G950 NXi LPV vs L/VNAV

2 Answers

1. 
   

   
   

   John D Collins on Apr 24, 2020

   There isn’t an official or explicit answer to this from the FAA, but AC 90-105A, Appendix A, A.6 Aircraft Eligibility for RNP APCH Operations states:

   “A.6.4 LNAV/VNAV Line of Minima Qualification.
   A.6.4.1 Stand-Alone Systems. Stand-alone TSO-C146 Class 2 or 3 systems meet the aircraft qualification requirements for RNP APCH operations using the LNAV/VNAV line of minima provided that the installations meet at least the performance and functional requirements of this AC.”

   An RNAV (GPS) procedure with LPV, LNAV/VNAV, and LNAV minimums available on the same procedure will only annunciate LPV when the the HAL (40 meters) and VAL (50 or 35 meters) integrity limits are satisfied by the HPL and VPL respectively. When the HPL and/or VPL do not meet that criteria one minute prior to the FAF, the procedure will be downgraded to an LNAV annunciation as long as the HPL is at or below the HAL value (556 Meters) for LNAV . An HPL value greater than 556 meters will require the approach be abandoned and other means be used. The point being, there is no way for the annunciation to ever be L/VNAV on such a procedure as it is either LPV or LNAV or none of the above. The HAL and VAL limits for LNAV/VNAV are 566 meters and 50 meters respectively and therefore when the most accurate level of service supported is annunciated as LPV, it is better than or equal to that required for an L/VNAV annunciation in all cases.

   So does the annunciation of LPV as the most accurate level of service supported by the WAAS receiver preclude using LNAV/VNAV or LNAV minimums? I think not. That would preclude the pilot from choosing the LNAV minimums when the visibility requirements for a vertically guided procedure exceed the requirements for LNAV or the pilot chooses to circle to land using the LNAV circling minimums. In my opinion, it also does not preclude a pilot from using the LNAV/VNAV minimums. This has generated a long standing argument as to the meaning of the annunciation of the most accurate level of service supported by the WAAS receiver and whether this in any way precludes the use of minimums requiring a less accurate and lower level of service approach options of the same approach and if they may be used by the pilot. I suggested wording be added to the AIM to clarify this argument. No one objected to the wording or its meaning, but it was not considered at the time worthy of needing clarification in the AIM.

   The following is the current AIM guidance wording, 1−1−18. Wide Area Augmentation System (WAAS)

   2. When an approach procedure is selected and active, the receiver will notify the pilot of the most accurate level of service supported by the combination of the WAAS signal, the receiver, and the selected approach, using the naming conventions on the minima lines of the selected approach procedure. For example, if an approach is published with LPV minima and the receiver is only certified for LNAV/VNAV, the equipment would indicate “LNAV/VNAV available,” even though the WAAS signal would support LPV. If flying an existing LNAV/VNAV procedure with no LPV minima, the receiver will notify the pilot “LNAV/VNAV
   available,” even if the receiver is certified for LPV and the signal supports LPV. If the signal does not support vertical guidance on procedures with LPV and/or LNAV/VNAV minima, the receiver annunciation will read “LNAV available.” On lateral only procedures with LP and LNAV minima the receiver will indicate “LP available” or “LNAV available” based on the level of lateral service available. Once the level of service notification has been given, the receiver will operate in this mode for the duration of the approach procedure, unless that level of service becomes unavailable. The receiver cannot change back to a more accurate level of service until the next time an approach is activated.
   NOTE−

   Receivers do not “fail down” to lower levels of service once the approach has been activated. If only the vertical off flag appears, the pilot may elect to use the LNAV minima if the rules under which the flight is operating allow changing the type of approach being flown
   after commencing the procedure. If the lateral integrity limit is exceeded on an LP approach, a missed approach will be necessary since there is no way to reset the lateral
   alarm limit while the approach is active.

   I suggested adding the following to the NOTE:

   “Occasionally, it may be to the pilot’s advantage to fly a procedure which requires a lower level than the most accurate level of service that is annunciated. Some examples of this would include 1) when circling to a different runway is indicated; or 2) when the visibility requirements of a vertically guided approach exceed the prevailing visibility, but the lower visibility requirements of a LNAV procedure are well below the prevailing visibility. When flying an RNAV (GPS) approach with a WAAS GPS, the pilot may follow any of the procedures and minima lines published on the same approach chart that are supported by the most accurate level of service that is annunciated, so if LPV is annunciated, the pilot may choose to fly the LPV, or LNAV/VNAV, or LNAV, or LNAV circling procedures and minima, if LNAV/VNAV is annunciated, they may choose the LNAV/VNAV, or LNAV, or LNAV circling procedures and minima, and if LNAV is annunciated, they may only fly the LNAV or LNAV circling procedures and minima.”

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2. 
   

   
   

   Mark Kolber on Apr 25, 2020

   Have you tried contacting Garmin to see if there is a way to supress LPV while not suppressing LNAV/VNAV? Seems unlikely since, unless you have baro-VNAV capability it’s the same system providing your vertical guidance, but you never know.

   I don’t pretend to understand the rationale for your policy, especially since they a are using the same system and are several approaches with LNAV/VNAV minimums lower than LPV, but that’s not my decision.

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