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3 Answers

Written question (Use of GPS for DME or ADF at an alternate)

Asked by: 690 views Instrument Rating

Q. GPS systems certified for IFR operations can not be used as an substitute for ADF and DME receivers

A) to determine aircraft position or a DME fix a alititude above 24,000 feet

B) to fly a DME arc

C) in selection of an alternate that requires use of ADF or DME

The correct answer is C, and the explanation for the answer is: "Because GPS is not authorized as a substitute means of navigation guidance when conducting a conventional approach at an alternate airport, if the approach procedure requires either DME or ADF, the aircraft must be equipped with the appropriate DME or ADF avionics in order to use the approach as an alternate."

I'm not persuaded by this explanation, because according to the section C in AIM 1-2-3 and NOTE 1 right below it, suitable RNAV (which includes TSO-C129/-C145/-146 GPS) can be used to "determine aircraft position relative to, or distance from a VOR, TACAN, NDB, DME fix," and that "applies even when a facility is identified as required on a procedure (for example, "Note ADF Required")."

What do you think? Does the fact that the approach is an alternative make difference to whether or not a suitable RNAV can be used to replace a DME or ADF?


3 Answers

  1. John D Collins on Jan 28, 2017

    There have been changes in the use of GPS for an alternate and this test question has not kept up with them. For non WAAS GPS, you can depend on the GPS for either the destination or the alternated, but not both. Using the GPS to substitute for the ADF or DME is considered as being dependent on the GPS. In the old days the rule was that you could not depend on GPS for the alternate, in which case C would be correct. But since the change, if your destination has no dependencies on the GPS, the alternate can be dependent on the GPS. So with the current policy, the answer is ambiguous, as it depends.

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  2. Mark Kolber on Jan 28, 2017

    John is absolutely correct. Things do change and a correct answer involving GPS is often a bit off the mark and behind the times.

    But, “C” is still clearly the right answer. A key to multiple guess questions is, often they are looking for the best answer among the choices. A & B are just outright wrong. C is at least sometimes right.

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  3. Lemontree on Jan 28, 2017

    Thank you John and Mark for your replies. It would have been much clearer If the question included some remark like “You’re planning to shoot a GPS approach in your original destination” in the sentence as a condition.

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