We covered something new today (5/21), DME Arcs. Nick spend about 20-30 minutes educating me on the finer points of DME Arcs, and then took me into the Sim for a bit of hands-on practice. What is a DME Arc you ask? According to the FAA, DME Arcs provide transitions to the approach course, but are actually part of the approach segment. Said another way, they are a portion of Instrument Approach Procedure (IAP) that transition the aircraft from the enroute phase of flight to the intermediate or final segments of the approach. DME Arcs are specified as distances from a VOR-DME, and never based on and ILS or LOC DME as they do not provide omnidirectional course information.
Flying a DME Arc is akin to flying a control line model; Think of the airplane as having a string, of a specified length, from the wing to the VOR-DME. You simply fly a segment of a circle, called an arc, a fixed distance from the VOR-DME. Using the Redding (RDD) LOC/DME BC RWY 16 approach as an example (below), from either of the initial approach fixes (IAF), FOLDS or GARSA, you would intercept and fly the 17nm DME Arc to the approach course of 162°. Sounds easy, right?
In concept a DME Arc was easy enough for me to understand, but the actual flying of one, while not overly difficult, seems to require a bit of knowledge and skill. Rather than trying to fly a giant smooth turn, the arc is flown as a sequence of flat segments by using the Turn-10 Twist-10 method of tracking the course. In a nutshell, Nick explained that as I approach 0.5nm of the desired distance I should begin a turn onto the DME arc, flying until the initial heading until the CDI needle is displaced 5-dots (10°) to the side, then I would initiate a turn of 10° in the direction of the arc, and twist the OBS 10°, which if done correctly centered the CDI again. I then fly this new heading until the CDI is again displaced 5-dots, at which time I would repeat the Turn-10 Twist-10 process again and again, until I have intercepted the appropriate radial. For example, suppose you were on a heading of 360° while on the 180-Radial, and you are told by ATC to “Arc East on the 20 DME from LIN, Track the 100 radial inbound”. To get this all straight, I needed to visualize it, so I drew it on my pad as such.
My diagram shows the aircraft more than 20nm South of the station, and flying directly TO the VOR on a heading of 360° with the CDI centered. The arc which I have been instructed to fly arcs towards the East until it intercepts the 100-Radial, which I have been directed to fly inbound. To fly this DME arc I would fly my present heading (360°, CDI centered) until I am 20.5nm on the DME, where I would initiate a 90° right turn to a heading of 090° (East), which if done correctly should result in me being roughly 20nm (DME) from the station at the completion of the turn. I would then fly the 090° heading until the CDI, which was centered at the beginning of the turn, displaces 5-dots to the side, at which time I would initiate a 10° left turn to a heading of 080° and twist the OBS to 350. If I have flown it correctly, at the point where the needle is displaced 5-dots, the DME should also read 20nm and twisting the OBS to 350 should center the CDI. If not, I may have to correct by adjusting my next 10° left turn a few degrees plus or minus to bring me back to the 20nm DME goal.
Continuing with the example, on a heading of 080° I would repeat the process by flying until the CDI again displaces 5-dots, followed by a left 10° turn to a heading of 070° and twisting 340 into the OBS. The Turn-10 Twist-10 process repeats itself until you intercept the radial you wish to intercept with the one exception being that I am told to Twist-20 at the last turn point before the final intercept. By twisting 20° at the last turning point, it should be easier for me to intercept the intended radial by looking for a centered CDI rather than a 5-dot displacement. In this example, as I arrive at the 110 Radial (OBS 300 - TO) with a 5-dot CDI displacement, I would turn 10° left and twist the OBS from 300 to 280. I know, it sounds confusing, and maybe I am not explaining it correctly, but it does work. This is a fairly simplistic explanation of what we covered today, and in the interest of time and space my example assumed a no-wind situation and didn’t go into the corrective actions I should take if I found myself deviating from the intended distance.
With the theory discussion behind us it was time for us to head to the sim so I could get a bit of hands-on exposure. The sim is perfect for this type of stuff as you don’t waste a bunch of avgas getting yourself set-up for a particular situation. Instead, Nick just clicks and drags the little virtual airplane to a spot on the map and I start flying right from there. Just think of all the money it is saving me! 
The first few scenarios went fairly well, but I found that I really needed to sketch the situation (current position, course and intended arc) for me to visualize and plan what I needed to do. Nick would issue me the instruction as if he were ATC, and I would pause the sim as I sketched them on my knee board or the white board on the wall before attempting to fly the maneuver. This helped, but since the real aircraft can’t be paused, I’ve got to be careful not to rely on the “freeze” button too much. The first few scenarios he gave me were all arcs to intercept inbound coursed (like my example above), and once I had exhibited some level of competence he started introducing outbound intercepts. In the hour that I spent with the sim, I probably flew 6 to 8 different DME Arc scenarios, some of which actually looked pretty good when viewed on the map.
My Stage 1 check is in a few days, and while I’m not overly concerned about it, I’ve got some studying to do for the ground portion of the check. Wish me luck!
Thanks for the posting on DME arcs. This is a very helpful explanation.
I don’t know if I got it right, but you are welcome.