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9 Answers

Why GA airplane does not training LNAV/VNAV even WAAS Equipped?

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Instrument Rating

Hello. I've a question about RNAV GPS Approach.

Currently, I'm training IFR course in GA.

I don't know exactly this training system is only limited in our flight school..... but anyway... whatever.

 

Usually, Our 2-hours of IFR training per each lessons, is consist like that.

  1. LPV or ILS
  2. Partial panel after passing IAF, then execute with LNAV until 1.3NM from Threshold and circling.
  3. VOR DME ARC and Full-stop.

The thing is, I've never experienced LNAV/VNAV Approach.

As you know, lots of airport in USA has RNAV-GPS approach, and LNAV/VNAV and LNAV are almost basic. (even some runway does not have LPV, but LNAV/VNAV and LNAV are still exist.)

and As far as I know, WAAS Equipped and certificated aircraft can do a LNAV/VNAV without Baro-Altitude.

I read some articles on web-site, but other IFR student, also does not training with LNAV/VNAV, even G500/G650 installed A/C. (Of course, WAAS Equipped.)

So my question is, Is there any specific reason that General Aviation IFR course usually not training with LNAV/VNAV Approach ?

Technically, we can do a LNAV/VNAV, but.... why ? so curious about that.

Thanks for your answer :D

9 Answers



  1. Russ Roslewski on Mar 15, 2019

    Obviously I can’t answer specific questions about your school’s training methods or syllabus. However, in the general case, the most often reason is that you CAN’T.

    On an approach with the normal LPV, LNAV/VNAV and LNAV lines of minima, on most WAAS GPSes I’ve used, there is NO WAY to fly the LNAV/VNAV line. The WAAS GPS will default to the LPV if WAAS is available. If not (or if WAAS is turned off), it cannot fly the LNAV/VNAV, so it will downgrade to the LNAV.

    However, there are procedures where there are only LNAV/VNAV and LNAV minimums, no LPV. If you fly one of these, the WAAS GPS will default to LNAV/VNAV. These are getting harder to find, but they are still out there. If there are none near you, then you would not have seen them in training.

    However, if you can fly an LPV, you can fly an LNAV/VNAV, so I wouldn’t worry about it. The signal course is wider but other than that, there’s no real difference from the pilot perspective – keep the needles centered, go missed when you reach DA.

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  2. John D Collins on Mar 15, 2019

    Russ,

    My understanding in discussions with FAA HQ is that as long as the annunciation indicates an accuracy level greater than that required for the procedure minimums, it may be flown. So if LPV is annunciated and there are LPV, LNAV/VNAV, LNAV and LNAV circle minimums, any of the lines of minimum may be used. If LNAV./VNAV is annunciated, LNAV/VNAV, LNAV and LNAV circle minimums may be used. I wrote a suggested text to be added to the AIM to clarify this, but did not push it. So if all 4 sets of minimums are presented on a procedure and the LPV is annunciated, it is the pilot’s choice. Some pilots do argue with this point.

    The TSO C145/146 navigators base the annunciation on the values of HPL and VPL. For HPL, the value must not exceed 40 meters, for LNAV/VNAV or LNAV, it must not exceed 556 meters. For VPL, it must not exceed 50 meters (DH 250+) or 35 meters (DH 200 to 249). For LNAV/VNAV it must not exceed 50 meters. AC 90-105A says:

    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.

    So an annunciation of LPV in all cases is good enough to support the lower accuracy minimums. In most cases, when a procedure has LPV, it will also have LNAV/VNAV minimums, with the LPV usually having the lowest DA, but not always. This is due to differences in how the DA is determined by the two TERPS specifications, particularly close to the runway. More common is a lower MDA or visibility using an LNAV or a need because of conditions to circle. A strict interpretation would not permit using a lower LNAV straight in or circling minimum or LNAV/VNAV minimums, but this is unreasonable in my opinion.

    There are some LNAV/VNAV procedures that have the chart note: WAAS VNAV NA (TJSJ – RNAV(GPS) RWY8), but in this case there is never an annunciation of LPV or LNAV/VNAV as the airport is outside reliable WAAS vertical coverage.

    LNAV/VNAV minimums in many cases preceded LPV at runway ends. LPV is now added to most of the approaches that had LNAV/VNAV. The LPV is an additional procedure option and rarely does the LNAV/VNAV change. So the day before the LPV was added, it was legal to use a WAAS GPS to fly the LNAV/VNAV and it is still legal to fly it on any day after the LPV is added. All that is changed is the annunciation to a higher standard. This is mostly academic as the LPV in the vast number of cases is going to have a lower DA.

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  3. Mark Kolber on Mar 15, 2019

    I think the answer is pretty simple. Your flight school is probably limiting the types of approaches you fly for efficiency and due to what is available in the area..

    LNAV/VNAV vs LPV. In terms of what you do and how you do it, there is literally no difference between flying an LPV approach and an LNAV/VNAV approach. Given that LNAV/VNAV minimums are typically higher than LPV, you are not likely to see those minimums annunciated on your GPS. There may simply be none in your area.

