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CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

    2. Flight instrument systems and their operating characteristics

      b. Distance measuring equipment (DME)
        1)Description
          DME a) Aircraft DME transmits interrogating radio frequency (RF) pulses that trigger ground receiver equipment to transmit reply pulses back to the aircraft DME
          b) Aircraft DME measures time between sending RF pulse to receiving reply and converts it to distance in nautical miles (NM) from the ground station
          c) Some receivers provide groundspeed in knots, accurate only when tracking directly to or from the station
        2)DME components
          a) Ground station may be collocated with other navaids to provide course and distance information as VOR/DME, VORTAC, ILS/DME and LOC/DME
            i) Frequency pairing plan provides for automatic selection of DME frequency when this collocated navaid is selected
            ii) DME frequencies are UHF between 962 MHz and 1213 MHz
            iii) Some aircraft DME must be separately tuned
          b) Airborne equipment
            i) Antenna and receiver
            ii) Pilot controlled receiver features
            • Channel frequency selector
            • On/Off/Volume switch
            • Mode switch (distance, groundspeed or time to station may be selected)
            • Hold function - holds current DME channel (useful for ILS approach when DME is nearby but not collocated)
            • Altitude - some DME correct for slant-range error
        3)Operational use
          a) DME allows accurate determination of distance of aircraft from ground DME station
          b) Flying DME arcs
            i) Flying track that is constant distance from station
            ii) General method
            • Track inbound (or outbound) on a radial to intercept the DME arc
            • Lead turn by 0.5 NM with GS < 150 knots
            • Turn approximately 90°
            • Monitor DME to time rollout to intercept arc
            iii) DME arc with wind
            • Wind-drift correction constantly changes throughout the arc
            • Important to maintain situational awareness
            • Maintaining arc is simplified by keeping slightly inside the curve
            iv) Flying DME arc using VOR
            • Center the CDI needle upon completion of 90° arc-interception turn, then
            • Note aircraft heading at left or right side of VOR
            • Adjust aircraft heading to compensate for wind or to correct for distance
            • Re-center CDI and note new primary heading indicated whenever the CDI has moved 2-4° from center
            v) Flying DME arc using VOR and 10-10 method
            • Set OBS to radial 10° ahead (FROM)
            • When CDI centers, reset OBS to radial 10° ahead
            • Check DME (at each CDI center/OBS reset)
              • On arc => turn 10°
              • Inside => maintain heading
              • Outsite arc => turn 2 x 10 = 20°
            vi) Flying DME arc using RMI
            • If no wind, can theoretically fly precise arc by maintaining relative bearing of 90° or 270°
            • In actual practice, fly a series of short legs
            • With RMI bearing pointer on wingtip reference (90° or 270° position), maintain heading and allow bearing pointer to move 5° to 10° behind wingtip
            • Turn toward station until pointer points 5° to 10° ahead of wingtip
            • Continue these last two steps to maintain arc
            • If crosswind drifts aircraft away, turn in until bearing pointer points ahead of wingtip
            • If crosswind drifts aircraft closer, turn out until bearing pointer points behind wingtip
            • Change relative bearing 10° to 20° for each 0.5 NM deviation from arc
          c) Intercepting lead radials
            i) Lead radial is radial at which turn is started to transition from DME arc to inbound course
            ii) Lead may be < 5° at < 150 knots
            iii) With RMI, set course of radial to be intercepted and determine approximate lead
            iv) Intercepting localizer from DME arc is similar
            v) If depicted lead radial provides 7° of lead, then half-standard-rate turn should be used initially
          d) GPS substitution for DME (or ADF)
        4)Operational DME errors
          a) DME/DME fixes (from two DME stations) are more accurate than VOR and DME
          b) DME signals are line-of-sight, so DME distance is slant-range distance
            i) Slant-range error is greatest when aircraft is directly over ground station when DME will then display altitude in NM
            ii) Slant-range error is negligible if aircraft is > 1 mile form station for each 1,000 feet of altitude above station

References
    Instrument Flying Handbook, FAA-H-8083-15, 1999
    AIM 1-1-7

Greg Gordon MD, CFII
Updated: