DME Arc

[lownslow]

Alright, this is embarrassing and to be honest I was hesitant to post but I truly suck at flying DME arcs. Does anyone have any advise on how to improve or perhaps different techniques to use?

LnS.

[Shiny Side Up]

What specifically do you have problems with? I find that people usually have problems with them if their VOR situational picture is poor to begin with. It is also far easier to accomplish if you have 2 VORs set up during it, they're a bit of a pain if you only have one.

[KK7]

As SSU said, we need a bit more info about what issues you have before we can help without teaching it all from scratch. It also helps if we know what equipment you're using, like 1 or 2 VORs, and if you have an RMI it makes them incredibly easier.

I suggest practicing in a sim on autopilot to start with, even if it's on your own computer (although I don't know what sim software is good these days). They are a time consuming procedure and cost a lot of money to practice in the airplane, or even in a sim at a school that you rent. But luckily you can practice them at home, get a good understanding on how to work them in different situations, and really just start by focusing on procedure with the autopilot on, rather than worrying about keeping the sim straight and level. Once you got them, then you can hand fly them, then finally if you have one near where you fly, go out and fly one.

[iflyforpie]

If you have an RMI with a co-located VOR (or VORTAC), or an ADF with a co-located NDB, it is very easy. Just keep the needle on your wingtip and monitor the readout. Memorize your formula for intercepting the arc. Some people find it easy to fly 'sectors', changing heading every ten or twenty degrees. I always preferred to let the aircraft drift slightly in the direction of the arc.

I don't believe there are any arcs published without a VOR or NDB, since bearings/radials are also what defines the arc plus the lead bearing/radial to turn to intercept. I can see where it would be very hard with a standard CDI to keep situational awareness though.

[Big Pistons Forever]

As pie said if you have a 2 needle RMI than you just keep the turning to keep the needle at a 90 deg relative bearing. Since most training aircraft just have a HSI and a VOR head this method is not an option. I have tried all the fancy formulas and OBS knob twiddling, but could never get the practice to match the theory, so I finally just ended up using the "Yay" method

. Set the No 2 VOR track bar to lead radial and continue to track towards the VOR with the number HSI When you are 1/2 mile from the DME arc distance (for the average light twin) turn 90 degrees in the direction of the arc and reset the HSI track bar to the inbound track of the approach. The DME distance will be at or close to the published arc distance. Fly on until the DME increases by .3 and then turn in 10 degrees wait until the distance stops decreasing and then starts to increase again and turn in another 10 degrees. In other words "Yay" the amount of turn based on whether the DME is increasing or decreasing. When you get to the lead radial turn to the procedure turn in bound heading (ie 45 deg) until intercepting the final approach track

[lownslow]

Thanks for all the input so far. As for the nav equipment I have, there's two NAV/COMMs; ADF; a DME (duh); and a GPS, which is probably where my problems started. All along I've been flying arcs on the GPS, which is easy to the point of boring and I've realized I need to do it the old school way if I'm to take myself seriously.

Shiny Side Up/KK7, I think situational awareness might be a part of my problem. Typically I can keep a very good 'air picture' of what's going on in IMC or under the hood but I think I'm over thinking the whole procedure when it comes to flying the arc. I'm not necessarily getting lost (this is while simming, btw) but either getting too far behind or maybe too far ahead and ending up way off the arc, or at least far enough that I'm not happy.

BPF, I'm going to try out the 'yay' method in the next couple days. Simple is good.

LnS.

[rjonno]

In my experience DME arcs are one of the few things out there that are actually made more difficult by using GPS. assuming you have an HSI/RMI:

-Figure out how early you should start the rate 1 turn to intercept the arc, obviously this will vary depending on what aircraft you are flying/at what speed.
-Once your established on the arc, let your RMI needle fall about 10* behind your wing, then make a turn into the arc, bringing the needle 10 degrees ahead of your wing. Then let the needle fall to 10* behind you again. Continue this staying within a couple 10ths of a mile of the published arc. If you're too close in, let the needle fall more than 10* behind your wing. If you're too far out, turn into the arc more than 10* to correct.
-As soon as you're established on the arc, set your HSI to the inbound course for the approach
-Every few seconds I like to just mentally review the published lead-radial where i'm going to start the turn on to the inbound course for the approach.

Presumably this will work the same way using a fixed card ADF with an NDB co-located at the DME. Though you're better off with an RMI. Once you get the hang of doing it in 20* sectors like I described above, its easy to just establish a couple degrees of bank and fly the DME arc right around the circle nice and smooth

[Gannet167]

What do you mean "too far behind" or "too far ahead" ? Once you're on the arc, fly straight and level holding an accurate heading. When the needle is 10 degrees behind your wingtip, alter your heading by 20 degrees to put it 10 degrees ahead of your wing tip. It's mindlessly easy this way.

Once you're good with that, do it with winds. If the wind is pushing you towards the station, put the needle 5 degrees head of the wing tip and wait for it to be 15 degrees behind the wing tip before changing heading again. Of course the wind component changes as you go around the station. If the winds are strong, put the needle on the wing tip and then wait for it to be 20 degrees behind.

[Big Pistons Forever]

lownslow said he doesn't have an RMI.......

