DME Arcs
Courtesy of Andrew Ayers.

The DME arc is a procedure used to transition from the enroute environment to an instrument approach. It's not a particulary difficult procedure, but it's difficult to explain. As the name implies, we'll need DME, so if you're aircraft is not equipped with DME, this won't apply to you. An arc is simply a segment of a circle and you'll see on the chart that the plotted arc is exactly that, a segment of an imaginary circle who's radius is defined by a DME distance from the VOR. Arcs can come in all sizes and don't necessarily have to be based on the facility which is the navaid for the approach. The ILS to Fort Pierce, Fl also has a DME arc based on the Vero Beach VOR. In demonstrating this procedure, we'll be using a new piece of equipment, the Radio Magnetic Indicator (RMI). The interpretation of the RMI is simple, it's needle always points to the station. Most RMIs have two needles, one ADF and one VOR so be sure you are using the VOR needle. Because of the initial complexity in learning this manuever, the FAA recommends that we use an RMI unless we are highly proficient with the use of the HSI/OBS. So, today we'll use both and you'll see first, how much easier it really is with an RMI and second, that it's not all that hard without it. Don't be intimidated by the arc. It seems like an odd and abstract procedure, but it can be useful in certain situations.

We've seen this chart before, so I'll only brief the parts pertinent to executing the arc. First, make sure you've got the DME set to receive the VOR, not a GPS point if you're using a GPS-equipped aircraft. At "A" and "B", we see two IAFs for the two arcs. "A" would be the IAF for the arc south, since we would be flying south to intercept course on the arc. This point lies at 7DME along the 345 radial, which also defines V3 between VRB and MLB.

So, if we were coming southbound along V3, we could go directly from the airway onto the arc to begin the approach. "B" is the IAF for the arc north, lying at 7DME along the 147 radial, which defines V295 between VRB and STOOP intersection. Today we'll be coming north (from West Palm Beach or someplace down south) along V295 and we'll fly the arc from there.

"C" tells us that this is a 7DME arc, which means that all along the arc, we'll be maintaining 7DME from the VOR. "C" also tells us that we'll be proceeding along the arc at 1500ft MSL. Next to that, you'll see "NoPT", which means "No Procedure Turn". Since we'll be using the arc to intercept the course inbound, there's no need for us to make a course reversal. Once we intercept the course, we'll just proceed straight in. "D" shows us our approximate turning point to intercept our course. You see that both arcs "funnel" into the approach course.

Somewhere along the arc, we'll have to stop flying the arc and start intercepting our course. Some charts actually depict "lead" radials which tell you when you should begin your turn. In the abscence of them, we will use 5 degrees prior to course, so, in this case, when we're crossing the 295 radial FROM VRB, we'll start a turn to intercept the 300 radial inbound (120 TO). OK, so I've gone backwards here a little bit. I've told you how to get off the arc, but how do we get on and stay on? Well, let's go flying and find out...

The weather today will be 500ft overcast, visibility 4 miles. You'll do best to practice arcs with no wind first. Well, we'll be proceeding directly to the VOR as we fly northbound. We would have already gotten the ATIS and contacted Miami Center with our request to make the VOR 11R approach, arc north transition. Center will then tell us to "descend and maintain 1500ft, report established on the arc." In order to establish ourselves at 7DME from the VOR and not get any closer, we'd have to be flying exactly parallel to it at that moment. In other words, we'll have to turn 90 degrees from our course. If you look at "B", you'll see that the arc starts exactly 90 degrees from the course to the VOR. We can't turn on a dime though, so we'll have to lead our turn. The suggested distance is to begin our 0.5DME before the arc at speeds below 150kts, so, since we're intercepting from the outside, we should begin our turn at 7.5DME from the VOR.

Figure 1:
Approaching the IAF for the arc north transition. Heading 327 and 7.5 DME. Start the turn.

This is position "1" marked on the chart above and the map-view below.


So, when we reach about 7.5DME, we'll begin a standard rate turn to the left 90 degrees and start our descent to 1500ft. Since we're flying inbound to the VOR on a heading of 327 degrees, we're going to turn to a heading of 237 (327-90) degrees. At this point, since we're now 90 degrees to the VOR, the RMI needle should be just about directly off our right wing and we should be just about 7DME from the VOR. Right there, you can see that the RMI gives us an instant picture of where we are in relation to the VOR and the arc. You get 0.5DME on either side of the arc before ATC will start warning you. OK, so what do we do with the HSI/OBS? Well, first, let's think about what we're trying to do. This is the meat of the manuever and it gets a little tricky here. We want to fly a constant radius around the VOR. We could do that by flying a continuous shallow bank all the way around, but holding in a small bank angle while trying to maintain 7DME would take a lot of attention, attention we'll need for other things. Instead, what we're going to do is fly small straight-line segments around the arc of about 20 degrees each. In this case, we're going to be flying across increasingly greater radials until we reach our turn radial of 295 degrees. For our first segment though, we'll only fly a 10 degree straight segment so we can see if we're properly established on the arc quickly. Since we just turned off the 147 radial, we'll fly a straight line on our heading of 237 until we cross the 157 radial. So, what we need to do is turn the HSI needle/OBS card until we've got the head set on 157. As we fly along, the needle will start to center as we begin to cross the 157 radial.

