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Teaching and Learning Flying - Airplane Instrument Rating

DME Arc


DEFINITION

Flying a DME arc involves intercepting and flying part of a circular track at a constant distance from a VOR/DME or VORTAC station.

SAFETY FACTORS

Accurate flying of DME arcs allows safe obstacle clearance on selected instrument approach procedures.

TOLERANCES

Instrument Rating PTS (FAA-S-8081-4D) V.A. To determine that the applicant
  1. Exhibits adequate knowledge of the elements related to intercepting and tracking navigational systems and DME arcs.
  2. Tunes and correctly identifies the navigation facility.
  3. Sets and correctly orients the course to be intercepted into the course selector or correctly identifies the course on the RMI.
  4. Intercepts the specified course at a predetermined angle, inbound or outbound from a navigational facility.
  5. Maintains the airspeed within +/-10 knots, altitude within +/-100 feet, and selected headings within +/-5°.
  6. Applies proper correction to maintain a course, allowing no more than ¾-scale deflection of the CDI or within +/-10° in case of an RMI.
  7. Determines the aircraft position relative to the navigational facility or from a waypoint in the case of GPS.
  8. Intercepts a DME arc and maintains that arc within +/- 1 nautical mile.
  9. Recognizes navigational receiver or facility failure, and when required reports the failure to ATC.

OBJECTIVES

To encourage mastery of DME arcs to increase pilot proficiency and enhance the safety of flight
To develop the student's knowledge and skill in DME arcs to meet instrument pilot PTS

PROCEDURES

Preflight
Discuss:
  • DME equipment
  • DME arcs
      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
      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
      Flying DME arc using VOR
      • Center the CDI needle (FROM) 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
      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°
      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
  • Intercepting lead radials
      Lead radial is radial at which turn is started to transition from DME arc to inbound course
      Lead may be < 5° at < 150 knots
      With RMI, set course of radial to be intercepted and determine approximate lead
      Intercepting localizer from DME arc is similar
      If depicted lead radial provides 7° of lead, then half-standard-rate turn should be used initially
  • GPS
Inflight
  • Demonstrate
  • Coach student practice
  • Intercepting DME arc
  • Flying DME arc with VOR
  • Lead radial out of DME arc
  • Flying DME arc approach with GPS
Postflight
  • Critique student performance

COMMON ERRORS

  • Improper identification of VOR/DME station
  • Improper interception of DME arc
      Lead turn by 0.5 NM with GS < 150 knots
  • Poor maintainance of DME arc distance
      Easier to fly slightly inside arc
      Wind correction angle constanly changes around arc
      Cross check DME to maintain arc within +/- 1 NM
  • Poor selection of lead radial on leaving arc, e.g., to join final approach segment
      Lead may be < 5 at < 150 knots
      Lead radial of 7°' - use ½-standard rate turn initially
References
    14 CFR parts 61, 91
    AIM
    Instrument Flying Handbook FAA-H-8083-15


Greg Gordon MD, CFII
Updated: