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5 Answers

COP, no DME onboard

Asked by: 2356 views Instrument Rating


So i'm planning an IFR flight here and my aircraft is without gps and dme.  How do I determine the change over point between vor stations?  There is actually no COP for that segment, and I know of course that means I switch halfway between stations, but how do I know when I'm there?  If I had to guess I'd say to go by the estimated flight time and divide by two, but then I am trusting totally the winds aloft forecast.  Is there a better way?



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5 Answers

  1. Bobby on May 26, 2013

    Either use 1/2 of leg time, or a radial off another VOR (if available).

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  2. Mark Kolber on May 26, 2013

    Guesstimating the COP is fine. The sole purpose of a COP (charted or midpoint) is to ensure a usable signal; if you’re getting one, there’s no “you shall change navaids now!” involved. And, if you make the switch and get a weak signal and a wavering needle and flag, you’d switch back even if you were beyond the COP, wouldn’t you?

    And, of course, other equipment can be used to determine location..

    • Have 2 VORs? Why not simply tune in the second VOR to the next frequency. When it’s strong and consistent, make the switch. That’s what I did when flying no-DME/no-GPS aircraft.
    • With 2 VORs or an NDB, you can also use a crossing radial or bearing (just like you learned as a private pilot).
    • Have a handheld GPS for situational awareness? Use that.

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  3. Jason on May 26, 2013

    Got it, thanks guys. I guess I just thought it strange that it’s not really covered in the instrument procedures, or instrument flying handbooks. Anyways, sounds like it will work out fine. Thank you

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  4. Ale on May 31, 2013

    In addition to the valuable comments above, I suggest you make a receiver sensitivity check. How?

    When your time calculation “tells” you are about to reach the 1/2 way point, switch to the VOR ahead of you and check your on-board VOR receiver sensitivity (i.e. the closer you are to a VOR station, the faster the VOR needle moves). Compare the receiving sensitivity from the two VORs (the one ahead compared to the one behind) and keep the one with the stronger signals; obviously at some point, the VOR ahead will have stronger signal; this is the time to permanently change to it.

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  5. Ale on May 31, 2013

    Please note that changing to the next VOR just because you receive its signals is technically wrong. That is because the on-board VOR receive indication (i.e the needle) won’t be accurate enough when the signals are receivable but weak.

    This can cause a serious navigation inaccuracy which might result in flying out of the airway protected airspace and that is when the ATC alerts you about that (e.g. C172 you are flying 2 miles left of the airway boundaries).

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