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clayton4115

VNAV still not working properly

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Vnav doesnt work if when you are climbing you accidently and inadvertently put flap down and then up again, the plane will descend never to climb again unless you do V/S. The VNAV does not work.

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ClaytonAs you have no doubt seen from the earlier thread in which this was highlighted, something does happen. VNAV does not just stop working - it switches to a descent mode. This is perhaps understandable for a flap selection - the problem was that we could find no way of reverting the FMC back to the correct required climb mode once the flaps are retracted.RegardsJohn R

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I surely hope not ......!Just hope that someone with 777 experience is around to put us right. Back to my Dubai flight.John R

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There is something amiss with the VNAV. On my last flight from RJAA to KSFO, VNAV would not engage. I stopped the flight and started over again four times to make sure that it wasn't any mistake I made to cause this, but everytime VNAV never engaged. The autothrottle mode on the flight mode annunciation display remained blank and the vertical cue of the flight director remained still. I either lost significant speed to the point of stalling or the plane started to descend dangerously low, indicating to me VNAV was not engaged. If I hadn't switched from VNAV to FLCH, the plane would have stalled or gone straight down to the ground on its own. After the fourth time with the same result, I did a control flight(a successful flight I made from PHNL to KLAX a day before)and had no problems with VNAV. Past FL015, I had a THRREF message on the autopilot mode annunciation. The vertical cue of the FD was also working properly and I could see the VNAV was engaged. This has happened to me before the patch. On some routes, not all, VNAV would not engage on my initial climb. I've also read the post by PSOLK on his experience with losing VNAV during s/c, so it leads me to believe there is stil something wrong with the VNAV even after the patch.-Ely

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Hi all, It would certainly seem from your posts that there is something amiss with VNAV in the FMS.I want to preface my remarks by saying that I am learning the PEGASUS FMC, so I would greatly appreciate some understanding if some of my questions sound more obvious than others. I am using the technique of "rule out" to see if there is actually something buggy or if some of this is just the nature of this particular FMC.OK, regarding Clayton's remarks that if you inadvertently extend and retract flaps, it seems to change VNAV mode from climb to descent mode and you can't get it back (paraphrasing...Clayton, is this interpretation correct?)I am looking at the systems manual and the FMC Guide and one particular point does seem to stand out: VNAV goes CLB, CRZ, DES (rather obvious, right?). What is not so obvious is the fact that VNAV modes are changed based on certain aircraft actions. I could not find any statement in any of the guides specifically referencing flap extension in the climb, but logic dictates that if the flaps are retracted then extended, the VNAV mode should advance forward. Once it advances forward, it cannot return back to CLB mode. I must stress that this is a theory based on logic; I cannot find anything in the systems manuals that speaks to it.Suggestion: If any of you have the LevelD 767, set up the same flight and operate it the same way you did with the 777; extend the flaps as you did accidently and see if it changes the VNAV from CLB to DES. My sense is that if it is a FMC logic issue, you will not be able to revert back to VNAV climb. Yes, the 767 has a different FMC than the 777, but that's the point: If the fault is repeated, then its a logic issue. If you can't, then you can assume 2 things: a) its the PEGASUS FMC that has the problem or :( there's a bug in the VNAV programming in the PSS777 FMC.You might say, why should I do all this testing...shouldn't PSS do it as customer support? The answer is yes. PSS has an obligation to attempt to repeat the failure and if they can repeat it (this is the critical point), determine if it is a logic problem or a bug. If we as a community can help them by attempting the same maneuver on different aircraft, it will speed up the process, since certain things can be ruled out based on our testing. I will be doing another test flight later today with some of these considerations, plus try to separate out what I reported in my flight post...what fault was the computer, what fault was pilot driven and what fault is software glitch.Thankfully, we have enough pilots on this forum who recognize when something doesn't go right. That speaks volumes to the quality of the virtual pilots who fly on this forum. That's probably the most positive point.Finally, I sincerely hope that I am not coming across as "preaching" on this subject. I am spending a lot of time on this right now (disability has a way of giving me more time than I really want), so I hope that what I post is contributing to the discussion. If any of you think differently, send me a PM with your thoughts (and yes, I know my posts are lengthy..but I prefer detail in these discussions to brevity). I do appreciate feedback, even if it is not favorable.After recent events, let me say I'm just a little sensitive about rattling other people's sensitivities and I don't want to make the same mistake twice.Many thanks and kind regards,Dave Lamb

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Dave,Not sure of the why of your last sentence. To me, a very sensible post. PSS - Norman and Steven etc - are busy enough at the moment answering the more straightforward questions appearing this weekend without worrying about the more esoteric qustions that we are raising.I have had no problems with PSS VNAV until I threw in the odd pilot error like dropping flaps by mistake, and I agree with your logic that this should advance the VNAV mode. Question is - is there any way to revert back? I feel that there should be - wish I could get into a real 777 sim to sort it out!Best WishesJohn Rooum

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>Question>is - is there any way to revert back? Revert back to what John?

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Hi Norman,Drop flaps in the climb and you are stuck in a VNAV descent mode - which the aircraft will fly at about 1500 fpm down whatever you do. Of course it easy to change to VS climb etc etc but we could find no way to revert back to a VNAV climb mode - and the subsequent VNAV cruise and descent modes.Purely as an interesting question as a result of a pilot pigs!RegardsJohn

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I also have this problem. I had to revert to flaps 1 during climb out as the target speed and the flaps up speed were the same and when the wind changed the acft speed went back into flaps 1 area. That was it then, the FMC went into DEC mode and I could not recover. Very annoying when I'd spent over an hour setting up the flight to have to start again within the first 20 mins.Paul Gardner

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Clayton,One solution (given to me by a pilot):On initial climb out, use HDG SEL and SPD+V/S or FLCH (V/S is better if you have to maintain significant vertical speed control). Do not engage LNAV until near the first waypoint and VNAV until above 10000ft. In this way, flap retraction (or possible extension) will not create the situation of VNAV advancing to DES mode.This is a very different approach. In reading the CO FOM for the 777-200ER, it states to arm both LNAV/VNAV on the ground and they will auto engage at 400ft. Of course, if I don't derate the takeoff and hit VNAV, the thing will climb like a rocket...and sometimes that's more than I can handle when I'm doing three other things to maintain control.Just a thought.Dave Lamb

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that sounds like a good idea Davebut it still begs the q's does the real 777 AC behave in this way, i still don't think so.

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Clayton,According to the my interpretation of the flight manual it does...but I know a pilot whom I can ask and get the straight information. I'll fire off an e-mail to him and get back to you on the forum.Kind regards,Dave

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