DME Arc question

SixPapaCharlie

SixPapaCharlie

May the force be with you
Joined
Aug 8, 2013
Messages
16,018
Display Name
Display name:
Sixer
I am being taught, twist 10 turn 10.

I have not tried this yet but could one not just turn the OBS to the final desired radial and make the arc based on distance only? it feels like I could.

I am not sure what the value of dialing in the 10 degree intervals is.
 
"Twist"ing the OBS 10 degrees at a time keep a current reference heading to which you can apply wind or distance corrections.

If you have an RMI, it does the "twist"ing for you.
 
The only downside to that is that you don’t know where you are along the arc until the end.

I haven’t done one in a while, but I do remember using the DME distance to control heading to stay on the arc, but still twisting the VOR regularly so I knew how far I had to go.
 
bluesky74656 said:
The only downside to that is that you don’t know where you are along the arc until the end.

I haven’t done one in a while, but I do remember using the DME distance to control heading to stay on the arc, but still twisting the VOR regularly so I knew how far I had to go.
Click to expand...

If you have an RBI ( relative bearing indicator) then you know where you are at all times relative to the VOR, hold your distance and you are golden.
 
Thanks for asking the question Briany, I had the same one. Follow up, couldn't I just tune the VOR to the lead radial and just keep the arc distance based on my DME until I get there?
 
The "turn 10 twist 10" method is what I was taught and it makes adequate sense to me.

However, I have also tried to teach it, and it makes virtually no sense to a lot of people. To some, it is excessively complicated. It takes numerous tries to figure it out and get it right.

So I started teaching just using the DME distance, and set the OBS to the radial you're going to use at the end of the arc. Using this method, most people are well within PTS standards on the first attempt, and understand it well. So, needless to say, I have switched to this method for all.

I have tried it myself and it is so much easier and requires fewer mental gymnastics.

If you're too far out, turn 10 degrees toward the inside of the turn. If you're too far in, keep your heading until the distance starts increasing. See if the distance changes and adjust. That's it for the majority of cases, unless you get really off.

Once the needle starts to come in at the end, you have plenty of time in typical training airplanes and typical arc distances to capture it. The DG itself gives you a good idea of how far along the arc you are.
 
Last edited:
Ravioli said:
Do you watch any of these guys videos?
Click to expand...

That's the "standard" way that I find needlessly complicated.

Also, he starts turning in at the Lead Radial, which may be too much for light airplanes. It's calculated to be 2nm from the ILS course. An airplane moving at 90 knots has a turning radius of about 0.5 nm, so if you turn at the LR you are going to spend a long time intercepting the ILS course.

Also, his GS needle at the end moves the wrong way...
 
Last edited:
SixPapaCharlie said:
I am being taught, twist 10 turn 10.

I have not tried this yet but could one not just turn the OBS to the final desired radial and make the arc based on distance only? it feels like I could.

I am not sure what the value of dialing in the 10 degree intervals is.
Click to expand...
The theory of dialing in the 10-degree radials is to keep track of your progress around the arc. Obviously not a big deal with GPS and doubtful if it's ever really been necessary. But it is one of those things which is taught that way because it's always been taught that way. My more cynical side says it's taught that way to make sure you understand this is supposed to be complicated, even though it's easy. (Although I am sure that it's necessary for some pilots to have a rote mantra)

When I received my instrument training DME arcs were book-only. The airplane I trained in didn't have DME. So I ended up teaching to to myself a bit later and the lazy part of me (about 92.47%) said, "this "turn 10, twist 10" thing is nonsense. So I came up with my own method. The downside is that a DPE might not like it because it doesn't include the "10-10"mantra.

