DME Arc Intercept and Tracking

A DME arc is exactly what it sounds like — you fly a constant distance from a DME station along a curved track, so the station stays abeam the whole time. It's most commonly used on the initial approach segment, linking the Initial Approach Fix (IAF) to the Final Approach Course.

It sounds trivial until you try to join one: you're inbound on a radial, the arc is curving away from your nose, and if you turn at the arc DME you'll blow straight through it. Get the intercept right and the rest is a sequence of tiny heading nudges.

The core idea: don't wait until DME reads the arc value. Lead the turn so you roll out tangent to the arc, not cutting across it. The technique below is the standard taught on instrument-rating courses and published in ICAO Doc 8168 (PANS-OPS) Vol I for arc arrivals.

Clockwise vs counter-clockwise

A DME arc is always flown in one direction around the station, dictated by the chart (e.g. "intercept the 12 DME arc from R-090, counter-clockwise to R-180"). The direction tells you which side the station sits on:

• Clockwise around the station — viewed from above, the aircraft orbits the station clockwise. The station stays on the aircraft's right the whole way around.
• Counter-clockwise around the station — aircraft orbits counter-clockwise. The station stays on the aircraft's left.

It also tells you which way to turn at the intercept: a 90° turn off the inbound radial, left for clockwise (so the station ends up on your right), right for counter-clockwise (station on your left).

Intercepting the arc — step by step

Assume the chart tells you to "intercept the 12 DME arc from the 090° radial, counter-clockwise around VOR XYZ". You're inbound on the 090° radial, so your current heading is 270° (reciprocal of 090°).

1. Track inbound on the published radial. Centre the CDI, monitor DME closing from (say) 20 NM toward 12.
2. Identify the lead distance. Simple rule: 1% of groundspeed in NM (180 kt → 1.8 NM; 240 kt → 2.4 NM). Errs on the side of leading early, which is safer. For light aircraft a flat 0.5–1 NM works just as well.
3. Start the turn when DME reads the arc value plus your lead. Turn 90°:
   • Left turn for a clockwise arc.
   • Right turn for a counter-clockwise arc.
4. Roll out on the tangent heading — the inbound radial plus or minus 90°. In this example (inbound on R-090°, counter-clockwise), that's heading 000°. The station is now abeam on your left and stays there for the whole arc.

Maintaining the arc — the 10° rule

Flying a true circle with straight-line flight is impossible, so pilots approximate an arc with a series of short straight segments, each 10° of radial apart.

1. Keep the heading bug 10° ahead of the radial you're crossing, in the direction you're going around the station.

   Start of segment

   Mid-segment

   End of segment → turn cue

2. Watch the bearing pointer. If your HSI shows a pointer to the DME station, it gives you the exact turn cue:

   • Just after a turn — pointer sits 5° ahead of the abeam mark (RB ≈ 85° on a clockwise arc, 275° on counter-clockwise).
   • Mid-segment — pointer is directly abeam (90° / 270°).
   • End of segment — pointer has drifted 5° behind abeam (95° / 265°). Turn 10°. The pointer snaps back to 5° ahead and the cycle repeats.

3. At each 10° radial crossing, turn 10° in the direction of travel.

4. Expect the DME to dip slightly between turns as the straight segment cuts the corner — it should settle back to the arc value before the next turn. Don't chase the dip.

5. If DME is out of tolerance at the next turn cue (typically ±0.5 NM):

   • DME increasing → drifting outside → turn toward the station.
   • DME decreasing → drifting inside → turn away from the station.

6. Bank stays small. There is no standard-rate bank for an arc — use whatever actually holds the DME.

Simple mental rule

"DME increasing, turn towards the station. DME decreasing, turn away."

That's the whole arc-maintenance loop. The 10° bug, the tangent heading, and the lead distance just set how big the turn is, not which way.

Wind correction

Wind pushes you off the arc like any other track, but the relative direction keeps flipping as you go around the station.

• Drifting inside the arc (DME decreasing against expectation) → wind is pushing you toward the station → crab outward.
• Drifting outside (DME increasing) → wind is pushing you away → crab inward.
• Expect 5–10° of crab on most arcs — more than on a straight airway, because the wind component rotates through head/cross/tail as you arc.

Four things pilots get wrong

1. Turning at the arc DME. No lead means you'll blow through by a mile and spend the next two minutes chasing the arc from the outside.
2. Chasing the DME needle. The DME is meant to wander a few tenths between radial crossings. If you chase every twitch you'll end up S-turning around the arc instead of flying it.
3. Under-banking at high speed. On a 6 NM arc at 250 kt you need real bank — 8–12°. Pilots trained on 10 NM arcs at 180 kt often under-bank and drift outside.
4. Forgetting the heading bug update. The whole technique relies on the bug being 10° ahead. Lose track of it and you'll fly straight off the arc on a tangent.

Why it matters

DME arcs appear on DGCA instrument-rating exams and on type-rating sim profiles for any aircraft with a procedure that routes via one. In the real world they're still used on arrivals into mountainous airports and on non-radar IFR procedures where arc geometry keeps aircraft clear of terrain. The technique looks fiddly on paper but it's a one-minute hands-on skill once the 10° rule clicks.