    The three you are doing pretty much cover the universe of the three types of approaches one will come across on both a checkride and thereafter. An approach with vertical guidance using GPS or localizer/glideslope. An approach without vertical guidance. A circling approach without vertical guidance. Everything else is a minor variation of those three

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  4. Russ Roslewski on Mar 15, 2019

    John,
    I agree that, given an approach with LPV, LNAV/VNAV and LNAV minimums, if it is annunciating LPV and you want to just fly to the LNAV/VNAV DA anyway, that’s no problem.

    However, I can’t see that providing any training value, which is really the question here. First, it’s kind of forcing the training scenario – “okay, so it’s annunciating LPV, but for practice, let’s fly the LNAV/VNAV”. Why? The only difference is going to be the DA, and if you can go missed at one, you can go missed at another.

    A “real” LNAV/VNAV is going to have a wider CDI scaling than the “fake” LPV-LNAV/VNAV, so it’s not really practicing with the wider course width, either. In my opinion, just flying the LPV is perfectly adequate practice for flying an LNAV/VNAV. All a pilot really needs to know is that if LNAV/VNAV is the top line published, fly it just like an LPV.

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  5. John D Collins on Mar 16, 2019

    Russ,

    I agree with your analysis except for the CDI scaling. It is optional for the GPS designer to use either linear 0.3 NM scaling of the FSD (Full Scale Deflection) or angular scaling. Garmin and all the navigators I am familiar with have chosen the latter and use +/- 2 degrees or 0.3 NM, whichever is lower.

    So if you if you fly with a Garmin WAAS GPS with LNAV/VNAV annuncited and final approach course is less than 7 NM, it will be angular +/- 2 degrees. The same scaling is used when LNAV is annunciated. There was an issue some time ago where the angular +/- 2 degrees would be > than 0.3 NM at 7 NM and greater, but Garmin fixed that. Final approach courses in the US are rarely 7 NM long, so in the US, the FSD for LNAV/VNAV or LNAV is angular +/- 2 degrees on a WAAS GPS. If the same approach is flown by a non WAAS GPS, the FSD is fixed and is +/- 0.3 NM.

    It is really hard to find an approach now a days where there is an LNAV/VNAV but no LPV. KMDH has one for 18L. If you use one of the WAAS trainers, you can see the FSD is angular and not linear as you describe. Just have to deviate from the center line and see what the cross track indicates when you have a FSD indication on final.

    The FSD has no bearing on the obstacle protection or the HPL criteria for an LNAV/VNAV or LNAV, but the guidance doesn’t use all the available +/- 0.3 NM.

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  6. Mark Kolber on Mar 16, 2019

    Russ, one can always turn off SBAS and “force” LNAV.

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  7. Russ Roslewski on Mar 16, 2019

    John, thank you for the in-depth details on CDI scaling. There’s another pair of LNAV/VNAV approaches down the road from here, in Hinton, OK, 2O8. I’ve gone there occasionally with students for the express purpose of seeing how those approaches are different, really to show that they’re not.

    Mark, you bet, and I do that somewhat regularly so that the pilot can see what they will have to do if they lose WAAS, which happens occasionally. I view that as a valid, reasonable training scenario. But using WAAS and flying an LPV but using the LNAV/VNAV DA does not seem, to me, to have training value. Hence probably why the OP’s school doesn’t have them as a part of their syllabus.

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  8. John D Collins on Mar 17, 2019

    Although it is not totally uncommon for VPL/HPL to be above that required for WAAS vertical or LP/LPV lateral guidance, it is extremely rare for a loss of WAAS itself. I am aware of only a single system wide outage in the WAAS system in the entire history and it was the result of a software update that went bad and caused a multi hour system wide outage.

    The only other outage affected portions of Alaska when control of one of two WAAS satellites at the time was lost and it drifted east for over 8 months. It was still transmitting WAAS but not where is needed to be to cover Northern and Far Western Alaska. Control was finally regained after the satellite lost power due to the solar panels not pointing at the sun and it reset itself. It came back on line and the ground was able to slowly move it back into position. In the mean time a new satellite was launched and the current WAAS satellites have triple redundancy and only one needs to be operational to provide service to the lower 48.

    As long as WAAS is providing the correction and integrity information, WAAS is available and this allows use of a TSO C145/146 GPS to be used as the sole source of IFR navigation and meet 91.205(d)(2). That is different than the VPL or HPL integrity causing a downgrade from an LPV/LP on an approach to LNAV. If WAAS does fail, then the fall back is to standard GPS service, RAIM. Turning off WAAS can be used to simulate a downgrade by simulating a general WAAS failure.

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  9. Mark Kolber on Mar 21, 2019

    Russ, the training value of flying an LPV but using the LNAV/VNAV DA may depend. My home base has one of those approaches where the LNAV/VNAV minimums are lower than the LPV minimums.

    (BTW, The latest software seems to agree with the view that those are the correct minimums)

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