FAA Flight Instructor Airplane (FIA)Navigation and AirspaceHard
A pilot is tracking direct to a VOR/DME station and flying level at 6,076 feet AGL. Just as the aircraft passes directly over the station, the DME reads 1.0 NM instead of 0.0 NM. A CFI explains this reading is best attributed to:
- Aa malfunctioning DME receiver that requires maintenance
- Bslant-range error, since 6,076 feet of altitude equals approximately 1 NM of line-of-sight distance to the station
- Cstation passage causing a brief loss of DME signal lock
- Dthe pilot's failure to identify the correct station on frequency
Show answer & explanationAnswer & explanation
Correct answer: B. slant-range error, since 6,076 feet of altitude equals approximately 1 NM of line-of-sight distance to the station
DME measures slant-range (line-of-sight) distance, not ground distance. Since 6,076 feet equals one nautical mile, an aircraft directly over the station at that altitude will show a DME reading of about 1.0 NM even though ground distance is zero — this is normal slant-range error, not a malfunction.
Why the other options are wrong
- A. This reading is expected behavior, not a malfunction.
- C. DME does not typically lose lock during simple station passage at this range.
- D. Frequency identification errors would cause no signal or an unrelated station's distance, not this specific pattern.
DME Slant-Range Error
DME measures the direct line-of-sight (slant-range) distance to a station, not actual ground distance; this causes a reading discrepancy that is most pronounced directly over or very near the station at altitude.
- 6,076 feet = 1 nautical mile.
- Slant-range error is greatest when close to the station at high altitude.
- Error becomes negligible at greater horizontal distances from the station.
Memory trick: 'DME sees a slanted line, not a straight ground line — closer you are, bigger the lie.'