How to fly a DME arc
A DME arc is a curved track that keeps you at a constant distance from the station, so instead of flying along a radial you fly around the station. This guide covers joining the arc from a radial, the twist 10 and turn 10 method that keeps you on it, what the wind does as you go round, and how to leave the arc on the radial your clearance ends with.
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WHAT IS 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.
JOINING THE ARC
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.
STAYING ON THE 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 AND LEAVING THE ARC
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.