Can GPS be used on missed approach that has localizer backcourse?

4 Answers

1. Best Answer
   
   

   
   

   John D Collins on Oct 29, 2021

   Yes, but according to current AIM guidance, you must reference to the raw localizer guidance, IOW it must be displayed on a CDI of some sort.

   See AIM 1-2-3(c) Uses of Suitable RNAV Systems, note 2.

   “Note 2. These operations do not include lateral navigation on localizer−based courses (including localizer back−course guidance) without reference to raw localizer data.”

   In the draft AC 90-119, there is a change in the guidance that if it makes through to release of the AC, hopefully next year is a good improvement. It is not yet approved guidance, but it reads as:

   “11.4.2 Authorized Uses of Suitable RNAV Systems. Operators may use a suitable RNAV system to:
   6. Navigate to the FAS of a conventional IAP.”

   “11.4.3.3 To Navigate a Localizer (LOC) Final Approach Course. Pilots may not use RNAV to fly a FAS defined by an LOC signal.”

   Taken together, these say it is OK to use GPS to navigate the localizer up to the FAS and the only restriction is on the actual FAS (Final Approach Segment) itself.

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   AviatorTrevor88 on Oct 29, 2021

   Thanks for the answer, John.

   Follow up question: I flew the KSEE LOC-D procedure today as a practice VFR approach with a student. We requested the procedure turn. We were coming from the west and were told “direct BARET, cleared for the approach.” When we got to BARET, we began to track the localizer outbound to begin the course-reversal/procedure turn. We were in GPS mode initially, and I wasn’t sure if I was legally required to be in VLOC mode at that point, so I just switched us to VLOC mode since I couldn’t be “wrong” in that mode.

   But my curiousity wants to know – could I have done the course reversal portion east of BARET using GPS only?

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   Mark Kolber on Oct 30, 2021

   Trevor, I think John answered your question,

   “Taken together, these say it is OK to use GPS to navigate the localizer up to the FAS and the only restriction is on the actual FAS (Final Approach Segment) itself.”

   GPS (with LOC needle also displayed under current guidance) until intercepting inbound on the extended FAC, then switch primary to LOC.

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4. 
   

   
   

   John D Collins on Oct 30, 2021

   The AFMS for the GTN 750, in which the limitations section is regulatory verses any AC or the AIM which is guidance, has the following limitation:

   “c) The navigation equipment required to join and fly an instrument approach procedure is indicated by the title of the procedure and notes on the IAP chart. Navigating the final approach segment (that segment from the final approach fix to the missed approach point) of an ILS, LOC, LOC-BC, LDA, SDF, MLS, TACAN approach, or any other type of approach not approved for GPS, is not authorized with GPS navigation guidance. GPS guidance can
   only be used for approach procedures with GPS or RNAV in the procedure title. When using the Garmin LOC/GS receivers to fly the final approach segment, LOC/GS navigation data must be selected and presented on the CDI of the pilot flying. When using the VOR or ADF receiver to fly the final approach segment of a VOR or NDB approach, GPS may be the selected navigation source so long as the VOR or NDB station is operational and the signal is monitored for final approach segment alignment.”

   I simulated this using the GTN Trainer. The approach started out at BARET using the magenta GPS needles for guidance and remained using GPS until crossing ZIRLU where they auto switched to using the green VLOC needles for guidance. So the Garmin ILS CDI Capture – Auto Switch believes you need to be in VLOC as soon as the FAF is the active fix. That doesn’t agree with the AIM guidance 100%, but does agree with the future guidance in AC 90-119 draft. The system is FAA approved to work this way.

   So my conclusion is, yes it is legal because it is specifically permitted in the AFMS limitations section, even though it does not agree with the AIM guidance. The AIM guidance will be updated at some point, probably when AC 90-119 is released and the other AC’s are cancelled.

   Also, the initial, intermediate, and missed approach segments are specified in common sections of the TERPS. Only the final approach segment is different based on the type of approach, RNAV (GPS), Localizer, VOR, or NDB.

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