Learn how to fly a DME arc: identify the station, lead the intercept, hold the published distance, correct for wind and exit on course.
To fly a DME arc, tune and identify the charted DME facility, intercept the published distance with a lead turn, then fly roughly tangent to the station while making small heading corrections to hold that distance. Begin the exit turn at the charted lead radial or fix and capture the next course.
In Aviation & Real-World Flying, a DME arc is a constant-distance segment centred on a specified DME transmitter. DME supplies distance, not directional guidance, so you also need VOR radials, a bearing pointer or an approved GPS/FMS position to determine where you are around the arc. See our explanation of how DME measures distance and why slant range matters if the display itself is unfamiliar.
How do you intercept and maintain a DME arc?
The basic method is to enter with a lead turn, establish a tangent heading and then bracket the published DME distance using small corrections.
- Brief the procedure: Find the arc radius, centre facility, direction of travel, minimum altitude, entry route, exit fix and any published lead radial. Confirm whether the distance comes from a VOR/DME, VORTAC, localiser-associated DME or another facility.
- Tune and identify: Select the correct frequency and listen to or verify the identifier. Check which receiver feeds the distance display; DME hold and dual-receiver installations can leave an apparently plausible distance from the wrong station.
- Approach the entry: Fly the published radial or feeder route towards the arc. Maintain a stable speed because groundspeed determines how early the turn must begin.
- Lead the turn: When entering from outside the arc, start before reaching the published radius. Roll towards a tangent heading rather than waiting for the exact DME figure and then making a sharp turn.
- Establish the tangent: In still air, a clockwise arc uses approximately the current radial plus 90 degrees; a counterclockwise arc uses the radial minus 90 degrees. The station should be on the inside of the turn.
- Bracket the distance: If DME increases, turn a few degrees towards the station. If it decreases, turn a few degrees away. As the trend stops, return towards a tangent heading rather than holding the correction until the aircraft crosses the desired distance again.
- Prepare the exit: Select and identify the next navigation source early. At the charted lead radial or termination fix, turn onto an intercept heading and capture the outbound or inbound course without descending below the published altitude.
How much should you lead the DME arc turn?
For a roughly 90-degree entry at normal instrument speeds, a useful starting estimate is half a nautical mile per 100 knots of groundspeed.
lead (NM) ≈ groundspeed (kt) × 0.005
At 120 knots groundspeed, that gives 0.6 NM. For a 12 DME arc approached from outside, begin near 12.6 DME. At 180 knots, the estimate is 0.9 NM.
This is only a rule of thumb. Intercept angle, wind, bank angle and aircraft speed change the required lead, while a published lead radial or database-coded turn takes precedence. In actual IFR operations, fly the charted procedure and approved operating guidance rather than improvising from the formula.
How do you keep the correct distance around the arc?
Use the DME trend as the primary radius check and use radial or bearing information to judge your progress around the station.
| Navigation display | Practical arc technique |
|---|---|
| RMI or bearing pointer | Keep the station generally abeam on the inside wing and correct according to the DME trend. With a crosswind, the pointer may not remain exactly 90 degrees from the nose. |
| VOR CDI or HSI | Select successive radials in small increments. On a clockwise arc the radials increase; on a counterclockwise arc they decrease. As each radial is crossed, adjust the heading around the arc and select the next radial. |
| GPS/FMS | Load the published procedure from the navigation database and verify the named DME source, arc radius and sequence of fixes. Monitor the aircraft rather than assuming the magenta line is correct. |
| DME only | You can approximate the radius once established, but DME alone cannot show your angular position or identify an exit radial. Another bearing or position source is required. |
The distinction between these indications is covered more fully in our comparison of VOR radial guidance and DME distance information.
A common raw-data method is “turn ten, twist ten”: select a radial about 10 degrees ahead, cross it, turn the heading roughly 10 degrees around the arc and select the next radial. Use smaller increments near the exit or whenever the arc radius is tight. Do not chase every tenth of a mile; repeated large corrections produce an S-shaped ground track.
Can the autopilot fly a DME arc?
An autopilot can follow a DME arc only when a compatible GPS or FMS has the procedure correctly coded and NAV mode is tracking that lateral path.
Traditional VOR navigation mode follows the selected radial; it does not understand a constant-distance arc. For a raw-data arc, use heading mode and make incremental heading changes yourself. If using GPS/FMS guidance, confirm the active navigation source, leg sequencing and annunciations; our overview of radio, GPS and FMS source selection explains why the displayed route and the CDI source can disagree.
What mistakes cause a poor DME arc?
Most poor arcs come from source-selection errors, late turns or excessive corrections rather than from the basic geometry.
- Using the wrong DME station: Verify the identifier and receiver source instead of trusting a believable-looking distance.
- Turning at the published radius: Without a lead, momentum carries the aircraft inside the arc.
- Ignoring groundspeed: The same indicated airspeed produces a larger turn radius with a tailwind and a smaller one with a headwind.
- Holding the bearing pointer exactly abeam: That works only without wind. In a crosswind, prioritise the DME trend and the required ground track.
- Chasing the display: Use corrections of roughly 5–10 degrees, then reassess. Large alternating turns make the distance oscillate.
- Exiting late: Waiting for the final course itself to centre may cause an overshoot. Use the published lead radial or fix.
- Correcting for slant range: Fly the charted indicated DME value. Do not subtract altitude or attempt to convert it into horizontal distance.
- Descending early: Being established on the arc does not cancel the charted altitude restrictions.