DME tells you how far you are from a station. Simple concept. But there’s a catch everyone learns in instrument training – slant range error.
What’s Happening
DME measures the actual distance from your aircraft to the station. Not ground distance. The straight line distance.
Picture a right triangle. Your altitude is one side. Ground distance is another. DME shows the hypotenuse – always longer than ground distance.
When It Matters
Close to the station at high altitude – big error. Flying at 10,000 feet directly over a station? DME shows roughly 2 miles even though ground distance is zero.
Far from the station – error becomes negligible. At 60 miles out, whether you’re at 5,000 or 35,000 feet barely changes the reading.
The Math
Pythagorean theorem. Slant range equals square root of (altitude squared plus ground distance squared).
At 6,000 feet AGL and 8,000 feet ground distance: sqrt(6000² + 8000²) = 10,000 feet slant range. That’s 25% error.
Practical Application
On approaches, be aware of it but don’t overthink it. Modern glass cockpits often display both DME and GPS distance.
The rule of thumb: slant range error is significant when altitude is more than half the ground distance. Otherwise, don’t worry about it.
Modern Fixes
GPS solves this entirely. Shows actual ground track distance. Most pilots use GPS primary and DME as backup these days.
