In the enroute/terminal navigation, the difference is well within the tolerance of the protected zone of the TERPS criteria for calculating the required obstacle clearance. (But who really cares?) But it also means we can't fly a VOR based approach with an OTS VOR since we are unable to monitor the raw nav data (ironically even though we can fly VOR airways with an OTS VOR since we can use GPS to substitute for VORs sole source enroute and terminal navigation).Īs for the difference in DME slant range versus actual GPS distance this is made irrelevant in one of two ways. But even this is outdated thanks to another AIM entry added more recently that clarifies we may navigate via GPS as long as we can monitor the raw Nav signal (see 1-2-3 c) note-5) This is something we can all do on a second CDI for VOR approaches which is nice but still does nothing legally to allow us to do NDB approaches with just GPS and no installed ADF equip. The note about not using substitution for primary lateral guidance past the FAF on approach is referring to VOR and NDB lateral guidance - not Arc's. Very good - you are well on your way to Ace'ing your CFII - but that circular, although the original source, is a bit dated and these days and it's better covered by the AIM in section 1-2-3 (which actually references 90-108).ĭME arcs are certainly covered, including the note worthy unique Martin State DME arc, where the actual Arc leads to the runway.
And for the regulatory guidance, check out AC 90-108 Basically it can substitute for the DME or ADF in any phase of flight, unless the DME is the primary lateral guidance on the final approach segment(I think the only one that would qualify is the Martin State VOR/DME approach all the others only use DME to identify fixes for step-downs and procedure turns.)
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