Figure 2:
Established on the arc, initial heading 237 and VOR at 3 o'clock. OBS set to 337 (157 radial)

This is position "2" on the chart and the map-view.


Now we're going to turn our heading 20 degrees north along the arc to a heading of 257 (100 degrees past the radial we just crossed) degrees and twist the needle 20 degrees to the 177 radial. Now we're established on the arc and it's time to report to Center. Miami would come back saying "9246F cleared VOR 11R approach. Contact tower VOR inbound." We would then continue on until the needle centers and then turn to a heading of 277 and turning the needle to 197 (then turn 297 twist 217, ect.). You can see that this is continually turning us north along the arc in twenty degree segments. So, this is how we fly the arc using the HSI/OBS. We follow the rule of "turn twenty, twist twenty" all the way around. Some people prefer to follow a "turn ten, twist ten" procedure instead to maintain a greater dgree of accuracy, but the twenty rule should work fine to keep you within a half mile either side.

Now, that's the hard way. The easy way is with the RMI (refer to 2 above). We said that when we made our first turn of 90 degrees that the needle should be 90 degrees off the right wing. Well, we've got a card on the RMI that shows us bearings in degrees. So, if we just fly in a straight line until the RMI needle swings to 10 degrees behind the right wing (pointing to 337), we must be on the 157 radial now. All we have to do at that point is turn twenty degrees right to put the needle ten degrees before the wing tip. No twisting using the RMI, just let it fall back ten degrees behind the wing and then turn to put it back in front of the wingtip ten degrees (ten behind plus ten ahead=twenty total degrees of turn). Now, if you follow either of these procedures in a no-wind situation, you should maintain 7DME all the way around the arc. But what happens when the wind blows (365 days/year)? Well, as we travel around the arc, the wind will be coming from various directions trying to blow us either to the inside or the outside of the arc. You may find yourself following the procedure but drifting slowly further so that your reach 7.1DME and then 7.2DME. So, you've got to make a correction. If you're drifting to the outside of the arc, you're going to have turn MORE to get back to it. The general rule is to turn an extra ten to twenty degrees to get you back into the arc. In relation to the RMI, this addition will place the needle twenty degrees BEFORE the wing now. As you can see below, we were starting to drift outside the arc (7.2DME), so we want to get back to the arc. We've added an extra ten degrees to my turn to make it thirty degrees. You can see that the RMI is now about twenty degrees ahead of the wing.

Once I get back to the arc, I'll go back to my normal procedure, adding in a few degrees wind correction as appropriate to maintain the arc. Remember, as you proceed along the arc, you will continually be changing your heading in relation to the wind, so your wind correction will always be different. Don't get bogged down in wind correction angles, just fly the procedure and make small changes when you see that your current track isn't keeping the arc. OK, so we're continuing along the arc and we're closing in on the 295 radial. Once you see that your next twist of the needle will take you to or past the 295 radial, turn it to the 295 radial and wait for the needle to center. Your RMI should now be pointing to 115.

Figure 3:
Crossing the 295 radial. Time to turn right to intercept final approach course of 120 (R-300) and begin descent to 1000.

Note position "3" on the chart and the map-view.


Now we're at our turning point, so, instead of turning twenty and twisting twenty, continue your turn in order to intercept the 300 radial inbound(120 course) and turn the needle around so the head points to 120 TO the VOR. Your standard rate turn should take you right onto the 300 radial. The HSI/OBS should now be centered with 120 TO selected and the RMI needle should be pointed directly off the nose(120 heading). And there you are, off the arc and onto the inbound course. Now you can just proceed inbound on the approach just like we did in the "VOR Approach" lesson.

So that's the arc. Like I said, it's not tough if you're really familiar with the OBS/HSI use and you're able to make yourself a good mental picture of where you are and what you're doing. The greater the radius of the arc, the easier it is to fly because your time between radials is greater. Try it first with no wind and just use the RMI. That way you don't have to think about where you're supposed to twist to. Just fly ten back, ten ahead and get an idea of what it looks like. Once you do that, try it with the HSI/OBS and use the RMI for backup. If you're feeling confident, just try it with the HSI/OBS and leave the RMI out. I know this is a difficult procedure to learn just from reading and looking, so don't hesitate to ask if you have any questions. Good luck!


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