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3 Answers

DME ARC distance from fix formula

Asked by: 5843 views , ,
Flight Instructor, Instrument Rating

Hello

 

am doing my basic IR training and i have a questions regard DME arc entry and leaving procedures, what are the formulas to calculate the distance of the dme arc from the fix in nm and degrees?

in another word, if dme arc not published, how to find when should i start circling on the arc from the fix point?

i saw alot of formulas online but couldnt understand any...appreciate the help

 

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3 Answers



  1. KDS on Sep 10, 2019

    I believe it is best to just used fixed numbers. If your aircraft is traveling below 160 KIAS, starting the turn about a half-mile before the arc to intercept and then about five degrees before the inbound course will work. I am just a simple minded pilot and I like to keep it as simple as possible. If you are flying something that requires a higher speed, you are probably going to have equipment that is going to do all those calculations for you.

    I find that the most common serious mistake people make is forgetting to turn inbound when they get near the course. They get so wrapped up trying to fine tune their arc following that they miss the inbound course.

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  2. Mark Kolber on Sep 11, 2019

    I am curious about the reason for the question because it doesn’t seem to have much to do with flying an DME arc (see below). Flying a DME arc (even without GPS) is simple (far simpler than the complicated way it is taught – KDS is probably right that 1/2 mile will work for 160 too). There’s no advanced math required beyond 1st grade. So I’m wondering if you are interested in the design of arcs, in which case, I’d refer you to TERMS or other FAA publications which go into excruciating design detail. I’m sure there are also some advanced arc math sites, but you already said you didn’t understand them. I’m guessing they are taking IAS, converting it to true, adding wind and mixing it all together with rate of turn. I’m also not sure what you mean by “if it’s not published,” but even if a roll-your-own arc (some instructors do there in areas where there are no published ones) , published or unpublished makes no difference.

    FWIW, here’s how I teach DME arcs. At ground speeds up to about 120 kts (that’s the number the FAA Instrument Flying Handbook uses), turn 90 degrees standard rate in the direction of the arc when 1/2 nm out. Faster airspeed obviously needs a proportionally earlier turn, but unless you are travelling better than 240 kts, it won’t be more than 1 nm. After the turn (good situational awareness meas you can do it during the turn but you don’t have to) make any necessary correction to give you the arc distance you want. Confirm you have turned in the correct direction with a 10 degree twist of the OBS.

    Once on the arc, it’s just a simple matter of adjusting and maintaining the DME distance and monitoring your progress until you get to the lead radial (published or not). 1/2 to 1 mile is acceptable, so you don’t have to go crazy. 10 NM arc, DME reads 10.6, turn toward the arc. Pretty basic. Unless you are in no-wind conditions, the amount of correction you will need will change as you go around the arc (another problem with trying to calculate the flying – it will continually change and require a new calculation) . Many pilots are taught “twist 10” to monitor progress, but it’s not really necessary. The way I fly it, all you really *need* are the radial you are using to join, a confirmation radial after the turn, a lead radial, and the radial you are going to turn to. All the others are gravy, although I usually toss in 1 or 2 intermediary monitoring ones to see how quickly I am coming around (unless your checkride is with some “must turn-10-twist-10” DPE :D)

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  3. Craig on Oct 10, 2019

    At a distance of 60 NM from the arc navaid reference, 1 degree of arc distance = 1 NM. so divide the distance of the arc from the navaid by 60 & multiply by the number of degrees of arc travelled. For example; if the arc is 18 NM from the reference navaid & one travels 20 degrees along that dme arc from one fix to another, the formula is (18/60)x20=6 NM traveled.

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