DME and DME arcs

DME measures the sloping distance to a transmitter

DME equipment sends interrogation pulses to a ground transponder and measures the timing of its replies. It converts that delay into nautical miles between the aircraft and the transmitter. The result is slant range, not horizontal map distance. DME supplies distance, while an associated VOR supplies azimuth; the two together can establish position relative to the facility.

The difference is most noticeable high above and close to the station. Directly overhead, DME is approximately the vertical separation in nautical miles rather than zero. At approximately 6,076 feet above the transmitter, that ideal overhead distance is 1 NM. Use height above the transmitter, not altitude above sea level or an assumption that the station is at sea level.

For the ideal triangle, slant range is √(horizontal distance² + vertical distance²). Thus √(3² + 1²) = √10 ≈ 3.16 NM. At larger horizontal distances compared with height, the difference becomes smaller. This explains the indication; it does not authorize changing a published DME fix or subtracting an estimated height from charted distances. Fly the published procedure with the authorized equipment and indications.

GPS distance to a database point is generally horizontal geographic distance. It can therefore differ from raw DME, especially close to the transmitter at height. Differences can also result from selecting different reference locations. A distance to the airport, runway threshold, next waypoint, or along-track terminator is not automatically distance to the charted DME facility.

AIM 1-1-7: DME interrogation and slant range · IFH 9-17–9-19: DME operation and slant-range errors · AIM 1-2-3: approved RNAV position/distance substitution

Verify which facility supplies the miles

Read the procedure’s distance reference and confirm the required facility. VOR/DME, VORTAC and ILS/LOC-DME may provide paired selections, while some installations select DME separately or choose which NAV radio channels it. A HOLD function can keep the previous DME channel when the NAV frequency changes. That is useful when planned, but hazardous if you assume the distance changed with the new course source.

Tune or channel the correct facility, identify the DME signal by the installed method, and confirm valid distance. The DME identifier has a higher audio tone than the associated VOR/localizer and is transmitted less often. A DME identifier approximately every 30 seconds can remain when the associated VOR or localizer is inoperative; distance availability does not prove azimuth guidance is usable. Check each required component.

Some DME units calculate groundspeed or time to station from the rate of range change. Those values are appropriate only when tracking directly toward or away from the station under the equipment’s conditions. On an arc, a nearly constant range can yield very low displayed DME groundspeed despite substantial actual ground movement. Use an appropriate groundspeed source or estimate for turn planning.

DME is also subject to line-of-sight, terrain and service-volume limits. Current AIM classes can differ between collocated VOR and DME. Check current coverage, restrictions and NOTAMs for the actual operation. Treat missing or unreasonable distance as a problem to assess; do not use a frozen or wrong-station distance because the course needle still looks good.

IFH 9-17: channel selection, HOLD, identification and groundspeed · AIM 1-1-7: DME pairing, identifier and distance limits · AIM 1-1-8: current service volumes and component differences

Keep the station on the correct side of the path

A DME arc follows a stated distance from a stated facility. Its course direction continuously changes as you go around the station. In a horizontal plan view, the desired path is approximately a circle; constant raw DME at a constant height corresponds to a slightly smaller horizontal radius because of slant range. For orientation, use the radial to locate yourself around the circle and DME to judge inside or outside it.

In no wind, the tangent direction is 90° to the radial at that point. For clockwise movement, add 90° to the radial and keep the station to the right; for counterclockwise movement, subtract 90° and keep the station to the left. These are tangent track directions. Heading must also account for wind and any correction needed to regain the desired distance.

Do not keep flying one heading around an arc. A tangent straight line gradually departs outward from the circle. Manual arc techniques use a controlled series of small straight segments and turns, while maintaining the distance and position picture. Neither holding the CDI centered on one fixed VOR course nor homing toward the station follows the arc.

IFH 9-17–9-19: arc orientation and tracking

Plan the entry before reaching the arc

Brief the facility, arc distance, entry radial or fix, direction, altitude, exit course and any charted lead radial. Maintain the cleared or published altitude and speed requirements. Establish the incoming path and a mental picture of the tangent direction you need at capture. Determine where the turn should begin using the actual groundspeed, aircraft turn capability, wind and procedure.

Illustrative entry: you approach a 10 DME arc inbound on the 090° radial, course 270°, and intend clockwise movement with the station on your right. The no-wind tangent at the east side is 180°, so the entry is a left turn from westbound toward southbound. Starting the turn only when the decreasing display reaches exactly 10.0 NM would generally carry you inside the arc while the airplane turns. Begin with an appropriate lead, then monitor distance and heading progress through the turn.

The IFH uses an example 0.5 NM entry lead at relatively low general-aviation groundspeeds. That is a teaching starting point, not a universal lead or an aircraft limit. A faster groundspeed, different bank/turn radius or wind calls for a different lead. Your instructor will establish a suitable technique for the actual aircraft and operation. For an outbound entry, the turn starts before the increasing distance reaches the arc, so the lead is subtracted rather than added.

If entry distance is developing incorrectly, adjust the planned rollout using the distance trend and position picture while preserving aircraft control and procedure protection. Avoid an abrupt large turn simply to correct a last decimal place. A good entry leaves you oriented with useful scan capacity for tracking and the approaching exit.

IFH 9-17–9-18: inbound/outbound arc-entry example and groundspeed lead

Control range while updating the tangent direction

Include attitude, altitude, airspeed, heading, DME distance and radial progress in the cross-check. Use small deliberate heading changes to follow the curve and correct distance. When outside the arc, a track component toward the station reduces range; when inside, a component away increases it. As you recover the desired range, reduce the correction and resume a suitable tangent/wind-corrected heading before crossing through to the other side.

