ATPL Navigation Test 2025 – 400 Free Practice Questions to Pass the Exam

The DME (Distance Measuring Equipment) indicator tends to show the greatest error at high altitudes close to the VORTAC due to the geometry of the signal reception. At high altitudes, the line-of-sight distance to the VORTAC increases significantly, which can lead to a situation known as "slant range error."

When an aircraft is at a high altitude, especially just a few nautical miles away from the VORTAC, the DME calculates distance based on the straight-line (slant) distance between the aircraft and the VORTAC, rather than the actual horizontal distance over the ground. This can result in a significant discrepancy between the indicated distance and the true ground distance to the VORTAC.

In contrast, at lower altitudes, especially when flying close to the VORTAC, the influence of slant range is minimized because the difference in vertical position is less pronounced compared to horizontal distance. Therefore, the DME readings become more accurate at lower altitudes, reducing the error experienced at high altitudes in proximity to the VORTAC.

Overall, the combination of altitude and proximity to the transmitter is a key factor in understanding why the DME indicator may show greater errors in those specific conditions.