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VNAV Profile and AT Mode

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I'm not sure if this is a VNAV problem or really an auto-throttle problem, but several times now VNAV has bust the vertical profile whilst on descent.


The required descent angle to maintain profile has been shallower than the aircraft has been flying, putting it below path. When it has done this, the thrust has been in HOLD mode. The AT system does not appear to want to engage to maintain sufficient thrust once it enters this mode.


If I manually add thrust (mode still HOLD) then the aircraft will level off and capture the path, but I'm basically in manual thrust at this point, which is not what I would expect.


VNAV SPD INTV makes no difference - it just alters how fast VNAV flies, but it has no material affect on the AT mode and whether it flies the required path.


Does anyone have any insight into what it is doing and why I'm essentially having to manage thrust manually?


I've also had it bust the vertical profile during RNAV approach segment, ending up too high. It's like VNAV doesn't really care about the path at all.


Best regards,


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The VNAV profile is created so that the thrust can be at idle most of the time throughout the whole approach. Lowering the VNAV speed should shallow the glide path.

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