FAA Instrument Flight Instructor (FII)Instruments and SystemsHard
A pilot is flying a DME arc procedure and wants to verify the DME is accurate enough for use. According to AIM guidance, what is the maximum allowable system error tolerance for airborne DME equipment used on such procedures?
- A2% of distance measured with no fixed component
- B1.0 NM fixed, regardless of distance
- C5% of distance measured plus 1.0 NM
- D0.5 NM plus 3% of the distance measured
Show answer & explanationAnswer & explanation
Correct answer: D. 0.5 NM plus 3% of the distance measured
AIM guidance states that DME equipment must be accurate within a tolerance of 0.5 NM plus 3% of the distance from the station. For example, at 10 NM from the station, the allowable error would be 0.5 + (0.03 × 10) = 0.8 NM, which pilots and controllers use to verify system integrity.
Why the other options are wrong
- A. A percentage-only tolerance omits the fixed 0.5 NM baseline component.
- B. A fixed 1.0 NM tolerance ignoring distance does not reflect actual DME accuracy standards.
- C. 5% plus 1.0 NM overstates the actual allowable tolerance.
DME Accuracy Tolerance
The accepted error tolerance for DME (Distance Measuring Equipment) readings, per AIM, is 0.5 NM plus 3% of the distance from the station.
- Formula: tolerance = 0.5 NM + (0.03 × distance)
- At 10 NM, allowable error is 0.8 NM
- Used to validate DME accuracy for arcs and step-down fixes
Memory trick: Half a mile base, plus 3% growing with distance — 'DME grows slack the farther you go.'