Question on GPS and LNAV/VNAV Approaches

Discussion in 'Pilot Training' started by Guy Morton, Nov 14, 2020.

  1. Guy Morton

    Guy Morton Filing Flight Plan

    Joined:
    Jul 30, 2020
    Messages:
    10
    Display Name:

    Display name:
    GM
    During a practice IFR test, I got the following question: While flying a GPS approach, you see the GPS display an LNAV/VNAV annunciation. What action should you take?

    The correct answer, apparently, is that the LNAV/VNAV annunciation indicates that both lateral and vertical guidance have failed and you need to execute a missed approach.

    But, wouldn’t it be the case, if the aircraft had LNAV/VNAV capability, and the airport offered both approaches, that if WAAS failed, the GPS would “fail down” to an LVAN/VNAV approach and you could fly that down the appropriate minimums?
     
  2. PaulS

    PaulS Touchdown! Greaser!

    Joined:
    May 29, 2007
    Messages:
    11,344
    Location:
    New England
    Display Name:

    Display name:
    PaulS
    Not enough info as far as what an annunciation is.

    For the G1000 let's say you are planning an LPV approach, one of the checks you are supposed to do is make sure that LPV appears on the HSI, if you see LNAV/VNAV, you can fly it as a non-precision if you can get your act together quickly enough, which would make the answer to your question affirmative.

    Those FAA questions drive me nuts.
     
    mcdewey likes this.
  3. chemgeek

    chemgeek Pattern Altitude

    Joined:
    Dec 5, 2009
    Messages:
    2,400
    Display Name:

    Display name:
    chemgeek
    Define "annunciation." When my GNS-430 gives an L/VNAV annunciation on its screen prior to the FAF, it means you can fly to LNAV/VNAV minimums. The annunciation on the GNS-430 means that the annunciated approach mode is verified for integrity, not that it has failed.
     
    midlifeflyer likes this.
  4. midlifeflyer

    midlifeflyer Touchdown! Greaser!

    Joined:
    May 25, 2006
    Messages:
    12,417
    Location:
    Chapel Hill NC
    Display Name:

    Display name:
    Mark
    Huh? Our WAAS boxes have LNAV/VNAV capability. It may simply be an approach with LNAV/VNAV but no LPV minimums like the PDK RNAV Y 21L approach. Intercept the glidepath and fly it down.

    what was the specific question?
     
  5. PaulS

    PaulS Touchdown! Greaser!

    Joined:
    May 29, 2007
    Messages:
    11,344
    Location:
    New England
    Display Name:

    Display name:
    PaulS
    I remember similar questions concerning gps down grading, say you are planning an LPV but for some reason the signal isn't good enough to maintain LPV integrity. I also remember the FAA questions not giving enough detail to really justify the choice they want you to make.
     
  6. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

    Joined:
    Jul 31, 2007
    Messages:
    20,349
    Location:
    Catawba, NC
    Display Name:

    Display name:
    FlyingRon
    Eh? The question is ill-worded. But in reality, it's no different than the old "What happens if I lose the GLIDESLOPE while flying an ILS" but even less so. Here you still have vertical guidance. So if the approach has LNAV/VNAV mimina, and you're above the DA, you continue the approach to that DA. If you're below the DA (and you don't have the required visibility and runway environment in sight), then you start the miss.
    If there's only LNAV minima left, then you'll have to stop the descent at the minimum altitude (unless you're already below it and in that case, I'd go miss) and then use the GPS only for lateral guidance.
     
  7. midlifeflyer

    midlifeflyer Touchdown! Greaser!

    Joined:
    May 25, 2006
    Messages:
    12,417
    Location:
    Chapel Hill NC
    Display Name:

    Display name:
    Mark
    That's why we need the full question. Unless you happen to have real Baro-VNAV equipment, fairly sure LNAV/VNAV and LPV are using the same GPS/WAAS/SBAS functionality. In the PDK approach, annunciating LNAV/VNAV would be normal. OTOH, if the system were annunciating LPV and, in the middle of the approach changed to LNAV/VNAV, I'd treat it as a system anomaly/loss of integrity and go missed. Perhaps that what they were getting at?
     
  8. RussR

    RussR Pattern Altitude

    Joined:
    Jan 12, 2011
    Messages:
    2,129
    Location:
    Oklahoma City, OK
    Display Name:

    Display name:
    Russ
    If we are talking about the standard GA IFR GPS units, such as the 430W/530W/650/750, or Avidyne IFD series, then I don't think you're GOING to ever see a downgrade from LPV to LNAV/VNAV. Flying the LNAV/VNAV line of minimums requires one of two things - either a WAAS capable GPS receiver, or a Baro-VNAV equipped FMS. Since most light GA aircraft don't have a Baro-VNAV FMS, then it requires WAAS. Therefore, in a standard installation, if WAAS doesn't exist, then you can't fly LPV OR LNAV/VNAV. Rather, the unit will downgrade straight from LPV to LNAV.

    The only time you should ever see it annunciate LNAV/VNAV is if the approach has LNAV/VNAV as its top line of minimums and no LPV. (Not to be confused with LNAV+V, which is a completely different thing.)
     
    midlifeflyer likes this.
  9. midlifeflyer

    midlifeflyer Touchdown! Greaser!

    Joined:
    May 25, 2006
    Messages:
    12,417
    Location:
    Chapel Hill NC
    Display Name:

    Display name:
    Mark
    Exactly. And this situation should be covered in the manual
    upload_2020-11-15_7-6-37.png

    That's why I said that if I did get a weird annunciation like LPV changing to L/VNAV in the middle of an approach, I'd suspect something was wrong,

    (BTW, "only time." I am a little surprised I don't get a L/VNAV annunciation when the L/VNAV minimums are lower than the LPV minimums. Any idea. Is the unit logic simply that LPV is superior?)
     
