DME Slant Range Correction
DME measures the direct (slant) distance to the beacon; the true ground distance is found by removing the height component.
Formula
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Example
Slant 10 nm, 6076 ft (1 nm) high: d = √(10²−1²) ≈ 9.95 nm
How it works
A DME does not measure the flat distance along the ground; it measures the straight-line distance from the aircraft to the station, which forms the hypotenuse of a right triangle. If you know the aircraft’s height above the station, you can remove that vertical part with the Pythagorean theorem to estimate the horizontal ground distance. This correction matters most when you are close to the beacon or flying at significant altitude, because the slant error becomes more noticeable.
When to use it
Use this when converting a DME readout into an approximate horizontal distance to the station, especially during approach, navigation fixes, or position checks. It is useful whenever the aircraft is not near the same altitude as the beacon.
Worked example, step by step
Suppose the DME shows a slant range of 8.0 nm and the aircraft is 1.5 nm above the station. Use d = √(D_s² − h²) Substitute the values: d = √(8.0² − 1.5²) Calculate the squares: d = √(64.00 − 2.25) Subtract: d = √61.75 Take the square root: d ≈ 7.86 nm Final answer: the ground distance is about 7.9 nm.
Common mistakes to avoid
- Using the slant range as if it were already the ground distance
- Forgetting to use matching units for height and distance
- Subtracting height directly from DME instead of subtracting the squared height term
Did You Know?
Directly overhead a DME station the slant range equals your altitude, so the cockpit reads your height above the beacon rather than zero — the classic “slant-range error”.
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