Help Visualizing setup for DME ARC

Discussion in 'Cleared for the Approach' started by USAF-LT-G, Dec 13, 2017.

  1. USAF-LT-G

    USAF-LT-G Pre-takeoff checklist

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    Hey guys,

    In my training, I'm having some difficulty in particular situations visualizing how to setup for a DME ARC and I'm wondering if either someone can help explain this better, or point me in the direction of some material that might help me nail this.

    Basically, in the sim (x-flight based), I'll be flying along, and they say, ok, well.... yer doing great... but now I'm going to drag your airplane to some random spot, and I want you to enter a 15 DME arc to the East on the 040 Radial of some VOR.

    Ok, so maybe it's the fact that we're not pausing for this setup, and I was dragged to some "unknown" point at this point.... not sure, but the way I'm working this out in my head is as follows:

    Step 1: Figure out where the heck I am. Do this, by centering up on the needle on the VOR with a TO indication and / or if the DME # is getting bigger or smaller .

    Step 2: Read out the RADIAL i'm currently tracking. In this example, Let's say I'm on on the 030 tracking outbound (FROM), and 25 DME.

    Step 3: Align Heading / BUG to 030 and keep needle centered for time being.

    Step 4: Interpret - Okay, so by what I said above.... the way I'm visualizing this, is that I'm North and slightly East of the VOR by 25 DME..... to get to and intercept the 040 Radial, I need to turn RIGHT to a heading somewhere South, South West to intercept the 040 and start to close the DME gap to 15.

    Step 5: Tune the OBS to the 040 Radial, and turn RIGHT to a South, South West direction to intercept the 040. Once the needle is centered track either the 040 outbound or the reciprocal 230 inbound until DME 15.5

    Step 6: Established on the 040, at 15.5, make a standard rate turn to the RIGHT (EAST) to 130, Twist the OBS right 10 degrees, roll out on 130.

    Step 7. Wait for the Needle to near the "doughnut", start clock, turn standard right for 10 seconds, stop clock, roll-out, Twist to OBS 10 degrees.

    Step 8. - If DME is getting smaller than 15, turn left of present heading by 10 degrees or more to halt or reverse decrease AND / OR shorten the next 10 second turn (8 seconds or whatever feels like the variance). if DME is getting bigger than 15, turn right of present heading by 10 degrees or more to halt or reverse increase AND / OR lengthen the next 10 second turn (12 seconds as an example).

    Repeat steps 7 and 8 until it's necessary to end the ARC

    I more concerned with making sure I understand how to visualize where the heck i'm at, get established on the right Radial, and making that first 90 degree turn. I think I'm confused there.... not sure if I have all that info right.
     
  2. midlifeflyer

    midlifeflyer Final Approach

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    Whew! That all sounds way too overthought. The DME arc is one of those procedures that sound complicated but is actually very easy. My recommendation is a ground session with a good CFII to simplify it. Good may be the catch. It's unfortunate that the standard way of teaching it is to over complicate it.
     
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  3. LDJones

    LDJones Touchdown! Greaser!

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    This! Take your starting radial and either track inbound on it or outbound on it depending on which direction you're coming from. About one mile from your DME arc distance (say the arc is at 14 DME, and you're coming from the VOR, 13 would be your initial turn point.) At that point just look at your DG and see what's at the 90 point in the direction of the arc then turn to that heading. As you're turning, twist the OBS to a radial 10 degrees from your initial (again, in the direction you're flying the arc.)

    From that point on, when the needle centers, turn 10 degrees toward the airport, and twist the OBS another 10 degrees away from you. Keep that up until you're at your lead-in radial at which point just turn to a nice 30 degree intercept to your final approach course.

    They truly are that darn simple.
     
    Last edited: Dec 14, 2017
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  4. hindsight2020

    hindsight2020 En-Route

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    OP, what you're describing having trouble with is not dme arcs, it's the baseline trigonometry behind the old school "fix to fix" navigation. The FAA did away with the legality of fix-to-fix, so you'll never be asked to demonstrate the ability to get from one radial/dme to another radial/dme by pilot-deduced trigonometry on an IR checkride. You'll be provided a vector to intercept the radial in question, then demonstrate a DME arc. Alternatively, you'll be cleared a fix as part of a published approach that includes a dme arc, and you'll arc accordingly.

    Do know how to come up with a lead-turn DME to intercept the target dme arc, since after all, your turn radius is never zero. 0.5nm is not always going to be the answer. 1% of your ground speed for a 90 degree turn is a good calculation for turn radius. This is nifty because it already accounts for winds, whereas utilizing true airspeed does not.
     