  10. John Collins

    John Collins Pattern Altitude

    Joined:
    Feb 29, 2008
    Messages:
    2,484
    Location:
    Charlotte, NC
    Display Name:

    Display name:
    John
    You should never get a downgrade from LPV to L/VNAV and if the annunciation changed from LPV to L/VNAV during a procedure, it would be a software glitch. The rare cases where the LPV DA is higher than the LNAV/VNAV DA, if the annunciation is LPV, you can choose which DA you wish to use, the LPV or the LNAV/VNAV. The difference in the DA has nothing to do with which is superior, it has to do with the TERPS differences for the two procedure types and where the obstacles are located. Both are approved to be flown by a WAAS GPS. There are times when flying an LNAV provides a lower MDA than either the LPV or LNAV/VNAV DA. You can fly to the MDA using the LNAV procedure minimums as long as the annunciation is LPV or LNAV/VNAV or LNAV. If you have Baro-VNAV and are using it for the approach vertical guidance, all that is needed is LNAV annunciation, but you are not going to find that capability on anything less than an FMS.
     
  11. chemgeek

    chemgeek Pattern Altitude

    Joined:
    Dec 5, 2009
    Messages:
    2,400
    Display Name:

    Display name:
    chemgeek
    I've never seen my WAAS box change from one approach mode to another during the final approach segment of the approach. What I have seen is that the box will not arm the LPV at the FAF, and give an alert that it is downgrading the approach to LNAV only. I encountered this issue when the vertical guidance board in my GNS430W failed, leading to an integrity flag for any LPV approach.
     
  12. IK04

    IK04 Pattern Altitude

    Joined:
    Nov 6, 2018
    Messages:
    2,388
    Location:
    Copperas Cove, Texas
    Display Name:

    Display name:
    LNXGUY
    If I remember correctly, the "INTEG" annunciator is the one you don't want to see during an approach. That's missed approach time.
     
  13. midlifeflyer

    midlifeflyer Touchdown! Greaser!

    Joined:
    May 25, 2006
    Messages:
    12,417
    Location:
    Chapel Hill NC
    Display Name:

    Display name:
    Mark
    As you know I agree with you on this (and there are those who don't) but my question was more about why the navigator doesn't annunciate the approach with the lowest minimums it is capable of.
     
  14. MauleSkinner

    MauleSkinner Final Approach

    Joined:
    Oct 25, 2005
    Messages:
    9,169
    Location:
    Wichita, KS
    Display Name:

    Display name:
    MauleSkinner
    If it annunciated “LNAV/VNAV”, how would you know whether or not you could use LPV minimums? Or whether the glide path that shows up is temperature-sensitive or not?
     
  15. John Collins

    John Collins Pattern Altitude

    Joined:
    Feb 29, 2008
    Messages:
    2,484
    Location:
    Charlotte, NC
    Display Name:

    Display name:
    John
    How would the navigator know the lowest minimums the pilot is using? Minimums are a complex set of involving category of aircraft, aircraft speed,visibility, alternate altimeter settings, and procedural options along with the intention of the pilot. The design of the annunciation is to present a level of service which translates to lateral accuracy and related vertical accuracy. Pilots have to understand what the level of service being annunciated means.
     
  16. midlifeflyer

    midlifeflyer Touchdown! Greaser!

    Joined:
    May 25, 2006
    Messages:
    12,417
    Location:
    Chapel Hill NC
    Display Name:

    Display name:
    Mark
    So I guess the simple answer is, the system follows an internal hierarchy in which LPV annunciation has priority over LNAV/VNAV annunciation. It will only annunciate LNAV/VNAV if the approach has no published LPV minimums.
     
  17. John Collins

    John Collins Pattern Altitude

    Joined:
    Feb 29, 2008
    Messages:
    2,484
    Location:
    Charlotte, NC
    Display Name:

    Display name:
    John
    That is the way it does work. Without an LPV option, an approach with LNAV/VNAV DA will only annunciate L/VNAV or LNAV. LPV will be annunciated if there is an LPV DA or LNAV. There are two levels of service for LPV (WAAS approach), but they share a common annunciation. LPV with a DH of 250 or geater use HPL of 40 meters and VPL of 50 meters to obtain the LPV service. LPV with a DH less than 250 feet use the same HPL of 40 metes, but the VPL must not be greater than 35 meters. For LNAV/VNAV HPL must be not above 556 meters (0.3 NM) and VPL must be at or below 50 meters. For LNAV, only HPL matters and it must be not above 556 meters (0.3 NM). LP is also a WAAS approach, but only requires HPL to be 40 meters or less. If VPL is below 50 meters, then +V can be provided to LP or LNAV. In fact boh Avidyne IFD and Garmin (GTN series) will now provide +V without using WAAS. All WAAS approaches have a FAS Block as part of the database entry, while the non WAAS approaches do not. An LNAV/VNAV procedure minimum may be flown by a WAAS GPS with the vertical guidance provided by WAAS, but it is not a WAAS procedure.
     
  18. John Collins

    John Collins Pattern Altitude

    Joined:
    Feb 29, 2008
    Messages:
    2,484
    Location:
    Charlotte, NC
    Display Name:

    Display name:
    John
    There are also some LPV and or LNAV/VNAV procedures that do not have an LNAV option, in which case, if you get an integrity fault with either VPL or HPL, you must abandon the approach. These procedures will also not have circling minimums.