DME Arc Frustration

sferguson524

sferguson524

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I'm an instrument student and am pulling my hair out over DME arcs. My II was telling me that there is a time that you turn away from the arc before turning onto it, and i have been drawing it out, flying it in the sim, and still can't wrap my head around it. We even went out and flew it in the airplane, and i got it then, but promptly forgot. HELP!!! :mad2::mad2::mad2::mad2:
 
if the number is getting smaller turn away from the VOR, if the number is getting bigger turn towards the VOR. make no more than 10 degrees of heading change at a time, then pause and wait to see how that change affects the response.
 
tonycondon said:
if the number is getting smaller turn away from the VOR, if the number is getting bigger turn towards the VOR. make no more than 10 degrees of heading change at a time, then pause and wait to see how that change affects the response.
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Precisely. Think about flying a many-sided polygon around the VOR since that's really the track you make using the "turn 10, twist 10" method. If you are to the inside of the arc (DME is going down), then either maintain heading or turn slightly away from the VOR depending on how far off you are. If you are outside of the arc (DME is going up), then turn further in toward the VOR and then re-establish yourself on the appropriate heading to maintain the arc when you're at the correct DME.
 
What book(s) are you using? The best one I've found is ASA's Instrument Flying for describing a DME arc. Sit down with the II and have him/her draw the procedure on paper, explaining as you go along.

And yes, you're actually flying away from the VOR the entire time until you turn to final. Why? Because you're flying little straight lines starting on a tangent to the 'arc' or circle around the VOR.
 
I'm using the Jepp books, and they really don't explain it all that well. I am just about ready for the checkride, and havent taken the written yet.
 
i guess my confusion was if you are inbound to the station, or flying outbound from it.. she told me to turn 90 deg away from the arc, before turning onto it.
 
sferguson524 said:
I'm an instrument student and am pulling my hair out over DME arcs. My II was telling me that there is a time that you turn away from the arc before turning onto it, and i have been drawing it out, flying it in the sim, and still can't wrap my head around it. We even went out and flew it in the airplane, and i got it then, but promptly forgot. HELP!!! :mad2::mad2::mad2::mad2:
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Two tips:

One thing that will help is to always fly a number (heading) and not just "that way". If you turn 10 degrees, turn 10 degrees TO something. Think, "Ok, 10 degrees right to 130." If you don't know what number you're supposed to be flying it makes it hard to recover if you let your heading wander a bit.

The second is the thing that I find the *most* useful when flying an arc. Watch the groundspeed readout on the DME. It essentially acts like a lateral VSI for maintaining your arc. If the groundspeed is zero you are flying directly parallel to the VOR, so you will maintain your current distance from the station. If you need to get closer, turn toward the VOR (as Tony said). You can watch your groundspeed on the DME to see the rate at which you are getting closer. Over time, you will learn to use that groundspeed, in combination with the distance readout, to nail the arc.

So, let's say that you're flying a 10 nm arc and you're right at 10.0 DME and your groundspeed is zero. You are exactly where you want to be. However, if you keep flying that heading, over time, you will fly outside of the arc and your groundspeed reading will start increasing ever so slightly. If it gets up to 3-5 kts, then turn 5-10 degrees toward the VOR to get the groundspeed reading back to zero. Obviously, if the distance number increments in one direction or the other, you will need to adjust for that as well.

My sadistic CFII had me fly the the "VOR/DME or TACAN Z RWY 15" into KMTN last night in the sim. It requires you to fly a 14.7 nm arc for around 22 miles while hitting your step downs at certain intersections/radials. Toward the end of the approach, I would just watch the groundspeed, and if it would start creeping up from zero, I would make a small adjustment to my heading to "fix" it. By the end of the arc, I was dead on at 14.7 and right at my altitude while crossing the MAP (this was my short-lived moment of elation moments before I completely botched the missed approach procedure :mad2:).

