Among all IFR procedures that exist, the DME ARC is probably the most intimidating one. However it is not as difficult as you could imagine. This short article explains how one flies a DME ARC.
For the seek of clarity, we will assume that we are flying towards the ABC VOR-DME on the 180° heading, with no wind. We want to enter the ARC DME which is an ABC 11.5 ARC, turning counter clockwise with the beginning of the ARC being on heading 270°. This means that we will have to execute a 90° right turn to start the first portion of the DME ARC.
The first question is when should you start to turn so that the turn ends on the first portion of the DME ARC at exactly 11.5 NM?
Obviously, if you wait until you read 11.5 NM on your DME indicator to start your turn, knowing that under IFR all the turns are performed at the standard rate of 3° per second, the 90° turn will take 30 seconds. During that time, the plane will fly closer to the ABC VOR-DME and you will be on the first portion of the DME ARC closer to the NavAid that you should normally be.
Here is a quick tip to calculate the anticipation distance when you need to execute a 90° entry turn to start the DME ARC procedure.
If you fly a Cessna 172 or a similar performance aircraft, the anticipation distance is calculated this way:
Ground speed (KNOTS) / 200= Anticipation distance (NM)
Let's assume that your current ground speed is 109 KNOTS, it gives 109/200=0.55 NM.
Since the DME ARC is at 11.5 NM from ABC VOR-DME, then add 0.55 NM to 11.5 which gives 12.05 NM that we can round to 12.1 NM. Therefore, start your right turn when the DME indicator shows 12.1 NM.
So now you are about to start your right turn taking into account the additional distance to anticipate the turn as calculated above. It is time to change your OBS setting to reflect the radials you want to track during the DME ARC procedure . So twist the OBS to display 360°. Your instruments look like the picture below:
Now to start the DME ARC procedure, twist your OBS by 10°. Therefore the radial that you want to display is R-350. At the same time, take a look at your VOR gauge:
The heading shown on your VOR gauge at 9 o'clock (because the VOR-DME is now on your left) is the heading you need to fly which is now 260° as shown below. So make a right turn to 260° heading. Because you are not on the R-260 yet, the needle on your VOR gauge will be fully deflected to the right:
Keep heading 260° until the needle is centered again as shown below.
At this point, follow the "Twist 10° and Turn 10°" method: twist your OBS by 10° less to track the R-340 and simultaneously make a 10° left turn to fly on heading 250°. Don't forget to regularly check your DME indicator to ensure you are still at exactly 11.5 NM from the ABC VOR-DME. Since your course should always be fully perpendicular to the VOR station during the DME ARC procedure, the relative speed to the VOR station should remain at 0 KNOTS. It is another way to ensure that you are flying the DME ARC correctly.
Fly on heading 250° until the needle of the VOR is back to the center position and your instruments should look like this:
Again, Twist 10° and Turn 10°: set your OBS to track the R-330° and turn 10° left on heading 240° (once again, the needle of the VOR gauge will go fully right):
Keep the 240° heading until the needle on the VOR gauge gets back to the center:
And............Twist 10° and Turn 10°............That is easy, isn't it?!
Note that you will not fly an exact rounded flight path, but it is more like a series of short successive straight lines as shown on the images above (the orange lines).
With a bit of training, I was able to fly an almost perfect DME ARC procedure (slight wind coming from the North East in this case):
Obviously, we are not in a perfect world and therefore, most of the time the wind keeps you busy and requires you to constantly correct your heading to counter act any cross-wind component effect. In that situation, your DME indicator is your best friend:
If it shows 11.7 NM for instance meaning that you are currently drifting away from the VOR because you have a left cross-wind component, add a bit more than 10° during your turn to get back to the 11.5 DME ARC.
In the opposite situation, your DME indicator is showing 11.2 NM because the plane has a right cross-wind component that pushes the plane closer to the VOR-DME: Your turn should be less than 10°.
Play with the heading, and pay a lot of attention to your DME indicator to remain on the 11.5 DME ARC until the end of the procedure.
Remember: Twist 10° and Turn 10°... this is magic!
Remark:
If your plane is equipped with a RMI (Radio Magnetic Indicator), that is really easy to execute an ARC DME. SImply keep the needle at 90° left or right angle on your instrument (taking into account the wind correction angle if any) and that's it :)
The limits to fly a DME ARC is +/- 2 NM that ensure a 1,000 feet clearance above the highest obstacles in the area.
An example in this video here is shown. I flew the Martin State KMTN VOR-DME Runway 15 in IMC that has one DME ARC in the approach and another one in the missed approach procedure (plane used was a Cessna Centurion CT210M).
TIPAH