DME Arcs? What do the real guys do?

[dirtydog1006]

I am having 'fun' polishing up my DME arcs. I've been told to use distance to cue my turn to the next segment. It seems clear, I am sure to everyone and now I get it too, that you could use DME groundspeed to cue turns for the next segment. As you get tangent to your circle around the VOR, the closure speed drops to zero, and increases as you proceed down the segment.
I would like to know how the real IFR guys do this? DO you use speed (relative to the VOR) as well as distance, or just distance, or what? I expect there may be a diversity of opinion, but I would appreciate some real-world advice.

Thanks

[louross]

Been a long time since I actually flew a DME Arc approach, but we did many of them. DME ground speed has no bearing at all, it was never a consideration. We flew the arc at reduced airspeed, example 140 kts ias. If I remember, we used 120 in the Beech 99. You initiate the arc at at 90* angle, using the RMI. So if the VOR is to your left, the needle will point 90* to the left. When the needle is 10* "behind" (or 100* left), you make a left turn to put the needle 10 * ahead, or when you roll level the needle is at 80*. Hold that heading, and again, when the needle has swung 20*, or is 10* "behind", make another left turn to put the needle ahead again by 10*. Do you follow that? Contrary to what I've read in various forums, you do not hold an exact distance of xx miles. It is segments done by arcs based on the RMI.
If that's clear to you, great. If it isn't, I'll lay out an example using actual headings and bearings.
I'm sure that you realize that today with the FMC and all, all you have to do is program it into the FMC and let the plane fly itiself.
lr.

[dirtydog1006]

The buttons got away from me. I would delete this if I knew how. Please see below.

[dirtydog1006]

Thanks Lou I do follow you: you are saying that relative bearing is the cue to turn to the next segment, using a dual-needle RMI. I will install one in Old BEtsy. Everything I have seen (and I have not seen everything) has talked about obsessing on the DME diatance: turn when off track by half a mile or so. Or they say you should keep centering the CDI, and point 90 degrees from it (assime no wind: yeah right). It is refreshing to hear you say that the distance is not critical. From simple geometry, the DME groundspeed should work, as it is a function of the relative bearing....... But I will follow your suggestion: it sounds real efficient. As for the FMS, if I wanted to be a passenger, I would ride in back.
Thanks! Anyone else have ideas?

[louross]

Coming off a hard working weekend, so this is quick- yeah, reread what I wrote and that's how we did it RW. DME ground speed for an ARC Approach??? Doesn't make sense to me. DME GS is flying to or from the DME- you do neither on an ARC approach. The segment is what will keep you within the limits of the distance.
Sometimes flight simmers come up with a lot of theories and ideas that don't have any practical value in the RW, or just don't work. Example, you walk into the dispatch and the Dispatcher gives you a computer print-out of the flight plan- times and fuel loads- exact. Not important. As Captain who has flown the trip a number of times, you know the time enroute and fuel needed. You know the wx. Trip will take xx hrs and I need xx lbs fuel. The exact numbers are academics and tests for pilots working on their licenses. Trip will take 2 hrs and we'll take 17,000 lbs.
Dispatcher wants 17,347, "Okay".
lr.
lr.

[dirtydog1006]

LOU MY MAN!!!
I popped a dual-needle RMI into the old bug-crusher, and following your advice did the prettiest 10nm arc around the BOS VOR. I made segment turns on the relative bearings as you suggested, and (given a good more-like-lucky intercept of the first tangent at 10NM) the distance sort of took care of itself. Now I cannot understand how anyone does without a 2-needle!
There is nothing like experience, it is one of the few good things about growing older. Thanks very much for sharing yours!

[louross]

Well, I'm real glad that helped you! Yep- experience- that's the thing. Growing old you learn things too. Like quality vs quantity. I drank whatever when I was a youngster. Now I can appreciate malt liquor and fine wine. Of course there is a cost involved, so I'm drinking less, but enjoying it more.
There are downsides tho. Nine women and 3 guys were taking a boat ride thru some islands. As fate would have it, the boat sunk. Well, you know, as fate would have it, the women wound up on one island and the three men on another, about 350 yards away. The twenty year old looked over to the women and said to his buddies, "Let's start swimming and we'll get there before dark and we can each make love to a woman." The forty year old guy said, "Let's wait till tomorrow morning and we can clean this old log up a bit, row over to them, and then each of us can make love to three women in the morning."
The sixty- two year old man said, "I don't see what all the fuss is about getting over there- we can see them from here."
hahahahahahah
Anyway, glad to have been able to help!
lr.