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 Flying Instructors & Examiners A place for instructors to communicate with one another because some of them get a bit tired of the attitude that instructing is the lowest form of aviation, as seems to prevail on some of the other forums!

 20th Apr 2000, 14:36 #1 (permalink) Teroc Guest   Posts: n/a DME Arc on a VOR - How ? Guys, 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 Teroc
 20th Apr 2000, 15:38 #2 (permalink) New Bloke Guest   Posts: n/a 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.
 20th Apr 2000, 17:48 #3 (permalink) RVR800 Guest   Posts: n/a Can be most easily done with an RMI VOR needle - behaves like an ADF needle
 21st Apr 2000, 03:10 #4 (permalink) Dan Winterland Guest   Posts: n/a 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)
 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.
 23rd Apr 2000, 02:47 #8 (permalink) Grandad Flyer Guest   Posts: n/a 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!
 23rd Apr 2000, 13:02 #9 (permalink) Fokjok Guest   Posts: n/a 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?
 23rd Apr 2000, 22:47 #11 (permalink) Grandad Flyer Guest   Posts: n/a 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.
 25th Apr 2000, 01:06 #12 (permalink) Tinstaafl Guest   Posts: n/a 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).]
 25th Apr 2000, 08:13 #13 (permalink) Oktas8 Guest   Posts: n/a Checkboard, 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).]
 25th Apr 2000, 14:49 #14 (permalink) Teroc Guest   Posts: n/a 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, Teroc
 27th Apr 2000, 20:06 #15 (permalink) Teroc Guest   Posts: n/a Guys, 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 T.
 19th Sep 2000, 02:21 #17 (permalink) Hew Jampton Guest   Posts: n/a 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'
 19th Sep 2000, 14:39 #18 (permalink) Oleo Guest   Posts: n/a 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.
 21st Sep 2000, 01:31 #19 (permalink) Tail Plane Guest   Posts: n/a Teroc, 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!
 14th Feb 2001, 06:54 #20 (permalink) CaptSensible Guest   Posts: n/a 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!