Flying DME Arc - Raw Data

[mfclearner]

Any tips on flying B737 DME Arc Raw Data. Having trouble deciding headings during arc and at what intervals i should change heading so that my RDMI always points at 90 degrees. Whats prefered mode VOR Or Centered VOR etc.

[aviatorhi]

You can do it in "cheat mode"...

FIX page, input the VOR you want to arc around, then put (for example) /10 to draw yourself a 10 mile circle on your MAP display. When you join the arc, put the A/P in CWS Roll mode and put your position trend vector "on top" of the circle you drew. The plane will do the rest. You can still monitor raw data on your "steam" RMI, there's nothing that says raw data is only available on the CRT.

If you want to do it in "real pilot mode"...

Use the RMI, and after joining the arc it all comes down to splitting hairs, taking into account the winds, make the appropriate correction (needle in front of/behind the wingtip) and then it just comes down to old fashioned flying "splitting hairs" to get the needle to stop moving from it's bearing off the wingtip.

At the speeds you should be flying at in the 737 it's much easier to keep a constant turn going rather than constantly going back and forth from -1 to +1 DME and making a heading change over and over, that technique is more for small aircraft which don't cover ground fast enough to make for easy arcing.

[italia458]

Hand flying an arc and keeping your needle at 90 degrees the whole time is virtually impossible. To do that it would be best if you had no wind, and then you'd have to fly the airplane in a precise bank with an accuracy of about +/- 0.05 degrees throughout the whole arc. If you did have wind, you'd have to know exactly what bank angle you needed at all times, and of course be able to maintain it with great accuracy. For a human pilot, that doesn't work too well.

I first start my students off in the sim with no wind. Then I get them to intercept the arc. If you're flying perpendicular to the arc as you intercept (90 degrees), start the turn 1% of your groundspeed in NM from the arc. So if you're grounding 140 kts, start your turn at 1.4 NM. If it's a 12 DME arc, you would start your turn at 13.4 DME. This is a rule of thumb but works pretty well.

Once you're on the arc and at the proper distance (12 DME), start with the needle at the 90 degree postion. Continue until the needle is 5 degrees behind you. Turn 10 degrees in the direction of the arc so that the needle is now 5 degrees in front of you. Continue on that heading until it is 5 degrees behind you and repeat until you finish flying the arc.

If you have wind, you'll need to be thinking about what angle you'll need to crab for every 10 degrees of heading that you turn. This should be what you're doing anytime you're tracking an airway or direct to a beacon or whatever - always consider where the wind is, turn into the wind a few degrees (whatever you consider appropriate), and then monitor your track to make sure you applied the correct crab angle.

[Checkboard]

Pah!

Make sure your display is in "Rose", not "Arc".
Start with your VOR needle at 90º
Fly a constant heading, watch the needle pointer drift "behind" you
When it is 10º behind you (i.e. on a relative bearing of 100º(RHS)/260º(LHS) ),
Turn towards the needle by 20º (so that it is 10º "in front" of you)
Fly a constant heading, watch the needle pointer drift "behind" you
repeat ad infinitum

[mfclearner]

Those tips are really useful... thanks.

[Deano777]

mfcleaner

Contrary to what's said above you can keep the arc to well within 0.1 of a DME accuracy quite easily. Checkboard & italia458 has it about right, except you need to be "wind aware" which will change the basics slightly. I wrote a DME tutorial for one of my students once, if you'd like it send me a PM and I'll see if I can find it. I am sure it's on a hard drive somewhere.

D777

[Sciolistes]

at what intervals i should change heading so that my RDMI always points at 90 degrees.
As said above, you're not trying to keep the needle at 90º. All good tips, but personally I find chasing the VOR needle too much like hard work on a windy day, it needs to look sensible, but working with your DME distance and rate of closure pretty much corrects for wind and is for me much easier to assess.

I intercept the arc, turn, fly straight and see what happens. If you got your heading right you'll drift 0.2 DME inside the arc and then 0.2 DME outside the arc. When it reaches that point turn and fly straight to repeat. If you get to 0.3 DME inside or outside the arc, correct the heading. If you have a functioning FMC on the 737, then the wind arrow on the EHSI/ND removes the guesswork.