The groundspeed number really helps when you're trying to recover from a botched arc. Let's say you mess up somehow and get .5-.7 DME away from your arc and you need to get back to it. Well, you're going to take a big fat cut toward the arc. Say, 30-40 degrees. Without the groudspeed number it's hard to know the rate at which you're closing in on your arc. However, if you pay attention to the groundspeed reading it will tell you everything you need to know. Let it get up to 30-35 kts while you close the gap. Then, when you're about .2 away, start lessening the cut and watch the groundspeed number decrease. Your goal being to get the groundspeed back to zero right as you get back to your distance number.
 
sferguson524 said:
i guess my confusion was if you are inbound to the station, or flying outbound from it.. she told me to turn 90 deg away from the arc, before turning onto it.
Click to expand...

It sounds like you're over-thinking it.

If you look at the plate you'll see right there which direction you need to turn to enter the arc. Generally (but not always) your turn onto an arc will be a 90 degree turn. Start that turn about .5 DME away from your number and turn 90ish degrees to acquire the arc.

So, let's say you're flying east on the 090 radial and you're about to enter the 15 DME arc. You're either going to be flying right or left when you get there. At 14.5 DME look at your heading and start a 90 degree turn toward the arc. You may undershoot it or overshoot it, but you can use my groundspeed tip (above) to help you dial it in.

Don't overcomplicate it. It's just like flying a VOR needle. You're either right or left of your "course". Just track it like you would fly a needle inbound to a VOR. The DME distance is your needle offset and the groundspeed readout is your clue to the as to how much you are flying toward the needle (just like your heading is the rate at which you're closing in on your OBS setting).
 
tonycondon said:
if the number is getting smaller turn away from the VOR, if the number is getting bigger turn towards the VOR. make no more than 10 degrees of heading change at a time, then pause and wait to see how that change affects the response.
Click to expand...

I did the turn/twist for years, now I am a big fan of this method. Much easier and gets the job done every time. I usually start off by turning 90-100 degrees into the arc(know what the wind is doing), then just set a bank angle and fly the plane....Keep it simple !
 
taters said:
I did the turn/twist for years, now I am a big fan of this method. Much easier and gets the job done every time. I usually start off by turning 90-100 degrees into the arc(know what the wind is doing), then just set a bank angle and fly the plane....Keep it simple !
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Turn 10-twist 10 is a nice training method but it tends to instill rote behavior that distracts you from the more obvious making turns based on the DME increasing or decreasing.

FWIW, the method I use 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.

DME arcs are a little like holds - much easier to fly than the way they tend to be taught.
 
FlyNE said:
My sadistic CFII had me fly the the "VOR/DME or TACAN Z RWY 15" into KMTN last night in the sim. It requires you to fly a 14.7 nm arc for around 22 miles while hitting your step downs at certain intersections/radials.
Click to expand...

what an A-Hole
 
FlyNE said:
I couldn't agree more.

This approach was easy compared with the ILS 13 at KPVU (beginning at FFU) which I also had the pleasure of flying last night.
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They both look like fun, time to book some sim time I think.
 
TangoWhiskey said:
LOL! Jason, thanks for the DME groundspeed trick. I've not heard that little tidbit before. I'll have to pay attention to that next time I fly or practice an arc.
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It really is as simple as it sounds. Once it clicked, DME arcs were cake. My $.02 is that "turn 10 twist 10" and similar systems where you break the arc down into tangents of straight lines are just WAY too much work in relationship to how simple DME arcs really are.

I got this trick from Day 7 of Joe Campbell's "Diary of an IFR Ticket". It's a fantastic read for aspiring instrument pilots. Hell, I'd argue that it's a great read for seasoned instrument pilots. I had read it before (a long time ago) and didn't "get" much of the content...but bookmarked it. I went back to read it after about 10 hours of instrument training and couldn't stop reading it.
 
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FlyNE said:
It really is as simple as it sounds. Once it clicked, DME arcs were cake. My $.02 is that "turn 10 twist 10" and similar systems where you break the arc down into tangents of straight lines are just WAY too much work in relationship to how simple DME arcs really are.

