Guys thanks for the answers so far, maybe some misunderstand. I am completely aware of changes to the LNAV track adjusting the distance to go and hence altering my relative position with regards to the VNAV Path. My question is more about why such a change often screws up VNAV. For instance, an example would be with 60nm to go to touchdown, passing 18,000ft at about 270kts, with little wind component, I would say that that is 'in the ball park', but I have had VNAV tell me I am 2500ft off profile (high or low!).
It seems that the re-calculation once you have executed a 'direct-to' is what triggers this mis-calculation. I am NOT talking about the inevitable high or low on profile because you have lost or gained track mileage. I mean when the track mileage, speed, wind component, deceleration and everything else fit the passing Flight Level/ Altitude. FMC is U10.6.
in deference I have always assumed it knows more than me
Not something I am prone to doing as I think, an FMC doesn't.
sometimes ....... abandoned me and left me to sort out its mistake.
These are the times I am asking about. I understand if it just can't do it due to a suddenly increased tailwind etc, but I mean when it really gets its knickers in a twist simply due to a forced re-calculation as the result of an LNAV update.
PP