DME Arc Techniques

[airplanerik]

How about....load the GPS overlay (with arc transition), arm autopilot, GPS steer

[ppragman]

I actually don't care for the turn 10 twist 10 because it dramatically increases your workload, I use the RMI and then just correct as required, it cuts down on the work load. I load the lead radial into the OBS, then wait for it to center, rather than dink with the turn ten twist ten

[BuickCFI]

Originally Posted by ppragman

I actually don't care for the turn 10 twist 10 because it dramatically increases your workload, I use the RMI and then just correct as required, it cuts down on the work load. I load the lead radial into the OBS, then wait for it to center, rather than dink with the turn ten twist ten

works great if you have an RMI

[MidlifeFlyer]

Originally Posted by BuickCFI

works great if you have an RMI

Yeah but "turn 10 twist 10" unnecessarily increases you workload =without= an RMI also.

[B767Driver]

Originally Posted by ppragman

I actually don't care for the turn 10 twist 10 because it dramatically increases your workload, I use the RMI and then just correct as required, it cuts down on the work load. I load the lead radial into the OBS, then wait for it to center, rather than dink with the turn ten twist ten

With an RMI...it's a no brainer. Set the inbound course on the HSI/OBS and keep the needle at 90 degrees. In Nothern Michigan, up around Marquette, there is no radar coverage (at least 12 years ago)...so you'd always get the DME arc to the ILS. Those were the days....NOT.

[ppragman]

we get them all the time in ak, no radar coverage

[fish314]

Originally Posted by MidlifeFlyer

Yeah but "turn 10 twist 10" unnecessarily increases you workload =without= an RMI also.

I don't know about that (the part about the workload increase being "unnecessary, that is). I think that the whole point of the "Turn 10, twist 10" is to give you some kind of a reference for what heading you would need to fly to fly tangent to the arc if you haven't got anything but a course needle, a DME readout, and a DG or compass.

Let me see if I can explain. Let's say you're supposed to fly 3/4'ths of the way around the 15 DME arc off of XYZ navaid, starting at the 090 radial and proceeding counterclockwise to the 180 radial.

You can easily calculate a lead DME to turn onto the 15 DME arc, and let's say that you do that and begin by holding a heading of 360. A little while into the ARC you notice that you are now at 15.5 DME, so you know that you need to turn left. But you don't really know how much, because you don't know exactly where you are on the ARC. Until you twist in something to the CDI (or use some other method of obtaining what radial you are on, like a bearing pointer), there's no way to determine what direction is tangent to the arc. Since you can't determine what heading should hold your DME constant, it's also impossible to tell what headings will result in a correction IN, and what headings will result in a correction OUT. (Of course, you could always make just a REALLY large correction, like 45 degrees of heading change and watch the DME, but then how much of the correction do you take back OUT once the DME reads 15 again?)

But if you use the CDI, and center it, you can now get the radial that you are on. Let's say in the above example I am still reading 15.5 DME, and I'm still heading 360, but the CDI centers up with a "TO" indication on the 070. I now know that by turning to 340 (90 degrees off of 070) I should have a pretty good no wind heading that will be tangent to the ARC at this instant. In other words, with no wind, a 340 heading shouldn't result in a drift into or away from the ARC at that particular instant. Since I want to correct from 15.5 back to 15, however, I need to use a heading that's to the LEFT of 340 (based on going counterclockwise around the arc). As a rule of thumb, 10 degrees of correction for each 1/2 mile outside the arc, or 5 degrees correction for each 1/2 mile inside the arc is a popular one.

So I'll use a heading of 330, which should start to correct me back towards the arc (asuming no wind). But I can only get this information if I know what radial I'm currently on. Also, once my DME reads 15 again, I can re-center the CDI and then fly a heading that's 90 degrees off of that. So let's say that when I get to 15 DME again, the CDI centers to 062. 90 degrees from that is 332, so that heading should keep me from drifting at that particular instant. (Of course in just a couple of seconds, I'll be on the 060 or the 055, so headings of 330 or 325 would be better at keeping me on the ARC).

[USMC-SGT]

wow...i havent flown an arc since my instrument rating...I think I need to dust one off if I can find one in the actual "system"

I know they are out there, just nowhere that I have been too recently

*please dont send me links to the plates of every DME arc in the northeast

[MidlifeFlyer]

Originally Posted by fish314

I don't know about that (the part about the workload increase being "unnecessary, that is). I think that the whole point of the "Turn 10, twist 10" is to give you some kind of a reference for what heading you would need to fly to fly tangent to the arc if you haven't got anything but a course needle, a DME readout, and a DG or compass.

Let me see if I can explain. Let's say you're supposed to fly 3/4'ths of the way around the 15 DME arc off of XYZ navaid, starting at the 090 radial and proceeding counterclockwise to the 180 radial.

