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4 Answers

LNAV/VNAV Minimums on a LNAV only approach?

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Instrument Rating

I am a CFI-I and one of the airplanes that I instruct in is a WAAS capable C172 with the G1000. We do some LPV and LNAV/VNAV approaches at nearby airports. However, on some GPS approaches at our local airports the G1000 gives a glide sloped and indicates that we are shooting a LNAV/VNAV approach. However, if you look at the approach plate for these different approaches, there are no LNAV/VNAV or LPV minimums, only LNAV. Why does the G1000 have LNAV/VNAV approaches that don't exist according to the approach plate?

4 Answers

  1. Matthew Waugh on Jul 27, 2010

    Throwing caution to the winds – does the approach have LPV minima published? I think if that was the case then the unit would have to show LPV to meet those minima, but I’m not going to claim to be the font of all knowledge on GPS approaches.

    Can you tell us what airport/approach plate this is occurring with? Plus you sometimes find that NACO (am I showing my age, didn’t they change the name) and Jepps will show different approaches. If there is a published NOTAM then Jepps (and presumably Garmin) will add that information to their plates/software distribution while NACO holds onto the past as if one day buggy whips may make a miracle resurgence.

    There is a relatively old Garmin notice about vertical guidance on approaches that had been downgraded to LNAV only – but I think that was related to their synthetic flight path display and not glideslope so I don’t think it’s relevant (but who knows).

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  2. Earl Kessler on Jan 18, 2011

    The magic of Garmin! Although it may be a LNAV, Garmin can make it a LNAV+V approach, which is technically a non-precision approach, but the glide path is painted on your PFD and will permit you to fly it like an ILS.

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  3. Kent Shook on Jan 19, 2011

    I think Earl might have it. What does the annunciator on the HSI say? I’m guessing it says LNAV+V which is not LNAV/VNAV, it is LNAV with advisory vertical guidance. On an approach that actually has LNAV/VNAV minimums, the annunciation should instead be L/VNAV, on a true WAAS approach it’ll say LPV. You should use the minimums indicated by the mode of the Garmin as annunciated in the HSI, just to the right of where it says “GPS”.

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  4. John D. Collins on Jun 17, 2011

    As Earl and Kent indicated, the annunciation was most likely LNAV+V.  LNAV+V indicates that there is an advisory glidepath provided by the manufacturer, it is not actually part of the official approach and is not charted as a part of the procedure.  The purpose of the advisory glidepath is to provide the pilot with a tool to aid in flying a stabilized approach profile. The pilot can choose to ignore the advisory glidepath and fly the approach the traditional way with the dive and drive method. If the pilot follows the advisory glidepath, they are still required to follow the charted procedure, which includes not flying below an minimum altitudes, step-downs, or the MDA without the runway environment in sight.  The advisory glidepath looks like the indication provided on an ILS or LPV, but flying it different in several respects. 


    First, following the glidepath below the MDA is not permitted and no obstacle clearance is provided in the visual segment below the MDA.  On an approach that has a DA (ILS, LPV, LNAV/VNAV), the decision is made at the DA, so momentum of the aircraft will carry the aircraft below the DA (sink thru) before a go around can be initiated.  This is permitted with a DA, but not with a MDA.  If the runway environment is not in sight, there is no sink thru allowed for a MDA. Also, unlike an approach with a DA, if it has a MDA, the approach is not over once the MDA is reached, but the pilot needs to level off and continue to the MAP. 

    Second, the advisory glidepath is relatively fixed in space, but temperature affects the indicated altitude on the altimeter.  The pilot is still required to observe the minimum altitudes at all times, particularly when the glidepath is below the required minimum.  This may require leveling off above the glidepath to satisfy the minimum altitude requirement. For all approach types, step-downs may be included before the FAF, but on a vertically guided approach (ILS, LPV, LNAV/VNAV), there are no step-downs between the charted glidepath intercept point and the DA.  With a LNAV approach, there can be a step-down between the FAF and the MAP.  If the GPS manufacturer provides the advisory glidepath on a LNAV approach, the pilot must not descend below the step-down altitude, even if they remain on the advisory glidepath. 


    In conclusion, the advisory glidepath (LNAV+V) is a great aid, but be aware of the limitations when using it. 

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