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.
Plain English
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.
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.
Why Pilots Care
Overreads actual distance, most noticeably when close to the station at altitude, affecting position fixes and timing.
Analogy
Like measuring the slanted side of a triangle instead of its flat base on the ground.
Grounding Statement
Imagine standing at the top of a 100-foot tower with a tape measure running down to a person 100 feet away on the ground. The tape doesn't read 100 feet — it reads about 141, because it's measuring the diagonal. DME does the same thing.
Intuition Check
Do not assume the DME number is always the flat map distance. With slant range error, the DME is showing the diagonal distance from the aircraft to the station.
Example Sentence 1
Crossing directly over the VOR/DME at 12,000 feet, the pilot noted the DME read 2.0 nautical miles rather than zero — a textbook example of slant range error.
Example Sentence 2
The pilot accounted for slant range error before using the DME distance to time the approach segment.