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VNAV ALT to VNAV SPD

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    VNAV ALT to VNAV SPD

    Hi all,
    ATC clears me to a certain altitude to where I climb with VNAV SPD. Some hundred feet before reaching this altitude the climb mode switches from VNAV SPD to VNAV ALT. I use VoxATC which tends to give further climb clearance quite late, so often VNAV changes already to VNAV ALT and then I get further climb clearance. So irl in this case how do you change back to VNAV SPD again? I press LVL CHG button and then after a few seconds VNAV button.
    Thomas Kuligk
    simming since FSII

    #2
    Instead of pressing LVL CHG try pressing VNAV..
    Cristian Caicedo A.

    Comment


      #3
      Generally speaking in real life ATC would,if the airspace permits,try to avoid leveling off situation's when ever possible.
      Rarely would you get full VNAV descend from TOD to IAF,but in 2020 due to Corona virus and limited air travel I saw couple of situations with full unrestricted VNAV descend all the way to the IAF.
      STEFAN DJORDJEVICH

      Comment


        #4
        Originally posted by Coolegg View Post
        Hi all,
        ATC clears me to a certain altitude to where I climb with VNAV SPD. Some hundred feet before reaching this altitude the climb mode switches from VNAV SPD to VNAV ALT. I use VoxATC which tends to give further climb clearance quite late, so often VNAV changes already to VNAV ALT and then I get further climb clearance. So irl in this case how do you change back to VNAV SPD again? I press LVL CHG button and then after a few seconds VNAV button.
        You would need to press the ALT INTV (stands for Altitude intervention) just beside the ALT knob and above the V/S button on the MCP. That will then change the FMA to VNAV SPD and the plane will continue its climb to the selected MCP Altitude up until it reaches CRZ ALT when it would change to VNAV PTH or level off on an intermediate ALT and it would engage in VNAV ALT again.
        Balint

        Comment


          #5
          SPD INTV should switch you to vnav spd altough a good manner would be to go to V/S when approaching altitude, 2000fpm when 2k feet to go, 1k fpm when 1k feet to go. Both in climb and descent.
          Matthew Chalupniczak

          Comment


            #6
            Originally posted by Dzosef View Post
            SPD INTV should switch you to vnav spd altough a good manner would be to go to V/S when approaching altitude, 2000fpm when 2k feet to go, 1k fpm when 1k feet to go. Both in climb and descent.
            V/S is NEVER a good mode as you don’t have speed protection. VNAV is quite able to intercept any altitude, it is just what is is made for.
            René Moelaert EHLE

            Comment


              #7
              Nop, not working, only pressing VNAV or ALT INTV not working either, the only option is to go FLCH and then back to VNAV…
              Cristian Caicedo A.

              Comment


                #8
                Originally posted by Fleng200 View Post

                V/S is NEVER a good mode as you don’t have speed protection. VNAV is quite able to intercept any altitude, it is just what is is made for.
                It is, but if you are screaming up (or down) at 3000+fpm in VNAV/FLCH with an aircraft 1000ft above/below your target level you will very likely generate a TCAS RA, which ruins everybody’s day. Hence the guidance to reduce rates of climb/descent when approaching the cleared level, which will normally be done in V/S. There should be no concern over speed protection or lack of in this scenario as you are reducing an excess of performance for a brief period approaching a level off which you should be monitoring closely anyway.

                V/S is an extremely useful mode which provides the pilot a great deal of control and finesse, if used appropriately (just like every other AFDS mode).
                Simon Kelsey

                Comment


                  #9
                  Thank you Cristian for confirming that FLCH and then VNAV button is the only way to get from VNAV ALT to VNAV SPD. I assume then this is how it’s done in real life.
                  And thank you also for the interesting side discussion on V/S vs VNAV!
                  Always happy landings to you all.
                  Thomas Kuligk
                  simming since FSII

