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DME behavior near VOR

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Instrument Rating

I understand that the DME measures the straight-line distance from the aircraft to the ground station. This is called "slant range" and is slightly more than the actual horizontal distance because of the difference in elevation between the aircraft and the station. The most extreme case of "slant range error" occurs when the aircraft passes directly over the station; instead of reading zero, the DME shows the altitude of the airplane above the station in nautical miles. In the airplane that I fly, however, the DME stops working, when in proximity (approx..+/-2 miles) of a VOR.  This is inconvenient, because, if there is a change of heading at the VOR, I like to anticipate the turn at a specific distance from the VOR that depends on the magnitude of the change of heading and the airplane speed.  The GPS  provides guidance on this, but sometimes I keep it off for training purposes.  Is perhaps my understanding of how the DME should work incorrect or something is wrong with my DME?

Thanks

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3 Answers



  1. Russ Roslewski on Jan 09, 2017

    If your DME stops reading anything within 2 miles of the VOR, I suggest you need to have it repaired. It should not behave like that, and should continue giving you distance as you pass over the station itself. (Showing, for example, 0.5 nm if you are 3000 feet over the station.)

    It’s a safety consideration, too. For instance, if you ever fly VOR/DME approaches it could be a problem. Consider the KSPI VOR/DME RWY 4 approach (or actually, any of the VOR/DME approaches at KSPI):

    https://skyvector.com/files/tpp/1701/pdf/00518VD4.PDF

    If your DME doesn’t read when you’re within 2 nm, you would have no way to identify the MAP on this approach at 0.4 DME, the VDP at 1.4 DME, and you might not be able to identify the stepdown fix at 2.0 DME.

    It needs to be taken to an avionics shop, tested, and repaired as necessary, or placarded INOP.

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  2. Skyfox on Jan 09, 2017

    I completely agree with Russ that your DME should continue working no matter how close you are to the station. The only thing I can think of beyond the radio needing repair would be if the DME antenna is mounted on the roof of the aircraft, so as you get closer to the VOR the signal is becoming too obscured by the fuselage and it simply can’t transmit and/or receive with the ground station. If that’s the case, when you take the thing to a repair shop for service and testing, have them move the antenna to the belly of the plane.

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  3. Charles22 on Jan 09, 2017

    Russ, Skyfox,

    Thanks to both of you.

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