Wind correction changes around the arc because the desired track changes. A wind that initially pushes you outward can later become a headwind, tailwind or inward drift. Watch range trend and radial progress; do not carry one fixed correction around the circle. Distinguish a temporary heading to recover range from the heading needed to maintain it.

One traditional manual technique periodically recenters a VOR CDI to read the present radial, then turns in small steps as the radial changes. Another uses a bearing pointer near the wingtip reference. The familiar “turn ten, twist ten” shorthand is an introductory technique, not a requirement to turn exactly 10° at every location or a replacement for DME trend and wind correction. Correct course selection for the exit must be ready when needed.

Example: on an invented clockwise 10 NM arc near the 120° radial, the no-wind tangent is about 210°. DME increases steadily through 10.3 NM. Plan a controlled correction with an inward component, monitor its effect, then ease back toward a suitable tangent as the range recovers. The exact heading depends on wind and turn response. If it decreases through 9.7 NM, an outward component is needed instead. These examples teach direction, not approved control settings.

ACS V.A.S6 specifies maintaining a selected DME arc within ±1 NM. Treat that as a practical-test performance standard, not a planned wandering band or an obstacle-clearance guarantee. This lesson prepares you to explain corrections and recognize trends; demonstrating that accuracy requires instructor-led flight training and evaluation.

IFH 9-18–9-19: arc tracking, changing wind correction and lead radials · ACS V.A, printed page 12

Prepare the next course while keeping the arc

Identify the exit radial/course and the direction of the intercept turn before reaching it. An inbound course is the reciprocal of its radial. Set and orient to that course on the intended display, check TO/FROM and source, and anticipate the turn’s required lead. A published lead radial provides a reference for the procedure; where technique determines lead, it varies with arc radius, groundspeed, wind and the aircraft’s turn.

Continuing the example: a clockwise arc progresses toward the 180° radial and will join course 000° inbound to the station. Near the south side the no-wind arc tangent is 270°. The transition toward northbound needs a right turn, with appropriate lead before crossing the desired inbound line. Do not continue the arc simply because the distance remains perfect; radial progress determines when to leave it.

Complete chart review and major programming before the exit becomes urgent. If using one CDI to identify the current radial and the exit course, manage those selections deliberately; the selected value must not be mistaken for the current radial when the needle is deflected. A properly selected bearing pointer or map can assist orientation but cannot replace checking the approved primary guidance and its validity.

Maintain altitude and airspeed through the intercept. The arc is often part of a larger procedure; a successful lateral turn does not authorize a descent or a different leg. If distance, azimuth or required integrity is lost, maintain control and assess the published procedure, remaining approved capability and ATC assistance. Do not fly the arc by elapsed time alone after losing its required position information.

IFH 9-18: lead radials and exit interception · ACS V.A, printed page 12

Approved RNAV substitution has defined boundaries

Current AIM 1-2-3 allows a suitable approved RNAV system, operated within its installation and operational guidance, to determine position/distance relative to specified conventional facilities and fixes and to fly DME arcs. The allowance can apply even when a procedure notes DME required, or when the relevant DME is unavailable, subject to the actual conditions and restrictions. An unapproved tablet distance is not such a substitute.

Confirm the correct database facility or coded procedure, valid integrity, supported operation and applicable NOTAMs. Do not use distance to a nearby airport or another convenient waypoint in place of the stated DME reference. A conventional procedure identified NA without exception by NOTAM is not restored by drawing its path with GPS. DME/DME/IRU substitution without GPS/WAAS has additional specific-authorization restrictions in the current AIM.

The lateral final-approach aid has separate limits. You may not substitute RNAV for the NAVAID providing final-segment lateral guidance, except as allowed for procedures with “or GPS” in the title. The current AIM does permit suitable RNAV to navigate a conventional VOR/TACAN/NDB final segment when the underlying aid is operational and monitored for course alignment. That is different from replacing an inoperative final-segment aid. Localizer-based lateral courses, including back courses, require reference to raw localizer data under this policy.

These permissions require equipment-specific preparation; “GPS in lieu of DME” is not a blanket authorization to fly every conventional approach or bypass a restriction. For the actual aircraft, verify the approved supplement and current guidance with your instructor before relying on substitution.

AIM 1-2-3: suitable RNAV types, permitted uses, final-segment and NOTAM restrictions · AIM 1-1-17: GPS approval and database requirements

Use range and radial together in the study exercise

Draw a station, a 10 NM clockwise arc from the 090° radial to the 180° radial, an incoming 270° course, and an outgoing 000° inbound course. These are invented geometry targets, not a charted route. Mark the tangent directions at radials 090°, 120° and 180°. Explain why entry lead occurs outside the arc and exit lead occurs before the outgoing course.

1. Place the airplane at 10.4 NM on the arc near radial 120° and state which track component will reduce the range.
2. Place it at 9.6 NM and state which component will increase the range.
3. Add a wind from the north and explain why its effect changes as the arc curves.
4. Assume the NAV frequency is changed but DME remains in HOLD. Identify what must be verified before using its distance.
5. Replace DME with a suitable approved RNAV system. Name the reference, integrity, procedure and restriction checks that still apply.

Work in words and on the diagram before choosing numbers. A study answer demonstrates orientation and planning. It does not show that you can maintain the arc, workload and aircraft performance in IMC.

IFH 9-17–9-19: DME arcs · AIM 1-2-3: approved arc substitution · ACS V.A, printed page 12

FLIGHT SCENARIO

What would change your plan?