I got this trick from Day 7 of Joe Campbell's "Diary of an IFR Ticket". It's a fantastic read for aspiring instrument pilots. I had read it before a long time ago and didn't "get" much of the content...but bookmarked it. I went back to read it after about 10 hours of instrument training and couldn't stop reading it.
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I remember reading that diary when I was getting my ticket, but I don't remember the tip. And I totally agree, it's a great read! I might have to download it and read it again.
 
FlyNE said:
It really is as simple as it sounds. Once it clicked, DME arcs were cake. My $.02 is that "turn 10 twist 10" and similar systems where you break the arc down into tangents of straight lines are just WAY too much work in relationship to how simple DME arcs really are.
Click to expand...

If they are taught properly, DME arcs are not that complicated. My preferred method for DME arc completion is to use an RMI or a bearing pointer if available (keep the pointer on the left or right wingtip and watch the DME readout, adjust heading accordingly). I teach my instrument students how to fly a DME arc using a traditional nav receiver, an HSI, and an RMI. Drawing it out relative to the aircraft position is also handy (just like hold entries).

The instrument used to complete the arc isn't necessarily important but I find it worth while to use different methods (HSI vs. RMI) to reinforce the mechanics of the procedure. The flight path you want to make is the same regardless of what instrument you use to fly it.
 
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FlyNE said:
Two tips:



My sadistic CFII had me fly the the "VOR/DME or TACAN Z RWY 15" into KMTN last night in the sim. It requires you to fly a 14.7 nm arc for around 22 miles while hitting your step downs at certain intersections/radials. Toward the end of the approach, I would just watch the groundspeed, and if it would start creeping up from zero, I would make a small adjustment to my heading to "fix" it. By the end of the arc, I was dead on at 14.7 and right at my altitude while crossing the MAP (this was my short-lived moment of elation moments before I completely botched the missed approach procedure :mad2:).
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That approach is famous 'round these parts. I hold that it was created to make instrument students quit! I can't imagine anyone actually flying it. BWI is 5 minutes away--with lots of nice ILSs, GPSs, and even CAT III approaches!
 
There is always the arc around the VOR, and then you turn outbound. Very rare but it happens, I forget where the approach is, but IIRC, there is a 14DME Arc then turn away from the VOR. The MAP is at about the 22DME point.
 
Yeah, but that one is GPS. Just follow the MLOD :eek:


When I flew DME, I just set the OBS to whatever my final inbound radial was off the arc, and worried about the distance. If the VOR/DME/TAC is off to my left, and I show less than the DME distance, correct 10 to the right. If I show more than the DME distance, correct 10 to the left. Don't worry about the radials every 10 degrees, just include your CDI in your scan, and when it starts to center make your turn to the next segment.

I didn't worry about ground speed, I didn't worry about heading after joining, just altimeter and distance until the CDI centers. There's nothing else that needs to be referenced.
 
FlyNE said:
My sadistic CFII had me fly the the "VOR/DME or TACAN Z RWY 15" into KMTN last night in the sim. It requires you to fly a 14.7 nm arc for around 22 miles while hitting your step downs at certain intersections/radials. Toward the end of the approach, I would just watch the groundspeed, and if it would start creeping up from zero, I would make a small adjustment to my heading to "fix" it. By the end of the arc, I was dead on at 14.7 and right at my altitude while crossing the MAP (this was my short-lived moment of elation moments before I completely botched the missed approach procedure :mad2:
Click to expand...

Come visit and fly it for real... It's a fun approach. My original CFII used it as a final exam.
 
Look at the ILS LOC/DME to rwy 13 to Provo UT. Start at the IAF at Fairfield (FFU), plan your radios well, out bound on one VOR, arc from a second VOR, to intercept the ILS, the Missed needs two VORs and goes back to the first VOR. Add to that, the DME on final is on the LOC frequency.

VOR FFU, arc PVU, LOC DME I-PVU, missed VOR PVU, then to FFU.
 
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