But FWIW, the method I use (VOR and DME only, no RMI or GPS) is, after turning onto the arc, I "twist 10" as a cross check that I turned in the correct direction, then twist to somewhere along the arc as a progress check, then to a lead radial (which may or may not be the one depicted on the chart) before twisting to the intercept. That's a minimum of three twists before the final twist to the in bound radial. To me that's more than enough to remain situation ally aware...with an exception. If the winds are such that you are going to have a honking tailwind on the second half of the arc, you might want to toss in a few more twists.
 
SixPapaCharlie said:
I am being taught, twist 10 turn 10.
Click to expand...
Only the first cut is 10° and the rest are 20°. So, it's really:
Turn ten,
Twist twenty,
Turn twenty,
Twist twenty, etc.

See here, slides #15 to the end: http://www.avclicks.com/Flash2/Needle_Work_temp/index.html

May not render correctly except on a PC.

Back when DMEs had shaky pointers instead of digital readouts to the tenth of a mile, this kept you in the ballpark better than reading a quivering pointer needle.
 
Last edited:
Everybody keeps forgetting a critical info item.

It’s to keep track of your location on the arc without GPS or FiveFlight. VOR only.

That old timey stuff. :)

Yes you technically can fly it distance only and in fact glancing at the distance to see if it’s increasing or decreasing is quite useful.

You’re going to have a failed moving map. Visualization of location is your friend including knowing in your head how long you expect to be on the arc at your current speed. :)
 
Larry in TN said:
"Twist"ing the OBS 10 degrees at a time keep a current reference heading to which you can apply wind or distance corrections.

If you have an RMI, it does the "twist"ing for you.
Click to expand...

denverpilot said:
Everybody keeps forgetting a critical info item.

It’s to keep track of your location on the arc without GPS or FiveFlight. VOR only.

That old timey stuff. :)

Yes you technically can fly it distance only and in fact glancing at the distance to see if it’s increasing or decreasing is quite useful.

You’re going to have a failed moving map. Visualization of location is your friend including knowing in your head how long you expect to be on the arc at your current speed. :)
Click to expand...


I am talking about without a moving map.
Could it be done just using DME just as easily?
No map.
 
Last edited:
SixPapaCharlie said:
I am talking about without a moving map.
Could it be done just using DME just as easily?
No map.
Click to expand...

Kinda. You can talk to your instructor and play with it. Have them pretend your CDI failed. It’s certainly possible to do it, but you then don’t know where to get on, or more importantly, or off of the arc.

In a way you’re just centering the CDI to see what radial you’re on, if that helps visualize it. The 10 thing for both is just to keep an enormously shallow turn going.

Have you done one yet or just ground schooling it right now? Most are ten miles long and take an eternity in anything slow. Since most students are still working on aircraft control, straight and level, then a small turn, then straight and level... are things they know how to do.

Then they’re just adding in the 10 degree thing with the CDI as an additional task and feel overloaded at first. That feeling will go away as aircraft control becomes second nature. Then arcs in GA airplanes start to feel like they take an eternity.

Then they’re only interesting if you have like 30 knots of wind aloft blowing you off of the ground track.

Since ground wind is often generally in the same direction ... you get on the arc and about halfway around you feel good, zooming along with a tail wind and then get blown way off as you’re trying to complete it since that’s the point where a wind down the runway will mess with you the most as you remembered you forgot to dial up the ILS in a single Nav/CDI panel or simulated failure of all but one Nav radio. ;)

You think “It’s not time to turn toward the runway yet” and forget you’ll almost always need a crab angle on the part where you’re 90 degrees off of the runway heading.

You’ll have fun.

I’ll let ya in on another trick. You’ll probably also lose the CDI as one of the simulations. Your mag compass is now your friend, if you can hold level for a bit to let it settle between turns. :)

I’m ruining all of your CFIIs fun. LOL.

Yep. If you’re outside the arc on the DME or inside of it, fix it. Outside requires a big turn. Inside if not too far, just stay wings level in no wind. You’ll fly back out to it. Big turn to intercept then.

The whole “turn 10” thing is just a starter. You really have to turn as much as it takes to hold the distance.

