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Advisory step climb giving me bad information?

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    Advisory step climb giving me bad information?

    Hello,

    Two flights in a row I have been cruising at FL340 feet. Both times, I wanted to plan for a 4,000 foot stepclimb. I put FL380 into the step climb window, and once I reached the calculated S/C point, began my climb. Both times, the aircraft became very unstable toward FL380 and was virtually at it's aerodynamic ceiling, requiring me to descend to 360. The CRZ page also went blank, which I've never had happen after a stepclimb before. My question is: why is the FMC telling me it's OK to stepclimb, when in reality, it's not? I'm not a professional pilot, so there is a chance I'm just doing something wrong, but I never had this happen on the NGX.

    ThanksScreenshot_102.png
    Andrew DeForest

    #2
    Also, after descending back down to FL360, it's now telling me a S/C to FL380 won't be possible for ~160nm. Why did it tell me it was OK before?

    Screenshot_103.png
    Andrew DeForest

    Comment


      #3
      I know this may not answer your question, but if you use a flight planner like SimBrief or PFPX, and have a profile created with the proper weights\fuel for the NGXu, the flight plan should give you step climbs at specific way-points on longer flights.

      After using the exact fuel\weight data from these flight plans, entering the step advisory at said way-point always worked for me. (S\C can be done automatically in the B744 with this method too).

      It looks like the MAX altitude shows FL382. Optimum ALT is where you want to be (FL360). I can't speak on why it told you to climb before that.
      .
      Last edited by Tino; 23Nov2019, 02:41.
      Sante Sottile - CYVR

      Comment


        #4
        I do use SimBrief, but tend to rely on the FMC more to tell me when to stepclimb as it's more accurate (except in this case).
        Andrew DeForest

        Comment


          #5
          The first pic is the Climb page. It's blank because you can't climb anymore. You probably overlooked something and made a mistake. Your second pic the best altitude is FL360. Good rule of thumb: When the OPT altitude is 1000' above you, then climb 1000' above that and you should be ok.
          Tony Fontaine

          Comment


            #6
            Originally posted by captbillybob View Post
            The first pic is the Climb page. It's blank because you can't climb anymore. You probably overlooked something and made a mistake. Your second pic the best altitude is FL360. Good rule of thumb: When the OPT altitude is 1000' above you, then climb 1000' above that and you should be ok.
            Mistakenly posted CLB page instead of CRZ. There is a chance I made a mistake, but I followed the same procedure I followed a hundred times in the NGX with no problems.
            Andrew DeForest

            Comment


              #7
              Interesting. I don't think I've ever tried entering step climbs at 4000' intervals in the real thing. Just 2000'. I'll see what I can learn and report back on this. I think its incorrect behaviour.
              Last edited by MeatServo; 23Nov2019, 04:14.
              ATPL - Class I Instructor - Seaplane - B737 - BE1900D

              Comment


              • MeatServo
                MeatServo commented
                Editing a comment
                Far as I can see it should calculate your step climb correctly. What did it say for OPT/MAX altitude at your step point?
                Last edited by MeatServo; 23Nov2019, 04:46.

              #8
              Under RVSM rules shouldn't step climbs be in increments of 2000?
              Mark Crabtree AAL3 | VP-HR

              Comment


                #9
                We never use the Step Climb function in my airline, other than to find out how much fuel we could save.
                We do not climb above the Optimum unless we are really in still air. The 737 has such a little margin near the maximum levels, the slightest turbulence would through you into either overspeed or underspeed.
                Therefore we normally only start the step climb once the OPT reaches the new cruise level.

                FMC Step Climb logic unfortunately does not take care of this so we just don't use it.
                Greetings,
                Emanuel Hagen

                Comment


                  #10
                  It looks like your weight entered into the FMC is lower than your actual weight. Such a situation would create a scenario like the one that you describes.

                  Søren Geertsen
                  Søren Geertsen

                  Comment


                  • Emi
                    Emi commented
                    Editing a comment
                    I disagree, the FMC will round the stepclimb up to the next 1000ft, so if you are in 360, the OPT is 370 and the max 380 it would command the step climb.
                    Interestingly enough though, in our fleet this would be impossible. Our OPT and MAX are no more than 800ft apart and not 1500ft as in the NGXu as we fly with a fixed cruise CG.

                    However, if you fly the aircraft right up to the MAX level the shown screenshot seems correct to me. It does get this narrow, that's realistic for the 737 (and exactly the reason why we don't fly up to MAX).