Personally speaking, flying a tighter arc diverts too much attention from other activities like checking FMAs, descending, setting flaps, speeds, monitoring traffic, etc. Flying looser and you run the risk of drifting a 1nm off the arc too easily.

[italia458]

Deano777... I'd be interested in that tutorial if you happen to find it. I'm always interested in different ways to teach things!

Cheers!

[juliet]

Is this seriously a 737 pilot asking how to fly a DME arc? How do you get to fly a Boeing but not know an IFR fundamental like flying an arc???

[aviatorhi]

Sounds more like he's just having a hard time keeping up with the speed of the jet and trying to get some insight into how others do it.

While many stick to it, the 10-20 degree turn when the DME starts moving is not a good technique at 150+ knots.

[sheppey]

You don't need to be too precise with the DME.

Don't try that in New Zealand where the instrument rating test tolerance during a DME arc is plus or minus one DME....:ok:

[PantLoad]

At typical ground speeds for min clean speed for both the 737
and Bus....assuming the usual arc of 12 miles to 15 miles....
once established, hold maybe four or five degrees of bank....

No kidding...

More if a smaller arc....less if a bigger arc....more with higher
ground speed....less with lower ground speed....

But, around four degrees...plus or minus a bit....
will hold your arc plus or minus a few tenths.

Try it....


Fly safe,

PantLoad

[aterpster]

sheppy:

Don't try that in New Zealand where the instrument rating test tolerance during a DME arc is plus or minus one DME.

That's generous. It is 1/2 mile in the U.S.

[arismount]

In the U.S., for an instrument rating, the standard is +/- 1 NM on the DME Arc:
http://www.faa.gov/training_testing/testing/airmen/test_standards/media/faa-s-8081-4e.pdf

[FullWings]

I remember the first time I tried a DME arc during initial training, thinking I was doing pretty well... Until I heard muffled laughter coming from the instructor on the other side of the cockpit. I'd gone most of the way round the airfield and was now approaching the runway centreline from the other side - talk about fixation!

[Junkflyer]

go to faa.gov and look under the manuals section. The instrument flying handbook is in pdf form-chapter 7 for the dme arc, nicely presented.

[aterpster]

arismount:

In the U.S., for an instrument rating, the standard is +/- 1 NM on the DME Arc:

I stand corrected. I believe it was +/- .5 NM on the ATP practical a very long time ago. Note DME ARCs are no longer on the ATP PTS. Likely an ATA campaign.

[bubbers44]

We did these 15 DME arcs all the time arriving from the north at Kingston, Jamaica. It was a lot of fun in the 727 at 250 knotts. I just put my wing on the vor and pretended I was a Ucontrol airplane with a wire streched out from the wing. Staying a few 10ths of a mile from arc was easy at any speed. I agree 2.5 miles at 250 knots is a proper lead time as it worked for me all the time. I just loved the automation in that 727. It was so primitive it made flying fun.

Next I enjoyed the 757/767 era planes and am retired but loved the old stuff.

[CJ Driver]

I am slightly surprised that no-one has mentioned the basic technique that I was taught.

Keeping the VOR at 90 degress, plus the DME indicating the correct distance are the primary outcomes of a correct arc, but the performance indication that you need to actually look at is the DME speed (which is the rate of change of distance). If you are maintaining the arc, the speed will be zero. If you are off the arc, the speed will start to creep up - simply turn in (or out) as required to put it back to zero.

[aterpster]

CJ Driver:


Keeping the VOR at 90 degress, plus the DME indicating the correct distance are the primary outcomes of a correct arc, but the performance indication that you need to actually look at is the DME speed (which is the rate of change of distance). If you are maintaining the arc, the speed will be zero. If you are off the arc, the speed will start to creep up - simply turn in (or out) as required to put it back to zero.

These DME speed indicators are few and far between.

[Slasher]

I use the +5/-5 method from 090/270R. Works just fine.