The 10 & 10 thing is just a technique. Mostly because flying the tiny bank continuous turn needed would be a pain at bugsmasher speeds.
 
DME arcs are rare around here, there's one about 50 miles away, KPVD, if they are using non conflicting approaches then they let you do it. When I was working on mine there were 40 knot winds aloft just about every time, holds to. Made life interesting. The ten ten thing made more sense to me when I saw it drawn out, but at the end of the day it's just another flying estimate unless you are up on a 5 knot winds aloft day. A bearing indicator makes it much easier.
 
Bryan thats how I flew them and it works fine. If i was flying a 10dme arc once on it id set it to the final radial. If it went 10.1 then I turn in a bit and back to 10dme. No turn time twist heading bug praying to the faa or whatever required.
 
A lot of confusion on DME arcs comes from poorly written texts that try to teach some rote cookbook method for things.

The strategy for a DME arc are really no different than tracking any other course:
1. Assess whether you are right or left of course.
2. Take a guess at a correction.
3. Fly that for a short time to see if things are improving.

lather, rinse, repeat.

You'll typically start (unless you know there's substantial wind) by turning perpendicular to the radial that you're starting from. If the DME distance is larger than the arc radius, turn inbound a bit. If it's smaller, turn outbound a tad. As mentioned, periodically recentering the needle gives you an idea where on the arc you are and again what the suggested no-wind course would be.
 
denverpilot said:
Everybody keeps forgetting a critical info item.

It’s to keep track of your location on the arc without GPS or FiveFlight. VOR only.
Click to expand...

I think it's quite possible to keep track of where you are on the arc without doing the turn 10/twist 10 thing. If you're flying a clockwise arc, and you are headed East, you must be somewhere north of the station. As you fly if you find you are now headed SE, well you must now be NE of the station. Plus or minus some for wind, of course, but accuracy is not critical here. With your CDI lined up to the inbound (or outbound in rare cases) radial you're going to fly, once it starts moving you consider turning to intercept. That's really good enough for most training airplanes. The PTS standard of +/- 0.5 nm is pretty simple. And on a "real" DME arc you actually get a gigantic amount of obstacle protection - +/- 4 nm! And add to that 2nm more on each side of reduced obstacle protection. That's a lot of error!
 
RussR said:
I think it's quite possible to keep track of where you are on the arc without doing the turn 10/twist 10 thing.
Click to expand...

Which is exactly what I said in describing it as simply being a turn of the OBS to center it and see where you’re at.

You quoted the part where I was explaining that any moving maps will be failed — and he already stipulated that is the case in his training and he’s already at that stage.
 
SixPapaCharlie said:
I don;t hink I have an RMI. I thought that was the thing they purt on the privat test to throw you since it is only used in s
Click to expand...
If you are doing this in the Cirrus and it has a Sandel then you do have an RMI-like bearing pointer on the outer edge of the EHSI. (G1000 too but unless there's some news you haven't shared with us yet, I'm pretty sure that isn't what you are using).

If so, then the twisting is unnecessary. Just keep the pointer off the wing, adjusting fore/aft for winds.
 
SixPapaCharlie said:
I am being taught, twist 10 turn 10.

I have not tried this yet but could one not just turn the OBS to the final desired radial and make the arc based on distance only? it feels like I could.

I am not sure what the value of dialing in the 10 degree intervals is.
Click to expand...

The method has you making a decision about every 1-2 minutes while providing situational awareness.
 
Walboy said:
I think I pointed that out in post #3; situational awareness
Click to expand...

SA is a vague term that means a lot of things and can be misunderstood by a student but yeah, you did.

Specifically you’re using the OBS to see which radial you are currently on, and the DME for whether you’re inside or outside of the arc.

