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Old 20th Apr 2000, 14:36   #1 (permalink)
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Unhappy DME Arc on a VOR - How ?

This is probably outrageously simple..my apologies if it is but i havent been able to find any info on how to do it..

How do you do a DME ARC around an entire VOR station ?
I know how to do one around an NBD using the ADF but cant figure out how to do one on a VOR.
I recently got my PPL and am starting to take my first few steps into the wonderful world of pure Instrument flying so again apologies if this is real easy stuff..

Thanks in advance
Old 20th Apr 2000, 15:38   #2 (permalink)
New Bloke
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Okay, first off, I am not an instructor so I may be shot to bits about this, but this is how I do it.

First fly 90 degrees to the radial that you first intercept in the direction of the arc. Set the ODB for 10 degrees further on, wait until the needle centres then fly 90 degrees from that radial and so on and so on. It isn't a true arc, more like a polygon, but it works.
Old 20th Apr 2000, 17:48   #3 (permalink)
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Thumbs up

Can be most easily done with an RMI VOR needle - behaves like an ADF needle
Old 21st Apr 2000, 03:10   #4 (permalink)
Dan Winterland
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If using an RMI, the neddle should be pointing at the wingtip, ie at 90 degrees. (Imagination required for swept wing aircraft). If too close, put the needle just behind the wingtip. If too far away, put the needle ahead of the wingtip. When on the arc, just set a turn at a couple of degrees aob and use the TLAR method to maintain. Easy!

(TLAR = That Looks About Right)

Old 21st Apr 2000, 12:47   #5 (permalink)
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 3,772

The idea when flying a DME arc is, first, to stay within two miles, and preferrably second not to fly the arc with either a constant angle of bank, or many small angles.

Why not? Because a constant angle of bank will eventually cause the artificial horizon to re-erect in the turn, causing an error than will be a problem later. The artificial horizon is self-erecting, and in the turn the horizon will "erect" in the turn (as a balanced turn results in the total force being experienced through the floor of the aircraft, regardless of bank.)

How to avoid this? With an ADF needle, once you have turned onto the arc set the needle about 10 forward of the wing, then hold the aircraft level until the needle is 10 aft of the wing, then turn 20 and continue.

With a VOR display you do the same thing. Set the OBS to the initial radial you are on (ie centre the needle) then fly level until the needle is at full deflection (10 off the radial), turn the OBS 20 (ie swing the needle to the other side of the display) and fly level until the needle has crossed for that side of the display to the other side, and adjust the OBS another 20 etc.
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Old 22nd Apr 2000, 08:55   #6 (permalink)
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Ok Teroc I will try to keep it simple. If you can get hold of any approach plate with DME arc that will help you understand this, here we go:

1) Tune and identify the VOR/DME to be used.

2) Determine position and distance from the facility using all available equipment.

3) Check (minimum every 10-15 mins) and set your Direction Indicator with Mag Compass.

4) Track inbound or outbound on the specified radial.

5) Check the DME readout frequently while tracking to the arc.

6) When within 0.5 mile of the desired arc, commence a standard rate turn in the direction of the arc rotaion (clockwise or anti-clockwise).

7) Turn 90 degree from the radial specified in step 4.

8) Center the needle on the #2 VOR with a "FROM" indiacation. Advance the OBS 10 degrees in the desired direction.

9) Monitor the DME readout to hold the desired distance. If the readout increases turn into the arc by approx 10 degrees. If the readout decreases turn away from the arc.

10)While established on the arc, change the aircraft heading 10 degrees towards the inside of the arc each time #2 VOR needle centers.

11) Continue to reset the #2 VOR 10 degree each time the needle centre while changing the aircraft heading 10 degree as that radial is crossed.

12) Before crossing the lead-in radial(LR) for the approach, tune and identify the #1 VOR to the approproate frequency and adjust the CDI to the inbound approach course. If the approach does not have a lead-in radial, use a 3-5 degree lead radial.

13) Upon reaching the lead-in radial, turn to a 45 degree course intercept heading. COmplete the course intercept as the #1 navigation CDI begins to centre.

phhhheeewwwwwwww I hope Teroc all this make sense. The DME arc's are designed for specific approach and it could be and usually is different for different approach.

[This message has been edited by DesiPilot (edited 22 April 2000).]
Old 23rd Apr 2000, 00:34   #7 (permalink)
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Some other rules of thumb, with all turns at Rate 1.

1. Lead distance to turn onto the arc from an inbound or outbound radial/bearing (nil wind. Use groundspeed if head or tailwind is significant)

TAS / 200 = nm lead to commence the turn

eg inbound at 180 kts TAS = 0.9 nm lead DME dist to commence the 90 degree turn onto the ARC

2. Lead BRG to commence the turn inbound on final:

Using a lead distance determined using method 1.

60 / DME ARC dist x lead dist = degrees lead needed to commence the turn

eg. 120 kt TAS, therefore Lead dist = 0.6 nm, DME ARC = 12 nm

so, 60/12=5; 5 x .6 = 3.6 deg ie approx 5 deg (rounded). Start the inbound turn 5 deg of arc prior to the inbound radial or track.