[Gannet167]

I'm not sure how you'd fly an arc without an RMI, I guess you could continually be adjusting your VOR CDI needle. This seems almost ridiculous to me. I'd ask for vectors at that point or use my GPS to go somewhere direct. Get an RMI.

[lownslow]

I'm not sure how you'd fly an arc without an RMI, I guess you could continually be adjusting your VOR CDI needle. This seems almost ridiculous to me. I'd ask for vectors at that point or use my GPS to go somewhere direct. Get an RMI.

Not every airplane has an RMI, not every approach has vectors available. Luckily my GPS is capable of guiding me through any published arc but as I said, I want to be able to use ALL the equipment on board the airplane.

Back to BPF's method, here's what I'm getting:
Obviously I'll be coming up on the arc at 90* to the arc itself. At this point I'm tracking inbound to the navaid, but this part I knew. Sounds like about 15-20 seconds before I reach the arc I want to turn 90 degrees and fly that heading until I'm at the arc DME distance +.3 or so then turn 10*. Repeat as necessary until I'm at my lead radial or final approach course. If I'm understanding correctly, while I'm flying the arc itself I don't know or care exactly what radial I'm on, only that I'm not yet at my lead radial.

Am I close?

LnS.

[Big Pistons Forever]

Back to BPF's method, here's what I'm getting:
Obviously I'll be coming up on the arc at 90* to the arc itself. At this point I'm tracking inbound to the navaid, but this part I knew. Sounds like about 15-20 seconds before I reach the arc I want to turn 90 degrees and fly that heading until I'm at the arc DME distance +.3 or so then turn 10*. Repeat as necessary until I'm at my lead radial or final approach course. If I'm understanding correctly, while I'm flying the arc itself I don't know or care exactly what radial I'm on, only that I'm not yet at my lead radial.

Am I close?

LnS.

Yup don't worry about the radial you are passing just Yay the DME distance....

[KK7]

I'm not going to add much since BPF's method is spot on.

But I do want to throw out there that I am quite pleased that this is being practiced. Although DME arcs aren't used very often in N. America, overseas some places have nothing but. In my experience particularly the Middle East, unless flying into a major centre like Jeddah, there is no radar environment and one has to fly an arc to transition to the approach. I flew with quite a few people who had never even flown an arc before, despite holding an instrument rating.

[Gannet167]

If you want to be really nerdy about it, you can calculate your lead DME to turn onto the arc. For a rate 1 turn onto the arc, I believe it's 1/2 your TAS, which you can get by multiplying your altitude in 1000's by 4 above 10,000', or by 3 if under 200 kts.

So at 12,000' and 220 kts, you'd multiply 12 x 4 and add that to IAS to get 268 kts TAS. Plus or minus a wind component. Without wind, start your turn at 2.7 DME ahead of the arcing DME. For a 12 DME arc, that'd be 14.7 DME. All this is quite academic and useless if you know how to WAG it (wild ass guess). I think I'm remembering that right.

I suppose you can Yay your arc, but unless you're going quite slow and the winds are light, to me this is a recipe for getting confused and off the arc. An RMI is the clear way to go. If you don't have one, I wouldn't feel bad about not flying perfect arcs - and in real life I'd look for a better way to transition onto the approach with the kit I have.

[Dagwood]

What do you mean by "Yay" an arc? I haven't heard that terminology before...

[Doc]

I never do them. Keeps it simple.
You could write a book about it. Some have. See above.
Keep the fix to your left or right, maintain the proper DME + about .2 miles, and keep it there. You can't help turning. Then about 10 or so degrees from the inbound....in you go. That's it. No smoke and mirrors. KISS

[SuperchargedRS]

I use my range and ground speed, keep a steady turn with a GS of 0 and your shooting a spot on arc. range increases a little tighten it up slightly.

[Big Pistons Forever]

What do you mean by "Yay" an arc? I haven't heard that terminology before...

Yay = Use judgement and situational awareness to manage a procedure. So in this case when the DME is increasing you turn in and when it is steady or decreasing you hold your heading. The rate and magnitude of the DME range changes determine how/when to turn in.

Many flight schools use formulas to decide when to turn and and a complicated procedure of re-centering the OBS to stay on the arc. Personally I have found they brief well in the class room but don't work very well in the air. My personal feeling is that the time and brain cells used in doing mathematical formulas and constantly twiddling the OBS knob is better spent flying the airplane and maintaining situational awareness

[toelessjoe]

I have a procedure I follow religiously whenever I feel the need to do a DME arc approach that goes like this:

1) After convincing some silly bugger at ATC that I simply MUST do an approach that takes up shit loads of time and airspace for no apparent reason I proceed to grab the nearest heavy object and beat myself thoroughly about the head and shoulder area, paying particular attention to the frontal lobe.

2) Upon regaining my faculties, I then plug the stupid thing into the GPS and let the autopilot fly it.

3) Once set up, I request fellatio from the f/o. You know, since we apparently now have some time to kill.

Silly wabbit, arcs are for kids!

- Toeless.

[Gannet167]

Agreed, if you don't absolutely have to do one, why would you? In the case of training though, it's necessary and a good thing to know how to do and practice. This "yay" thing must be in a relatively slow aircraft. It's not practical in anything that moves quick.