FAA Instrument Rating Airplane (IRA)Navigation SystemsHard
An aircraft is flying at 8,000 feet MSL, 15 NM from a VOR/DME station. The DME display shows 15.2 NM. What is the approximate slant-range error in this reading?
- A0.2 NM
- B1.0 NM
- C0.8 NM
- D0.5 NM
Show answer & explanationAnswer & explanation
Correct answer: A. 0.2 NM
DME measures slant-range distance. Slant-range error is most significant when close to the station and at high altitudes. The difference between the displayed DME (slant range) and the horizontal distance is the slant range error. Here, the displayed 15.2 NM is the slant range. The horizontal distance is 15 NM. So, the error is 15.2 NM - 15 NM = 0.2 NM. This is a direct measure of the error, not a calculation of the theoretical slant range.
Why the other options are wrong
- B. This value is too high for the given scenario.
- C. This value is too high for the given scenario.
- D. This value is too high for the given scenario.
DME Slant-Range Error
The difference between the actual horizontal distance to a station and the DME displayed (slant range) distance.
- Most pronounced when close to the station and/or at high altitudes.
- Decreases as distance from the station increases.
- Becomes negligible (less than 1 NM) at distances greater than 1 NM for every 1,000 feet of altitude above the station.
Memory trick: Slant is always longer than horizontal.