This is particularly easy if you have an RMI or RBI. When the head of the needle is at or just after 5 deg above the abeam/wingtip position, commence the turn.

3. On the initial turn to establish on the ARC you will need to turn approx 10 deg more than 90 deg if joining from an outbound track, or approx 10 deg less than 90 deg if joining from an inbound track.

4. Once on the ARC, it can be flown entirely using the DME ie without reference to an azimuth aid. I refer ONLY to maintaining oneself on the arc, NOT determining position around the arc (although that is also possible).

As mentioned by Checkboard, the ARC should be flown as a series of straight segments. You should then find the following:

a. Mininum rate of closure will be when the aircraft's track made good is at 90 deg to the direct track to the DME. This corresponds to the minimum distance to the DME in that particular straight segment.

b. Commence your turn onto the next straight segment prior to reaching the DME ARC distance ie try to keep yourself inside the ARC. That way the arc is always curving to intercept your straight flight path. This means it is self correcting regarding containment within the tolerance. If you get onto the outside of the ARC then it is constantly deviating away from you. This will eventually cause you to exceed the tolerance - unless you adjust your HDG to correct it, of course.

c. You can determine if each of your HDG adjustments is sufficient because if it is, then you should see a rate of closure develop that then reduces as you close with item 'a.' above.

With practice you can contain yourself within the half nm or so inside the arc.

I rather like DME ARCs. Very easy & can be efficient if not too lengthy. With good design they can also have step down altitudes that get eliminate having to lose height in the holding pattern.

Hope this helps.
Old 23rd Apr 2000, 02:47   #8 (permalink)
Grandad Flyer
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Teroc, Congratulations on getting your PPL! I remember your very first posting on this forum....
DME arcs, most people hate them, but there are still a few airfields that have them in their procedures.
The best way to fly a dme arc? Buy an Airbus, select the relevant dme arc in the procedure at the airport, then sit back and watch in awe as the aircraft flys a beautiful dme arc with no variation on the distance at any time. Damn!

Old 23rd Apr 2000, 13:02   #9 (permalink)
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Grandad, you're quite right, but remember that there's no reason not to use a GPS for this if it is an approved one and you know what you're doing (and always back up with raw data).

Also, why not ask to route to the centreline fix, and save all that time wasted flying in turns when you could be going straight?
Old 23rd Apr 2000, 19:25   #10 (permalink)
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 3,772

G'day Tinstaafl.

With the TAS/200 lead distance thing, this works pretty well around the 100+ knot mark, but for a more accurate number you can use the circular slide rule:

For a rate one turn:

    TAS Radius of turn
    400 TAS

(TAS in knots, Radius is produced in Nautical Miles)

The radius of turn will also be your lead distance, although the above calculation assumes that you are already established in a rate one turn, so as it takes a few seconds to establish the turn you need to add a bit for higher accuracy. You also need to adjust a bit for a tail or headwind.

As you can see:
  • at 60 knots, the radius is 0.09 NM (TAS/200 gives 0.3 NM)
  • at 100 knots, the radius is 0.25 NM (TAS/200 gives 0.5 NM)
  • at 160 knots, the radius is 0.63 NM (TAS/200 gives 0.8 NM)
  • at 200 knots, the radius is 1 NM, (TAS/200 gives 1 NM)
  • at 250 knots, the radius is 1.8 NM. (TAS/200 gives 1.25 NM)

The TAS/200 approximation is simple, and as such works well in your head. At the lower speeds it also allows that little extra to roll into the turn, and as the required performance on the DME arc is 2 miles the extra accuracy really is academic...

...but then I am an academic kind of guy

P.S. I actually worked this out a while ago and my notes (scribbled on a scrap of paper) are at work, so I cannot remember if I used a rate one turn, or a 25 angle of bank turn (and I suspect the latter).
Tech Log forum moderator

[This message has been edited by Checkboard (edited 23 April 2000).]
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Old 23rd Apr 2000, 22:47   #11 (permalink)
Grandad Flyer
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FokJok - because sometimes slower airplanes get in the way (Fokkers, perhaps!!!), there is no radar at the airport and so we have no choice.
Of course if we can go direct to the centre fix we will, no-one flys a dme arc unless a) they have to or b) they are a masochist.
Old 25th Apr 2000, 01:06   #12 (permalink)
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A DME arc can save track miles depending on just where you can join the arc, compared to flying all the way to the aid, conducting a sector entry, flying the outbound leg etc.

Checkboard, nicely put. I also used to have calculations done at different speeds to justify the rule of thumb. Unfortunately they're now somewhere in storage half a world away.

Now, I wonder where I put my whip & handcuffs...

[This message has been edited by Tinstaafl (edited 24 April 2000).]
Old 25th Apr 2000, 08:13   #13 (permalink)
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Hmmm... I went and got interested in this, and then couldn't stop until I'd worked it out! Anyhow, your calculations do presuppose a fixed angle of bank.

According to the geometry of a banking aircraft:
v x v / r = g tan(AoB)

Your view of a flight computer shows a calculation in progress
v x v / r = 400
which indicates a fixed AoB (Angle of Bank).

