How to fly a DME arc

A DME arc is a curved track that keeps you at a constant distance from the station. Instead of flying along a radial, you fly around the station at, say, 10 NM.

You join the arc from one radial, fly it around, and leave it on another. In the real world arcs link a feeder route to an approach without sending you over the station.

You start already established on the entry radial, 3 NM before the arc, so the whole job is the arc itself. Watch the DME run toward the arc distance.

The turn onto the arc is 90 degrees and that needs room, so start it before the number comes up. How much depends on how fast you are going. A rough guide:

• Around 120 kt, typical training speeds: about 0.5 NM early.
• Up to 200 kt: about 1.5 NM.
• Above 200 kt, jet speeds in something like an A320: about 2 NM.

These are rules of thumb, not law. Fly a few and you will find your own numbers.

Flying outbound (away from the station) you turn the same way the arc runs. Flying inbound (toward the station) you turn the other way, because you are pointing at the station instead of away from it. The task card tells you which way.

Roll out with the bearing pointer on your wingtip. That is the picture you keep for the whole arc.

An arc is not flown as one long turn. It is a series of short straight legs that together make a curve. The classic method is twist 10, turn 10:

• Fly straight and let the bearing pointer fall 10 degrees behind your wingtip.
• Turn 10 degrees toward the station side, so the pointer sits back on the wingtip.
• Repeat all the way round.

Check the DME at every step. A little too far out, turn slightly toward the station. A little too close, turn slightly away. Small corrections, often.

Wind pushes you off the arc, and it changes as you go round: on one side it blows you out, on the other it pulls you in. Handle it with the size of your steps rather than with one big correction. On the side that drifts you out, turn a bit more. On the side that pulls you in, turn a bit less.

You do not turn off the arc on the radial you are intercepting. If you waited for it you would sail straight through. You turn a little before it instead, and that cue has a name: the lead radial. It sits about 2 NM back along the arc, so on a tight arc it is a wider angle than on a big one. Charts print it next to the arc as LR and the radial, for example LR-217.

Your task card gives you the lead radial for every exercise. When it comes up, start the turn and roll out on the radial from the clearance, tracking it inbound to the station.

Use SHOW SOLUTION on the map to compare your flown track with the ideal arc. Then hide it and fly the next one on instruments alone.

An arc is a transition. Approaches need to start from somewhere, and a feeder route rarely points straight down the final approach course. Rather than send everyone over the station and back out again, the designer draws a curve at a fixed distance and lets you slide round from wherever you arrived to wherever the approach begins.

That is also why arcs are being drawn less. An RNAV approach can put its initial fix anywhere and join it with a straight leg, so a modern procedure often has no arc where an older one would. They are still on plenty of charts, still in the exam, and still the best instrument exercise there is for reading two things at once.

A 90 degree turn at standard rate has a radius, and at 120 kt that radius is about six tenths of a mile. Start the turn when the DME reads the arc and you roll out six tenths of a mile outside it. So you start early, by about a turn radius, which is where the half mile rule of thumb comes from.

It scales with speed, because the radius does. Roughly: 0.5 NM at 120 kt, 1 NM around 160, 1.5 NM towards 200, 2 NM at jet speeds. Faster than you expected on the way in means an earlier turn, not a tighter one, because the bank angle is capped.

The same radius sets the lead radial for leaving the arc. The chart prints it, but the reason it sits where it does is that turn, and knowing that lets you sanity check a lead radial that looks wrong for your speed.

On a straight leg the wind is one problem with one answer. On an arc it changes the whole way round, because your heading keeps changing while the wind does not. A crosswind on the first radial is a headwind a quarter of the way on, and a tailwind a quarter after that.

Do not try to compute it. The twist 10 turn 10 method already contains the correction: when the wind pushes you out, the DME creeps up and you turn in a little more than ten; when it pushes you in, you turn in a little less. Watch the trend on the DME, not the number, and let the method absorb the wind.

What the wind does change is the pace. Downwind you cover the arc quickly and the radials come round fast, so the next twist is due sooner. Upwind everything slows. Set the OBS for the radial you are ABOUT to cross, not the one you are on, and you stay ahead of it either way.

A conventional arc is flown against a VOR/DME: you read the distance, you read the radial, and you steer the difference. An RNAV arc, called an RF leg on the chart, is flown by the navigator: it draws the curve and the autopilot follows it, and your job is to monitor.

The catch is in the details. An RF leg needs the equipment and the approval to fly it, and a box that is not certified for RF legs will simply not load the procedure, or will load it with the arc missing. Check what your navigator can do before you accept a procedure that has one.

Flying a conventional arc by hand is the skill that makes RNAV arcs safe: you know what the curve should look like, so you notice when the box draws something else.

Chasing the DME. The distance lags, and a correction made on every reading turns the arc into a zigzag. Make a correction, then wait for the next radial and see what it did.

Twisting the OBS to the radial you are on. Then the needle is centred and tells you nothing. Set the NEXT radial, so the needle is off to one side and swings through centre as you cross it. That swing is the cue.

Turning the wrong way onto the arc. Decide the direction round the arc before you reach it, from the chart, and say it. A 90 degree turn the wrong way puts you at 180 degrees to the arc with the station behind you.

Forgetting the lead radial. The arc does not end on the end radial, it ends on the lead radial, and the turn takes you to the end radial. Miss the lead and you fly through the final approach course.