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RNAV approaches

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    RNAV approaches

    Hey,
    I have started flying 737 after many, many hours spent with an A320 family. I have a few questions regarding RNP (RNAV) approaches:
    - When should I activate approach button and when should I leave it with LNAV/VNAV
    - Is 737 capable to do LPV approaches? If so, how can I tune them?
    All the best
    Mateusz Dybek
    Boeing 787 Dreamliner lover

    #2
    Mateusz - I’ll take a shot at your first bullet point question: The APP button is not used for an RNAV (non-ILS) approach. When on final approach, leave VNAV/LNAV enabled, set your altitude for field elevation, and manage your speed directly. Do not use the APP button.

    Disclaimer: I am not a RW commercial pilot, so I am definitely correctable. HTH
    Doug Miannay (KATL)
    i7-8700K (OC 5.0GHz - HT Off) | ASUS Maximus X Hero Z370 | Corsair Hydro H150i Pro Cooler | ASUS STRIX-RTX2080TI-11G | 32GB G.Skill DDR4 TridentZ RGB 4000Hz | Samsung 960 Pro 512GB M.2 (OS/Apps) | Samsung 960 Pro 1TB M.2 (Sim/Games) | Corsair H1000i PSU | Fractal Design Define R6 Blackout Case | Win11 Pro x64 (v22H2)

    Comment


    • Crabby
      Crabby commented
      Editing a comment
      APP is definitely used with the 737 for LNAV/VNAV approaches. I use it ALL the time because the 737 (at least mine are it may be an option in the equipment page) is IAN (Integrated Approach Navigation) equipped. During and RNAV approach the IAN system will calculate and display a FAC (final approach course) and GP (Glide Path). This is so pilots can basically fly the same instrument references for both ILS and LNAV/VNAV approaches. My understanding is that it is not used for RNP approaches with curved segments. Any who, do a search and grab a copy of the FCTM for the 737. It has a great area of information/instruction on using the IAN capabilities and the APP button.

    #3
    I have not found a way to tune the channel for LPV approaches. I have not seen it as an option either in the equipment settings. I could not do it in the NGXu either so I assume this version is the same.

    Comment


      #4
      I usually arm APP approach right before intercepting the IAF. I am not a real world 737 pilot either but by doing that for me, it activated FAC and GP which gives me both Lateral and Vertical Guidance when shooting a RNAV approach. Again I am no 737 pilot so I could be doing it wrong. Perhaps if any other one of the guys in the forum who are actual RW pilots for the 737 can let you know.
      Sergio Gutierrez

      Comment


        #5
        Most operators do not have IAN and thus do not use approach mode for non-ILS approaches; they use LNAV /VNAV.

        The limitation of IAN isn't just that it won't handle RF legs; it won't handle many non-straight in approaches very well.

        The 737 is not certified for LPV approaches, so technically speaking you cannot use LPV mins. Of course in the sim you can do what you want 😉.
        Andrew Crowley

        Comment


          #6
          Our IAN means a pretended ILS approach with a standard 3 degree glideslope. It will calculate the extended centerline and a glidepath that will look like a standard ILS. That‘s all, it‘s still a non-precision approach, so keep in mind: no autoland.

          RNAV approaches in LNAV/VNAV can do a lot more and I prefer them in these situations. They can follow a lateral path at the right descent rate and speed. In the 737 you need to set the MCP altitude below MSA or it will level off, once you reach your MSA you turn off all automation, set the MCP to the missed approach altitude and fly a visual approach. (I love to do that into Juneau (PAJN RW 08) in minimum meteorological conditions ^^ )
          Ryzen 9 5900X, RX 6900XT, 32GB DDR4 RAM @3600MHz, 4k
          Marc Eland
          GFO Beta

          Comment


          • Ephedrin
            Ephedrin commented
            Editing a comment
            That may totally be but as far as I know it‘s simply a definition, maybe a legal one or it’s a safety consideration. There‘s no autoland from an RNAV approach but afaik a GLS/GBAS approach which has a ground station allows auto land and is defined as a precision approach.

          • Crabby
            Crabby commented
            Editing a comment
            True no auto-land with a RNAV approach, non-precision. No auto-land with an ILS Cat1, precision approach. The terms precision and non-precision have lost their luster and truly only comes down to regulatory. Unfortunately, most people use the term to denote precision, and this is just not the case because you can fly many RNAV approaches to 200 ft (ok actually whatever the MDA that corresponds to 200 feet AGL). As you said GLS/GBAS is "considered" a precision approach only because the equipment on both ends of the equation are certified (read that as passed through all the muckety red tape and considerable and mostly unnecessary expense of getting certified) by some bureaucrat somewhere.

          • Ephedrin
            Ephedrin commented
            Editing a comment
            I "think" it rather has to do with the redundancy of different sources to predict and correct aircraft position. You can totally autoland on a CAT1 ILS, as long as the weather minima are CAT1. I would imagine that there is no guarantee that the GPS precision is as good close to the runway as it was before while a calibrated and protected ILS will not change its precision. Afaik this is also the reason why a GBAS approach is considered a precision approach. While the aircraft and the GPS/GNSS system is extremely precise by itself the GBAS station can correct discrepancies if there are any so the guidance remains guaranteed. With GPS and datalink technology you can even autoland an aircraft on a moving and pitching aircraft carrier with an angled flight deck. Technology is there, no doubt, I think the big "problem" is safety and redundancy in civil aviation. An RNAV approach depends fully depends on satellites.

          #7
          Originally posted by Ephedrin View Post
          In the 737 you need to set the MCP altitude below MSA or it will level off, once you reach your MSA you turn off all automation, set the MCP to the missed approach altitude and fly a visual approach
          You can set the missed approach altitude in the MCP with autopilot on, as long as you are at least 300ft below that altitude. Turn the knob quickly past your current altitude and the plane should continue descending in VNAV PTH without a hiccup.
          Matheus Mafra

          Comment


          • Ephedrin
            Ephedrin commented
            Editing a comment
            Will this still work if your missed approach altitude is for example 5000ft and you are at 4600 ft right now and there is an altitude constraint at let‘s say 2500ft with the runway at sea level? Will it respect the constraint if you set the altitude on the MCP that early? I know, special case but Juneau is similar.

          • Crabby
            Crabby commented
            Editing a comment
            This is true. It is even easier when using APP/IAN with a RNAV approach. Once I intercept the glide path, I bug up the MCP to the MA altitude.
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