But for slow aircraft sticking to a simple Rate 1 turn:
r = v / 188.5 using aviation units.

So the v / 200 suggested above will work reasonably well assuming the wind is not too strong.

[This message has been edited by Oktas8 (edited 25 April 2000).]
Old 25th Apr 2000, 14:49   #14 (permalink)
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Thanks all...very, very much Appreciated....

Hey Grandad Flyer,
Thanks for the Congrats..How are you ?
I too remember my first post..it seems like a long long time ago . Thanks to all the wonderful answers i received on this forum from yourself and the other good folk i managed to get through the theory and practical aspects of the PPL..still doing the odd bouncy landing though .next stop the CPL/IR and ATPL writtens... Anyone got a loan of 50,000 ?
Cheers all
Happy flying and clear skies,
Old 27th Apr 2000, 20:06   #15 (permalink)
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Thanks for that....Tried it out yesterday on a vor using the methods you suggested..
Was meant to hold it at 9 miles but varied between 8.4 and 9.3 all the way around...
Just needs some tightening..

Thanks again
Old 29th Apr 2000, 10:30   #16 (permalink)
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Flying the arc to +/- 0.1nm is fine if that is all you have to do. But it will mean that you are fixating to the detriment of other things. It is actually more practical to fly straighter legs and let the DME go to say +/- 1nm and use the extra time to maintain your general situational awareness.

For those of you who recommend 10 degrees ahead and behind the abeam, that is fine in nil wind. In real world terms, that may become 5 degrees ahead to 15 degrees behind or whatever, ie a straight segment that covers about 20 degrees of arc biassed into wind to cater for drift. Similarly, your turn radii need to be adjusted for wind as well as the time it takes to achieve a steady state turn.

In all of this, you need to be careful not to "chop wood with a scalpel", ie have some broad rules of thumb and a clear picture of the adjustments necessary to achieve the desired results. For example, turning on to the arc at 250KIAS with 25 kt of tailwind, start at about 2.2 miles and aim to roll out at 100 degrees from your inbound track. At about 45 degrees of turn to go, look at the DME and decide what your actual roll-out heading is going to be.

In many cases you will not be on the arc for very long, so don't let the exercise distract you from anticipating configuration changes and lead-in points. if you are like me, being inside the arc at the lead-in point just means you have even less time to correct your other mistakes.

Stay Alive,


[This message has been edited by 4dogs (edited 19 February 2001).]
Old 19th Sep 2000, 02:21   #17 (permalink)
Hew Jampton
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Try the article in Pilot magazine, September 1990, on DME arcs. Copies of the article available from Pilot, or it was reproduced in David Hoy's book, 'Instrument Flying'
Old 19th Sep 2000, 14:39   #18 (permalink)
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OK - some fine anorak contributions here.

My 2c worth is that when turning onto the arc you shouldn't turn 90 deg as this immediately puts you on a tangent to fly outside of the arc.

If you are intercepting from outside the arc, make the first turn 80 deg, and if entering from the inside, make the first turn 100 degrees. Then, by holding your heading, let the DME slowly count down to about 1 mile inside, and then count back up to the proper arc distance. This should conincide with full deflection, so twist another 10 degrees and turn another 10 degrees and so on.
Old 21st Sep 2000, 01:31   #19 (permalink)
Tail Plane
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You can guarantee that one day someone will want you to do it the other way round ie intercept a radial from an arc. Here's how: The 1 in 60 rule is the basis for this concept. The number of degrees to lead the turn by equals (60/DME of the arc) x 0.5% of the TAS. This assumes a rate one turn. eg with a 200 kt TAS flying round the 15 mile arc: 60/15 x 0.5% of 200 = 4 degrees.

For ease of MDR just remember that:

90kts requires anticipating by 27 deg/radius.

100kts requires anticipating by 30deg/radius.

120kts requires anticipating by 36deg/radius.

You Have Control!
Old 14th Feb 2001, 06:54   #20 (permalink)
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Thumbs down

I dug up this thread through the search function because I wanted some guidance for a briefing I was to do on DME Arcs on a 737 sim session.

Traditionally my training section has taught that a 1% of airspeed lead-in for arc intercept was used. That meant that at 210kts (737 min clean speed) the turn (rate 1....30deg bank) would start at 2.1NM from the desired arc.

However, your suggestion was based on the formula of TAS/200. And for 210kts this would equate to a 1.05NM lead in.

Today I went into the sim and tried this formula.

Sorry guys, but it doesn't work. Sounds good, but it's way wrong.

I set the 737-400 sim up at 45 tonnes. 210kts, clean, inbound to a VOR, to intercept a 20DME arc, flying at 3,000 feet, zero wind.
Using the TAS/200 formula the turn was comenced at 21.5DME (allowing an extra .5NM for the time required to roll into a 30degree bank).
The result was we overshot the arc by .5NM....in other words went through the arc due to insufficient lead in.

The exercise was then repeated using the 1% TAS formula (i.e. 2NM lead), and the arc was intercepted perfectly, without significant over/undershoot.

So how do you explain that? Your fancy formulas don't appear to work in the real world!
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