  5. denverpilot

    denverpilot Taxi to Parking

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    Students get hung up on the turn 10, twist 10 thing and start to think they’re following the OBS. The OBS is just telling you how far abound the arc you are. It’s NOT the controlling instrument for an arc. The DME is.

    For an oversimplification to get this point across, think about this... can you fly the arc without an OBS? Absolutely. How would you do it?

    Once you “get that” as a mental exercise, the rest can be added in logical building blocks.

    Summary for “the simplest arc you’ll ever fly”:

    1. Intercept the radial. Fly in or out as needed.
    2. Turn 90, as a minimum, no matter what at the DME arc distance. Maybe just a little more. (Intercept that arc from the inside, it’s a little easier. Once you get outside it’s bigger angle corrections to fix it.)
    3. DME distance is the primary instrument on the arc. Fly it. Make a continuous turn or a bunch of little straight legs out of it or whatever you like. You literally can fly this without the OBS if you want to. DME too low, fly out of the circle. DME too high? Turn harder to get back into the circle.
    4. Intercept the outbound radial or ILS, whatever it is. Don’t miss it.

    That’s it. You’re done. Don’t over-think it.

    Adding back some OBS twists to see which radial you’re on, or making a few all straight legs out of it, is easy to add back in, once you realize you can fly it as a continuous low rate turn watching the DME count up or down. ;)
     
  6. mscard88

    mscard88 Touchdown! Greaser!

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    :yeahthat: Yup!
     
  7. drjcustis

    drjcustis Pre-Flight

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    And don’t forget you can setup a DME arc approach in your GPS.


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  8. flyingron

    flyingron Touchdown! Greaser!

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    I too think the way DME is taught is rather odd. Tracking a DME isn't that much different than tracking anything else. Set a heading (it will be roughly perpendicular to the radial you are currently on). Fly it for a minute. If you're getting farther from the DME distance adjust it toward the station. If you're getting farther, adjust it away from the station. Lather, rinse, repeat. The only difference is that you'll never "get it right." Eventually the course will start to take you away from the station again. Just adjust when that happens.
     
  9. Clark1961

    Clark1961 Touchdown! Greaser!

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    Inop fixes the DME arc problem. Joking of course. Loren and Mark have simplified it. Arcs really are easy. Wind will mess with you a bit because the components change every time you turn.

    An educational DME arc is the VOR/DME RWY 30 at KLAR. At minimum altitude yer less than a thousand feet over the terrain. It teaches you not to mess around with instrument approaches in IMC in the hills because the 2,000 ft clearance we have enroute just isn’t there. Nothing to do with learning to fly an arc of course, just an arc I remember from training.
     
  10. aterpster

    aterpster Pattern Altitude

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    Picking at nits. It can't be less than 1,000 feet. DME ARCs are huge, 8 mile wide primary, plus 2 mile secondary each side. The highest obstacle in the primary area has to have at least 1,000 feet of required obstacle clearance, plus a 200-foot assumed adverse obstacle (AAO) additive, plus precipitous terrain additives where required. The AAO may be disregarded for a surveyed antenna.
     
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  11. labbadabba

    labbadabba Pattern Altitude

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    This is one of those scenarios that's perfect for hangar flying with a CFII. Pick a point in the room that is the VOR. Pick a point that is the IAF. When you get to the IAF (lets say you're flying a left arc), extend your left arm towards the VOR. Now, imagine you are the #2 VOR head in your panel. Your left hand will be pointing directly at radial you need to fly. Turn your #2 OBS 10 degrees left, rinse and repeat.

    As long as you're at a 90 degree angle to the VOR you should be right on.
     
  12. Clark1961

    Clark1961 Touchdown! Greaser!

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    A worthwhile nitpick. Looks like the arc minimum is 10,600' and the highest obstacle in the area is 9121' so not as close as I thought. My instructor made me look at the terrain while commenting how close we were.
     
  13. USAF-LT-G

    USAF-LT-G Pre-takeoff checklist

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    Just an FYI. Passed my checkride. Did in a disabled auto pilot g1000. DPE would not let me do the arc using the GPS. He picked a point on a plate closer than published 10 DME arc (5) and said arc at 5, without the GPS. So turn 10 twist 10. Kind of threw me off that he wouldn’t let me use GPS in the G1000.


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  14. aterpster

    aterpster Pattern Altitude

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    He should test to standards. The minimum DME ARC permitted by TERPs is 7 n.m.
     
  15. James331

    James331 Touchdown! Greaser!

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    You can also use the closing speed on the DME to make the turn, keep it at zero and the DME at the target distance and it's a perfect arc.