One of the other reasons for turn 10, is that many folks if they’ve flown along for a while by DME only will turn the OBS the wrong way and be confused when the needle stays pegged to one side. ;)

I have a tendency to do a scan, then turn it to center, then scan again... so it ends up more like I’m turning it nearly continuously to keep it centered until it’s time to get off the circular rail road track at the exit. ;)

Other tricks like if have a DG bug and no AP I’ll bug the final approach course so I can see my angle to it... so you have another hint other than memory of when it’s time to get off and start turning to intercept the final.
 
I’m currently in my IR training and we have two DME arc approaches close by so it’s assured it will be on the checkride. I’ve done a couple practice arcs with my CFII so far and the 10-10 thing really does seem confusing. Even though I understand why in principle I do feel task saturated.

Do DPEs care how you execute a task, so long as you stay within the parameters? Or will they expect you to use the 10-10 procedure since that is how it was taught to them?
 
I was wondering the same thing, what the DPE will expect. This is something your CFII should know.

FWIW I’ve not flown a DME arc since the checkride.
 
dtuuri said:
Won't care because they don't want to change the way you normally do things. Now if it was a CFII test...
Click to expand...

Btw thanks for the IR resource you have published. I’ve been looking through that on my free time.
 
SixPapaCharlie said:
I am talking about without a moving map.
Could it be done just using DME just as easily?
No map.
Click to expand...

Depending on your DPE, if you are tasked to fly a DME arc, you'd likely have one of 2 ways to do it, depending on the actual approach.

1) Use the IFR GPS only as a reference for DME to the reference navaid (i.e. Use the "Direct To" function and put in the navaid 3 letter ID) . Won't be any GPS overlay on the GPS moving map. That's usually the more difficult way. Fly it as others have written above. It typically works better if you visualize it as flying a series of short straight lines slightly inside of the arc. That way you're always heading back to intercept the arc rather than away from it. If you have to do it using the GPS only as a DME and you have a tablet with georeferenced IFR approach plates, follow the plane on the plates for "situational awareness" (if he doesn't make you turn off the aircraft being displayed on the plate). It'll be a heck of a lot easier. If allowed to do so, you could also set up the GPS DME as a straight leg b/w the navaid and airport, then activate that leg either to or from the navaid or airport depending on whether the navaid is inside or outside the arc. That would also give you a line to reference on the GPS when you're on our final approach segment. Plan your turn so you don't overshoot the final approach segment.

2) Use the IFR GPS for a GPS overlay of the DME arc....easy peasy, just follow the magenta line.
 
Last edited:
Turn onto the arc and when dme reads 10.1, turn in 10 degrees. Watch the dme go to 10.0 or maybe 9.9 and hold heading until dme creeps back to 10.1, then turn in another 10 degrees. Repeat as necessary to keep her at 10 dme. Set vor1 to your desired radial and set vor2 to a lead radial to give you a heads up on your arrival. Pretty simple to me.
 
SixPapaCharlie said:
I am talking about without a moving map.
Could it be done just using DME just as easily?
No map.
Click to expand...
When I got my instrument rating, we didn’t have GPS or moving map. I set my inbound course (and/or lead-in radial, I’d it exists), and fly DME. When the DME increases to the distance of the arc, I turn...it goes a little higher, then starts back down, dips below the arc distance, and then starts increasing...when it increases to the distance of the arc, I turn. Wash, rinse, repeat.

The VOR is going to be where the FAA put it, regardless of whether or not you have a needle pointing towards it. ;)
 
texasclouds said:
Turn onto the arc and when dme reads 10.1, turn in 10 degrees. Watch the dme go to 10.0 or maybe 9.9 and hold heading until dme creeps back to 10.1, then turn in another 10 degrees. Repeat as necessary to keep her at 10 dme. Set vor1 to your desired radial and set vor2 to a lead radial to give you a heads up on your arrival. Pretty simple to me.
Click to expand...

Half the time I forget but I prefer for most arcs to reverse Nav 1/2 and use 2 for the arc location, that way the ILS most arcs lead to can be already tuned in Nav 1 which used to be the only one with a glide slope needle and receiver in our airplane.