                  Comment


                    #10
                    Originally posted by Coolegg View Post
                    Thank you Cristian for confirming that FLCH and then VNAV button is the only way to get from VNAV ALT to VNAV SPD. I assume then this is how it’s done in real life.
                    And thank you also for the interesting side discussion on V/S vs VNAV!
                    Always happy landings to you all.
                    No. While LVL CHG will do the job it is not necessary. It's also not recommended as the SID may have further altitude restrictions that may not have been overriden by a "climb now" or a "climb unrestricted" clearance. VNAV SPD will respect those restrictions but LVL CHG will just bust through them. In real life we select the new higher clearled level and then just press ALT INTV once if climbing according to SID with restrictions, or multiple times if we're cleared for unrestricted climb. Each consecutive press of the ALT INTV button will clear one climb restriction (hard altitude). When we press ALT INTV to clear those restrictions, the PF is usually looking at the top right corner of the CLB page in the FMC.

                    For example, on this departure, if they clear you "Climb FL120" and they don't say "Climb now", if you use LVL CHG you will bust all those restrictions and will receive an unconfortable phone call.

                    Captura de pantalla 2022-01-06 073251.jpg


                    During a climb we ONLY use V/S when reaching a cleared level IF and ONLY IF the vertical speed is exceeding 3000fpm at 3 to go, 2000fpm at 2 to go and 1000fpm at 1 to go. This is to prevent triggering TCAS warnings. When using it we look closely at what the speed is doing and we look at the engines to see if there's excess thrust available. If the vertical speed excedance was caused by a headwind gust that is no longer there, we will quickly revert back to VNAV. There has been many cases of stick shaker and airpeed low events because people select V/S without paying attention to what the airplane is doing. Outside of this scenario, climbing in V/S is prohibited in most airlines' FCOMs. Using VS to prevent busts becomes less important and more risky the higher you are. I wouldn't select VS climbing through FL400 to FL410. It's very unlikely to find people higher than 410 and if you're doing more than 1000fpm that high, it's probably just because of a temporary wind change.
                    Attached Files
                    Last edited by Aeromar; 09Jan2022, 09:12.
                    Omar Josef
                    737 FO
                    757/767 rated
                    Spain

                    Comment


                      #11
                      Thanks a lot, especially for taking the time to prepare an example. Just tried it in a flight: after leveling out I selected the new cleared altitude in the MCP and pressed ALT INTV - AP switches back to VNAV SPD and aircraft continues climb. Works perfectly! - and now I'm sure that's the way it's done in real life.
                      Thomas Kuligk
                      simming since FSII

                      Comment


                        #12
                        Originally posted by Coolegg View Post
                        Thanks a lot, especially for taking the time to prepare an example. Just tried it in a flight: after leveling out I selected the new cleared altitude in the MCP and pressed ALT INTV - AP switches back to VNAV SPD and aircraft continues climb. Works perfectly! - and now I'm sure that's the way it's done in real life.
                        Yes, and if you really want to do it full SOP, pretend you're getting your colleage in the loop as well.

                        -ATC: Clears you further climb.
                        -PM: Reads back the climb clearance.
                        -PF: Selects the new cleared level, keeps his finger on the altitude selector knob and says "SET". (Notice that he will not say "FLxxx SET". He just says "SET" to avoid confirmation bias)
                        -PM: Looks at the selected altitude on his PFD and says "FLxxx Checked".
                        Only then:
                        -PF: Presses ALT INTV. Then he verifies the mode on his FMA and calls "VNAV".

                        We do this every time. Aviation SOPs are full of little protocols like this one to prevent events that have happened in the past. For example to avoid staying in HDG SEL when they tell us to resume navigation to a point. To prevent confirmation bias when selecting new altitudes. To prevent prolonged loss of communications, etc. You can call them Key Risk Operational Areas or KORAS.
                        Omar Josef
                        737 FO
                        757/767 rated
                        Spain

                        Comment


                        • Emi
                          Emi commented
                          Editing a comment
                          Must have been among my favorite SOPs back in the days at East Mids!

                        • Aeromar
                          Aeromar commented
                          Editing a comment
                          Yeah and it's crazy how naturally it all comes out now. Right?
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