                  #11
                  But a step-climb would take the plane to the optimum flight levels and not the maximum flight levels?
                  Søren Geertsen

                  Comment


                    #12
                    Originally posted by N119UA View Post
                    But a step-climb would take the plane to the optimum flight levels and not the maximum flight levels?
                    No, it generally takes you to 1000ft above optimum (assuming a 2000ft step size and that the B737NG FMC logic is the same as the B744) so that the optimum level comes up to you. Then when OPT is 1000ft above you it will recommend another step climb and so on. The idea being that you should always be close to (within 1000ft of) the optimum level.

                    As Emanuel says, you do have to sense-check it to see whether it is going to put you too close to or over the MAX level...
                    Simon Kelsey

                    Comment


                      #13
                      I fully agree. That is the logic. But, I have never seen (in the PMDG 744 though!) that operating under ICAO-rules (4,000 feet increment or even RVSM) that the Step climb would take the plane within 1,000 of the maximum flight level, which I understand is the issue here. I have experienced the same issue once where the actual weight of the aircraft was higher than the weight entered in the FMC. The step climb function would take the plane to 2,000 feet above optimum - around 2,000 feet below maximum altitude - but as the weight had been entered wrongly in the FMC the actual max altitude was actually lower than the optimum altitude so the aircraft actually stalled prior to reaching the optimum altitude (in this case). The step-climb function would not know this.

                      The same situation may be the issue here, otherwise I would have believed that the Step Climb would have a built-in protection not advising to take the plane into a dangerous situation ie. the optimum altitude is safely below (more than 1,000 feet) the max altitude?

                      I appreciate that you should never fully trust the systems - and do necessary sense-checks. But, if everything is entered correctly the plane should not recommend to take you into a dangerous altitude?
                      Søren Geertsen

                      Comment


                        #14
                        I think the big difference between the 744 and and the 737NG is that the Jumbo has a much wider range between OPT and MAX which means it's rarely an issue (one of the consequences of endlessly stretching the original 737 design?).

                        I agree that STEP TO should in principle never be >MAX but I don't know enough about the 737 (or have the appropriate documentation to check) to say with any level of certainty that the software is definitely that smart (and as MAX alt is dynamically calculated AFAIK taking in to account weight and sensed OAT, I guess it is also feasible that the MAX value could change when you get up there if the OAT is different to the FMC's assumption). I'm pretty certain that there is no requirement in the software for STEP TO or OPT to have any particular margin to MAX (much less 1000ft) -- even in the Jumbo the only limitation is that "all altitude calculations are limited by MAX altitude".
                        Simon Kelsey

                        Comment


                          #15
                          In our aircraft we have about 600-800ft between OPT and MAX (we use a fixed cruise CG while PMDG uses the actual). If we fly in FL360 and enter a step climb to FL380 the FMC will step up as soon as the OPT is above 370 and the MAX is FL380.
                          With our aircraft that means that by the time the MAX FL is 380 the OPT will be roughly 372-374.
                          The step climb function in the 737 will fly you right up to the coffin corner, so better be careful with it.
                          A trick which I always use is to take the difference between OPT and MAX and to add that to the altitude I want to climb to.
                          So in the above example, assuming a split of 800ft I would enter FL378 as Step Altitude, then the S/C point would be calculated at the position where the OPT becomes FL370, which is the point where we would request the climb.
                          Greetings,
                          Emanuel Hagen

                          Comment


                            #16
                            Thanks. That is very interesting. I had thought there would have been built-in bigger margins, but if the step-climb function can take you directly to the coffin corner then you have to exercise a great deal of caution.

                            Great to learn something new and happy to see if I can use your trick when flying the NGXu.
                            Søren Geertsen

                            Comment


                              #17
                              Originally posted by Emi View Post
                              (we use a fixed cruise CG while PMDG uses the actual)..
                              Hi Emanuel,

                              a bit off topic, but would you mind to explain what that means? Thanks a lot in advance.

                              best regards
                              Manolo Ruiz Carrió

                              Comment


                                #18
                                Alright, if you look at the top left corner you see the CRZ CG. The centre of gravity during cruise.
                                In my airline we have it fixed at 8% which is a very conservative value. It gives you lower OPT and MAX levels than the actual numbers.
                                Therefore we are always on the more conservative side with all our checks and indications.
                                PMDG does unfortunately not model changes to this number. I raised a ticket about this in the past and it's being looked into but no promises are made.

                                PERF INIT.png
                                Greetings,
                                Emanuel Hagen

                                Comment


                                  #19
                                  Ah, I see..thank you. Great to always learn something new !

                                  kind regards
                                  Manolo Ruiz Carrió

                                  Comment

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