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Flying DME-ARC with high speed aircraft

Asked by: 4246 views Commercial Pilot, Flight Instructor, Instrument Rating

hi , well... in approaches which have a DME-ARC transition , there is rule of thumb ( for general low speed aircraft) that says: intercept the arc 0.5 mile before the actual arc, and lead the inbound course by 2-4 radials what if your flying a high speed aircraft e.g CAT C or D and you don't have an FMS/GPS? what's the formula for leading the :  1. arc entry   and 2.inbound course entry  ? best regards.    

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  1. Mark Kolber on Jan 21, 2016

    Like all rules of thumb of this type, it is an easy shortcut that covers most situations, but is based on how far you are going to travel. The 1/2 mile rule of thumb for leading the arc is based on the idea that most light GA aircraft going to be flying about 100 KTS and that’s how much distance you will need to make a 90° turn without overshooting. The rule of thumb is really just a dumbed-down version of the rule of thumb (something about a dumbed down rule of thumb strikes me funny) based on a percentage of GS. I’ve heard variations on the theme, but the most common one is 0.5% of your ground speed. For a 100 KT GS, that’s 1/2 mile. For a 200 KT GS, that’s 1 mile. Since we are talking about GS, not AS and we are going to have to adjust anyway, close is good enough.

    Same is true for the lead radial, except a rule of thumb is even less reliable. The published lead radial for a turn to the inbound course is also based on GS, but it is also based on the arc distance (10 degrees on 20 mile arc takes more time and covers more distance than 10 degrees on a 10 mile arc, right?) and the relation of the arc to the final approach course. If you look at the approach charts that contain lead radials for different distance arcs, you will see that.

    An excellent illustration is the DME arc for the KSAF ILS RWY 2 (Santa Fe, NM). If you look at the chart you will see the VOR on which it is based in not associated with the FAC. So the lead radial for the 12 DME arc from the east has a much smaller lead radial than the one from the west.

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