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In IFR-GPS-equipped (/G) aircraft, a pilot flies most of the segments with GPS as primary navigational resource* even the route is along victor airways or VOR radials. However, when we start the approach, we have to switch the nav source from GPS to the nav source specified for the approach. In this context of using GPS for non-GPS approaches,

  1. When should a pilot switch away from GPS? Is it from an IAF, FAF or upon ATC vectors to intercept the final approach course?
  2. If the a pilot executes a missed approach, when can the pilot switch back to GPS? Is it when established on course direct to the holding fix?

*By primary navigational resource, I mean it provides the signal/data source for the CDI needle on a HSI, that a pilot, autopilot or flight director refers to make actions or action suggestions.

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An appropriately installed GPS may be used until crossing the final approach fix on a non-localizer approach, and until the segment requiring navigation via the localizer on a LOC, ILS, SDF, or LDA approach. It's generally approved and legal at any other time.

During the missed approach, a pilot may switch to GPS immediately, unless the missed approach requires navigating by using the localizer, in which case the ground-based navaid must continue to be used until the procedure no longer requires navigation by reference to the localizer.

The FAA covers this in AC 90-108:

An otherwise suitable RNAV system cannot be used for the following:
a. NOTAMed Procedures. Unless otherwise specified, navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
b. Substitution on a Final Approach Segment. Substitution for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.
c. Lateral Navigation on LOC-Based Courses. Lateral navigation on LOC-based courses (including LOC back-course guidance) without reference to raw LOC data.

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  • $\begingroup$ As a non-expert in aviation, I had no idea about this. It's more than a little concerning. Are aviators generally aware how insanely susceptible GPS is to not only jamming but also spoofing? $\endgroup$ – Lightness Races with Monica Apr 4 '16 at 0:59
  • $\begingroup$ @BarryTheHatchet It's Die Hard 2 all over again. :) $\endgroup$ – Iain Apr 4 '16 at 1:02
  • $\begingroup$ @BarryTheHatchet I'd love to discuss in more detail - care to talk about GPS approaches in chat? $\endgroup$ – NathanG Apr 4 '16 at 1:22
  • $\begingroup$ What if the NAVAID signal is monitorred, but the FD uses the GPS for input? (GPS overlay) $\endgroup$ – Vladimir F Apr 4 '16 at 9:05
  • $\begingroup$ Substitute and alternate have specific meanings in this context and for non-LOC approaches I don't know if this answer is correct. For LOC approaches the ability to monitor raw data could also influence what is allowed, although I believe the FAA urges/requires primary navigation using the localizer but I can't find this in writing. $\endgroup$ – sbooth Apr 10 '16 at 12:43

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