DME Arc

[ChrisM]

Hi...how do i fly a DME arc  Would like to know so i can do the arc on approach plates in planes without an FMC.  

Thanks....Chris

[Nexus]

takes practice and practice.
Depending on the radius of the arc, you need to bank at a certain angle to always keep the same distance from the navaid. And throw in some winds to the mix and you're in for an interesting approach to say the least. But the essential is obviously to have a  constant relative bearing to the Navaid, and the same DME aswell.

The procedure will have a marked "break in" point where you will steepen the turn to get aligned with the runway. But dont expect to succeed on the first attempt, because these appoaches are pretty darn hard I think.

[OTTOL]

A panel with an RMI(or a moving map GPS  ;D  ) is key to a painless arc approach.
With a Radio Magnetic Indicator(the thing that looks like an ADF indicator, if you're not familiar), you follow the needle straight to the station.
When you're within several miles of the arc range(two miles for most jets and one mile for props)begin the 90 degree turn to join the arc. The turn ends with the needle pointing to the wingtip(right or left depending on which direction you're arcing).
Nexus relayed what I think is the best method of performing the procedure(a constant bank turn).
You will hear a lot of people say that "it is not an arc but a series of small turns." These are people who have learned on a standard VOR indicator, in which case the OBS is advanced every ten degrees, making it appear (mechanically) as a small series of turns.
Once established on the arc, you have three elements to monitor: distance from station, proper altitude for the approach segment and lead in radial/final course.

-DME is necessary for the approach. In this "constant bank OR series of turns", bank is increased when distance is too great and decreased when distance is too little. This is the key reason that I think the "bank" method is simpler. Maintain about 3 degrees wing down toward the station increase or decrease two-four degrees, as necessary to maintain your distance.
- Altitude usually is a series of descents based on distance or radial. There can be minimum or mandatory altitudes on an approach.
-When you are close to the final approach course you will be reminded by a lead in radial. This will be established ten degrees from the final approach course. (ie: 360 degree final course(180radial) will have lead-in radials of 170 and/or 190. When you cross this radial you will begin the next 90 degree turn to join the final.
.................make small corrections in bank

    ........practice with no wind first(that's a whole different class all together!)  ;)

[ChrisM]

Thank you..

[OTTOL]

Thank you..  I think i have an understanding now.  Forgive me if you explained this, how do i do the harder one with the OBS on the standard VOR indicator?

Use the JAC (Jackson Hole) arc as an example.

We'll say that we're
approaching from the South at 13,000 feet. First we'll look at the MSA for the sector. Look at the MSA circle on the right side of the plate. At 13,000 feet, we'll be safe until we're established on the approach.  
I would like to be established before I'm on an approach segment(in real life, ATC would vector you), so I'll make a right turn and join the JAC 143 degree radial and if I'm within 25nm, I'll descend to 12,400(in accordance with the MSA). Set the reciprocal(323 course, so that you have proper sensing) using the OBS.  When the course needle centers, we'll make a left turn to about 320 degrees.  At this point we should be heading toward JAC-VOR on the 143 degree radial.
The DME will be my next focal point. When the DME indicates 17DME(I'll assume you're using a slow aircraft for your practice), I'll start a left turn to join the arc. If you have a heading bug, "life is easy". Most light training aircraft(in the US anyway)don't have a bug.   Set the bug to 233 degrees(a 90 degree left turn). This should put us on the arc over EBACA. Once the DME indicates 16 and the turn is complete we can also descend to the segment altitude of 11,900 feet.
This is where the big difference between VOR and RMI becomes evident.  
Use the OBS to set a 333(or round to 330) course. After the needle centers again, you'll rotate the OBS to indicate 340..then..350 ...and so on....and so on...  If you look on the opposite side of the arc, you'll see R-198 indicated. This is the lead-in radial and tells you that it's time to start another 90 degree turn to join the final course. I can't tell you why there isn't one on the East side(perhaps it has to do with a non-standard descent angle) but I would assume the 358 course(178 degree radial) as my led-in from the East side.
Once you've joined the final course, it's just a standard VOR approach.  

EDIT: and with the RMI, you make small corrections to keep that pointing at you wingtip, correct?

....with no wind...YES...BUT....the DME is still your primary distance indication. With a wind, the needle may be fore or aft of the 90 degree point and the amount of crab required will change depending on how much arc is travelled for the approach(a constant direction change relative to a fixed wind direction).

[ChrisM]

Thanks OTTOL

[Ben_M_K]

Wow, you guys are all WAY over my head!

[Boss_BlueAngels]

We were tought to do it tune 10 turn 10.  Every time you rotate the OBS 10*, you will turn 10*.  Another complication comes into hand whether you arc with a TO or FROM with the OBS.  I do it with a from, but also have done it a few times with a TO.  There are pros and cons to each.  When arcing with a FROM, you always know what radial you're on, and you turn the OBS in the same direction you make your turns.  Arcing with a TO you always know where to turn if you need to go direct to the VOR.  

As with the constant angle of bank... don't even think about trying that in a slow prop!!  At least with something like a 16 DME arc, those take forever.  My home airport has a 7DME arc and that baby goes by real quick.  

As for judging the turn to intercept the arc, or turning inbound, just take 50% of your groundspeed and move the decimal...  for a 100kt groundspeed, you want to make the turn at 0.5NM out.  Works perfectly.

It's not really that complicated once you've seen it done (or actually done it).  

And ugh, government charts... lol

[OTTOL]

Wow, you guys are all WAY over my head!

Despite the terminology overload, it's really pretty simple. You should try it. It's excellent instrument and situational awareness training.