Now we have two since the GPS went in up top and moved the ILS capable Nav radio down along with using the dual needle OBS for it. So less of an issue but nav 2 still works well if you’re using the GPSS for distance and eventual ILS.
 
Dave Theisen said:
Face it Bryan, you are that awkward age in aviation, too young to fly by raw data, but too old to “meow” on the radio. :)
Click to expand...

It is weird. I am bordering on the "Get off my lawn" era. but still young enough to know the lawn has changed and I need accept that people are going to be on it.
What irritates me the most about the meowing is it isn't original or funny.
If you are going to do something, make it unique.
I heard someone meow so I am going to meow...

What irritates me 2nd most is even I know better than screw around on the radio.
 
Bryan, In addition to learning how to hand fly this and get it Wong, make sure you get a chance to fly one using automation and autopilot.

1) to practice setting up the system correctly, and 2) verifying your system can do it without any surprises.

And if you need/want me as a safety pilot, I have lots of availability up to January 17.
 
I’d ether fly the GPS overlay

Or

Go off DME, getting close relax the turn, getting far tighten it up. I have never done the turn 10 thing in my life. Seems like a great way to complicate a simple thing.
 
Walboy said:
I just looked in my old The Instrument Flight Training Manual by Peter Dogan which was the gold standard instrument instruction manual for many years. Yep, sure enough, the turn/twist technique is in there for DME arcs.

The advent of moving maps with widespread use of GPS sort of represented a paradigm shift IMO. I bet a lot of the over 50 crowd that has had their instrument rating for a long time are pretty comfortable with situational awareness just looking at CDI/ADF needles. They had to be.
Click to expand...

Did my IFR with a old timer who just does IFR, most of my learning was done in a frasca sim with no windshield display, moving map or GPS, he mentioned the turn 10 but didn’t teach it, just distance and rate of turn
 
Walboy said:
I just looked in my old The Instrument Flight Training Manual by Peter Dogan which was the gold standard instrument instruction manual for many years. Yep, sure enough, the turn/twist technique is in there for DME arcs.

The advent of moving maps with widespread use of GPS sort of represented a paradigm shift IMO. I bet a lot of the over 50 crowd that has had their instrument rating for a long time are pretty comfortable with situational awareness just looking at CDI/ADF needles. They had to be.
Click to expand...

The reason it’s in every book is someone had to come up with a building block technique that was teachable vs saying “Just turn enough to stay on the arc.” :) After doing a few most folks just “get it” but for those who don’t, step by step instructions are needed.

As far as the over 50 crowd goes, I’m not and flew plenty prior to GPS and even remember a few folks who had money to blow buying LORAN. I continued to fly a /A airplane for a long time even after GPS was available. And there’s a lot of old ass working airplanes out there for those who go commercial that still don’t have a GPS, but it’s slowly fading.

Only one thought comes to mind though. If you’re flying something with an OBS you’d still better know how and be comfortable with using it by itself for SA. Crap fails. And most of us oldsters will cover your moving map and your iPad to make sure you know how to use everything on board to the fullest. Before you really need it one day when all of the automation in something you’re flying goes insane and you’re down to raw data and your brain is going to save your butt today. :)

Get comfortable with it if you aren’t. It doesn’t take too much practice but it does take a little. Evil CFIs will happy arrange practice sessions if you don’t ask for them. Hahaha.
 
SixPapaCharlie said:
I am talking about without a moving map.
Could it be done just using DME just as easily?
No map.
Click to expand...

Yes. You kinda keep a ‘map’ in your minds eye. The numbers, radial and distance, paint the picture for you. I assume you have access to a simulator. Do it without looking at any moving maps. But follow along with paper and pencil. Diagram what the numbers are telling you. You’ll get the hang of it soon enough
 
You must log in or register to reply here.
Back
Top