You can easily calculate a lead DME to turn onto the 15 DME arc, and let's say that you do that and begin by holding a heading of 360. A little while into the ARC you notice that you are now at 15.5 DME, so you know that you need to turn left. But you don't really know how much, because you don't know exactly where you are on the ARC. Until you twist in something to the CDI (or use some other method of obtaining what radial you are on, like a bearing pointer), there's no way to determine what direction is tangent to the arc. Since you can't determine what heading should hold your DME constant, it's also impossible to tell what headings will result in a correction IN, and what headings will result in a correction OUT. (Of course, you could always make just a REALLY large correction, like 45 degrees of heading change and watch the DME, but then how much of the correction do you take back OUT once the DME reads 15 again?)

But if you use the CDI, and center it, you can now get the radial that you are on. Let's say in the above example I am still reading 15.5 DME, and I'm still heading 360, but the CDI centers up with a "TO" indication on the 070. I now know that by turning to 340 (90 degrees off of 070) I should have a pretty good no wind heading that will be tangent to the ARC at this instant. In other words, with no wind, a 340 heading shouldn't result in a drift into or away from the ARC at that particular instant. Since I want to correct from 15.5 back to 15, however, I need to use a heading that's to the LEFT of 340 (based on going counterclockwise around the arc). As a rule of thumb, 10 degrees of correction for each 1/2 mile outside the arc, or 5 degrees correction for each 1/2 mile inside the arc is a popular one.

So I'll use a heading of 330, which should start to correct me back towards the arc (asuming no wind). But I can only get this information if I know what radial I'm currently on. Also, once my DME reads 15 again, I can re-center the CDI and then fly a heading that's 90 degrees off of that. So let's say that when I get to 15 DME again, the CDI centers to 062. 90 degrees from that is 332, so that heading should keep me from drifting at that particular instant. (Of course in just a couple of seconds, I'll be on the 060 or the 055, so headings of 330 or 325 would be better at keeping me on the ARC).

I don't know, but it sure =sounds= like a lot more work than moving the OBS ahead to a number you select for situational awareness depending on the approach and the weather conditions and making small course corrections to keep the DME readout within certain parameters.

You're starting with a tangent when you turn onto the arc. I'll even use your rule of thumb of 10 degrees of correction for each 1/2 mile outside the arc based on the intercept radial. Unless the wind is very strong, if I'm supposed to be on the 15 DME arc and the readout says 16, why do I have to know which radial I am on within 10 degrees to make the correction? And if the correction doesn't seem to be working out very well and I want to know exactly what radial I am on at the moment to choose a new reference heading, why not simply center the needle when I =need= to rather than 10 times that I don't?

Did you look at that approach I posted earlier? Now let's assume a good strong headwind for landing (tailwind for most of the arc).

[fish314]

Originally Posted by MidlifeFlyer

I don't know, but it sure =sounds= like a lot more work than moving the OBS ahead to a number you select for situational awareness depending on the approach and the weather conditions and making small course corrections to keep the DME readout within certain parameters. ...
... And if the correction doesn't seem to be working out very well and I want to know exactly what radial I am on at the moment to choose a new reference heading, why not simply center the needle when I =need= to rather than 10 times that I don't?

Oh, it sounds like we're saying the same thing sort of. I wasn't advocating centering the needle every 10 degrees, just that centering the needle is necessary occasionally if you want to know what heading you would need to fly to stay on the arc (or correct in or out, for that matter). If you are keeping track along the arc and have enough time to monitor the DME, then you could certainly just look at that and be able to work yourself around the arc. But I imagine that you'd have to focus on that quite a bit, to do it.

If you had to look away from the instruments for a few seconds to switch a radio or something, when you look back you have no way of telling how much correction you need to get back to or stay on the arc. Since you've been changing headings every few seconds, you probably wouldn't be able to tell if your heading drifted while you were messing with the radio, so to re-establish your heading you'd need to re-center the radial and figure it out.

[MidlifeFlyer]

Originally Posted by fish314

Oh, it sounds like we're saying the same thing sort of.

Seems that way. I wasn't suggesting that one should not twist the OBS to an upcoming radial; just that automatically doing it every 10 degrees by rote unnecessary increase in workload.

I like a minimum of three twists (no HSI; no GPS; and not the Martin State arc):

(1) "twist 10" immediately after the 90° turn onto the arc as a cross check that you turned in the correct direction

(2) twist somewhere around the midpoint as a check on progress around the arc; and

(3) the lead radial.

I like to do a couple of others, but when and where are based on what's happening.

[averyrm]

Set the lead in radial - turn towards the VOR if the number gets bigger, hold heading if it gets smaller.

Some of these seem really complicated, it's a DME arc not an ILS - if you're keeping it within 0.5 nm you're within the error of the system itself.