the line of measurement slopes downward from the aircraft to the station below.