1-Position the aircraft at Jackson Hole(KJAC) rwy19 and tune the JAC-VOR/115.4 on the No.1 NAV.

2-Takeoff and start a slow turn to the south(yellow line).

3-Climb to at least 12,400'.
..... I also created a map of the route over the MSA circle on the right,  to show can see which sector you'll be in.

4-Once you've climbed to 12,400' or more and at least 25DME-JAC, make a left turn to 060(purple line).

5-Use the OBS(VOR knob)to select the 323degree course.
....This will set  you up to join R-143(orange circle).

This may seem confusing at first. If you look at all of the depicted radials(R-143,167,188,198,209), they orient the pilot FROM the station. You aren't going to fly FROM(radial) the station right? You have to fly TO(course) the station to land don't you? The FINAL COURSE is a perfect illustration of this.
Look at the two green circles.  You must fly inbound on the 188 radial (R-188 ) but you have to set a 008 course (reciprocal)  to fly the approach if you want proper sensing on the VOR because you're heading 008 degrees.
6-When the VOR needle centers, make a left turn and join  R-143(323 course)(blue line).

7-At 17DME(unless you've chosen a fast airplane, then it'll be 18DME)start another left to join the arc over EBACA. Stop the turn when you're heading 230.
......As you were tracking inbound on R-143, you're heading should have been in the neighborhood of 320 degrees. A 90 degree left turn (320-90=230) puts us on a heading of 230.

8-Once you are heading 230 and the DME reads 16 you can descend to the altitude appropriate for this segment(magenta line), which is 11,900'.
...if you've never done this before or feel overwhelmed, I would pause the sim and setup for the next portion or just get your bearings straight
9- Advance the OBS another ten degrees (333 degrees).
.....obviously, if you've been paying attention 343 would be the next progression but R-167(347 degree course) is required to indentify the next segment (brown line) and descent, so I would just jump an additional four degrees(now 14 degrees instead of ten) and set 347 instead of 343.

10-After crossing R-167 descend to 10,500'.

11- Set the course indicator to 358.
....this will identify the 178degree radial(grey line)and let you know that you are only ten degrees from the final approach course.

12-When the needle centers on the 358 degree course, start a 90 degree turn to the right to join the final and set the final course of 008 degrees.

13-Once you're established on the final course, descend to 9,800 feet. Follow the 008 degree course and descend at each segment(DME distance) as the profile view(red box) calls out.

....don't be afraid to pause and check things out or run the sim at a slower speed

[OTTOL]

And ugh, government charts... lol

I don't use anything but JEPPS when I fly but when I'm at home and need to reference a plate, it's convenient just to grab one from fltplan.com.

And the best part.

My favorite word.....

                                   
[glb]F-R-E-E!   [/glb]

Sure beats the $1500 for Jeppview America's coverage!

[Boss_BlueAngels]

lol, I hear ya on the free!  And speaking of Jepp charts, for some reason i just stopped getting my Jepp chart updates in the mail... haven't gotten one since like mid Feb!  Would probably be a good idea to look into that.

[beefhole]

lol, I hear ya on the free!  And speaking of Jepp charts, for some reason i just stopped getting my Jepp chart updates in the mail... haven't gotten one since like mid Feb!  Would probably be a good idea to look into that.

What? Naaaah.  Just fly with old charts, like all the cool kids.  Don't you want to be cool?

[OTTOL]

What? Naaaah.  Just fly with old charts, like all the cool kids.  Don't you want to be cool?

....while on the subject.....I guess it's time for me to update my simulator!       I wanted to give my lesson a test run and found out that '02 still has the old Low Altitude frequency in the database. Ergo, if anyone is trying this at home and using an older version, 108.4 is the old JAC frequency and runway 19 used to be 18.

As with the constant angle of bank... don't even think about trying that in a slow prop!!  At least with something like a 16 DME arc, those take forever.  My home airport has a 7DME arc and that baby goes by real quick.  

 ...that's a tough call.   I tried it with the default Baron and flew the arc at 120kts and two degrees bank and maintained (+/-) .2nm. I can see how maintaining less the two degrees consistent bank might be a bit of a task(flying something slow like a 172).
The problem that I have with the ten and ten technique is that when you transition to larger and faster aircraft, it turns the approach into a sloppy mess. There are a lot of techniques, styles and equipment that I disregarded or bad mouthed as a young instructor, that I later found to be absolutely crucial in  bigger airplanes.
What that method creates is a  continuously fluctuating DME value. If the wind aloft is zero then the cycle can be predictable(on the arc initially, inside the arc and then on the arc again when the next ten degree point is reached).

Because the wind is usually never calm at altitude and the fact that it's relative direction changes throughout the approach means that what is essentially that technique's "acceptable" level of inherent deviation adds to the overall deviation created by dynamic factors. In laymans terms, "you'll be chasing the arc".
Try both techniques in something a little faster like the 737 or Lear45.

[Boss_BlueAngels]

Lol, holding 2 degrees angle of bank.  I'm talking about flying a Piper Warrior here, man!  Sure its possible to keep that kind of precision in smooth air, but as someone who is 99% through his instrument rating (RL) that 2 degrees is virtually impossible, especially having to hold it for 15-20 minutes at a time.  And where I fly, if there is ANY wind you're tossed around like a rag doll. :p  That kind of precision will have to come a bit later for me. lol

But, I'll try the constant thing next time I'm on a sim... certainly sounds easier than chasing the arc.  

[jrpilot]

Is it just me or does a DME Arc seem like alot of work in just a few minutes/secons  :o

From setting up the radios and such to the OBS and distances.  I guess in a slow aircraft it would not be as bad but suppose you are in a B1900D how does the pilots set this up in such a short period of time, also what about pilots who are by themselves?