How to fly a DME arc

What is a DME arc, and why do procedures use one?

DME (distance measuring equipment) gives a continuous readout of your distance from a ground beacon, usually paired with a VOR (VHF omnidirectional range) or an ILS (instrument landing system). An arc is simply a circle round that beacon at a published range: “the 12 DME arc”. Hold the reading steady and you are on it.

Procedure designers use arcs to bring you from an airway or an initial approach fix round onto the final approach course without a trip overhead the beacon and a procedure turn. The arc keeps you a known distance from terrain and other traffic, and it feeds you onto final at a manageable angle. You meet them mostly in approach transitions, and now and then in departures.

DME basics: nautical miles and slant range

The DME reads in nautical miles, measured as slant range: the straight line from the aircraft to the beacon, not the distance along the ground. High and close, the two differ a lot — directly over a beacon at 3,000 ft the DME still shows about half a mile. On an arc you are several miles out, so the difference is negligible and you can treat the reading as your distance.

Most DME units also show groundspeed and time to station, worked out from how fast the range is changing. That is only your real groundspeed when you are flying straight to or from the beacon. On an arc the range hardly changes, so the groundspeed readout sinks towards zero. It is not faulty; it is telling you the arc is being flown well.

Joining the arc: turn 90° and lead the turn

The arc is at right angles to every radial it crosses, so joining from a radial means a 90° turn, leaving the station off one wingtip. If you start the turn at the arc’s range you roll out outside it, because the turn itself takes room.

The room it takes is one turn radius. For a rate-one turn (3° a second) the radius in nautical miles is roughly 0.5% of your groundspeed: about half a mile at 100 kt, 0.6 NM at 120 kt. So flying outbound to an 8 NM arc at 100 kt, start the turn at 7.5 DME; flying inbound to it, at 8.5. The FAA’s handbook gives a similar figure: half a mile is enough up to about 150 kt, more beyond that. Where the chart gives its own turn point, fly the chart.

Flying the arc ten degrees at a time

A circle is hard to fly by hand; a series of short straight lines is easy. With the station exactly on the wingtip you are heading along the arc at that point. Fly straight and the station slowly moves behind the wing while the range starts to creep up. Turn 10° towards the station and you are back on a chord of the circle. On an 8 NM arc each 10° is about 1.4 NM of flying, so at 100 kt a turn comes round every 50 seconds or so.

With a bearing pointer (glass cockpit or RMI)

On a glass PFD (primary flight display) the bearing pointer on the HSI (horizontal situation indicator) points at the station; on older panels an RMI (radio magnetic indicator) needle does the same. Its tail reads the radial you are on.

1. Roll out on the arc with the pointer lying along the wing, on the wingtip mark.
2. Hold the heading. The pointer slips behind the wingtip.
3. When it is 5° behind, turn 10° towards the station. The pointer is now 5° ahead of the wingtip.
4. Hold the new heading until it is 5° behind again, and repeat.
5. Cross-check the DME after each leg: range going up, turn a few degrees more towards the station; going down, a few less.

Turning at 5° behind and putting it 5° ahead keeps each leg centred on the arc. Letting it go 10° behind and turning 10° only brings it back to the wingtip, so every leg ends a little further out.

On round dials: turn ten, twist ten

A basic panel may have a VOR indicator or HSI with no bearing pointer. The CDI (course deviation indicator) then becomes a marker for how far round the arc you are. Set the course 5° further round the arc than the radial you are on, with a FROM flag, and fly the heading. When the needle centres you are on that radial: turn 10° towards the station and twist the course selector 10° further round. Repeat all the way to the end.

Try it: Fly the DME arc lesson on the glass panel: outbound on radial 200, then the 8 NM arc round to radial 110

The lesson shows the method first with the animation above, then coaches you leg by leg in a 15 kt wind. It is scored on range within ±0.5 NM on the arc, the needle within half scale on the radials and altitude within ±100 ft. The same lesson on the round dials teaches turn ten, twist ten.

Correcting for wind on the arc

Going round, the wind blows you across the arc on some legs and along it on others, so the right heading keeps changing. You do not calculate it; the DME tells you. A range that keeps growing means you are being pushed outside: turn towards the station, by 10° or even 20° if you are well out. A range that keeps shrinking means you are drifting inside: do not turn away, just hold the heading for longer, or make the next turn only 5°, and the straight leg carries you back out to the arc. Then let it settle before correcting again.

Once you are holding a wind correction, the pointer no longer sits on the wingtip. With the wind blowing you outwards you are heading slightly towards the station, so it rides ahead of the wing; with the wind blowing you inwards it rides behind. That is fine: keep flying 10° legs from wherever it settles, and keep the DME steady. The idea of holding a heading off the track is the same one explained in the wind correction angle guide.

Leaving the arc: the lead radial

The turn off the arc onto the inbound course is another 90° turn, so it too starts early. The radial where you start it is the lead radial. Many charts publish one, designed to give at least 2 NM of lead: more than a light aircraft needs, so at training speeds turn there onto an intercept heading rather than straight onto the course. To work out your own, convert your turn radius into degrees of arc. One nautical mile of arc is 60 ÷ range degrees, so at 8 NM half a mile is only about 4°. The lesson uses 5°, which leaves a little room to roll out.

Have the inbound course ready before you get there: on the glass panel it is set on the last leg, while on the round dials the CDI is on the lead radial and the final course goes in as you turn. At the lead radial, turn towards the station onto the inbound course, let the needle come to the centre and lay off the drift as on any radial. The radial tracking itself is covered in the VOR navigation guide.

Real arcs are not always built on a VOR. At Exeter the runway 08 arc is flown at 12 miles on the localiser’s own DME, which gives range but no radials, so the bearing to the aerodrome’s locator beacon on the ADF (automatic direction finder) tells you how far round you are. The chart gives a point at 14 DME to turn onto the arc, a bearing after which you may descend, and a lead bearing for the turn onto the localiser. A localiser is narrow, so the early turn matters even more than on a VOR radial; the ILS guide picks up from there.

Try it: Fly the ILS joined from a DME arc: Exeter runway 08 from the south-west

Common mistakes on a DME arc

• Turning onto the arc at its exact range and rolling out half a mile outside it, then spending the first few miles cutting back in.
• Chasing every tenth of a mile. Make a correction of a few degrees, hold it, and give the DME time to show the result.
• Turning the wrong way. The station is always off the inside wing, and every routine turn on the arc is towards it.
• Losing track of where the arc ends and flying through the lead radial. Keep reading the radial (or bearing) as you go, and have the inbound course set before the last leg.
• Letting the altitude wander while you are busy with headings and the course selector. Trim, and check the altimeter on every leg.