Originally Posted by
BARKINGMAD
The autopilot was not at fault in the level off.
It was the trim system which was slow to react, so the 'frame would climb slightly above alt and eventually find itself back to datum.
Not quite. Whilst the trim wasn't brilliant and caused a certain amount of porposing, especially close to the barbers pole, the reason for the climb after altitude capture was due to a lack of PE correction in altitude hold. An increase in speed caused a climb and vice versa, one of the reasons for lack of RVSM approval.
We used to select an altitude 100' below the clearance and once captured the acceleration from cruise to climb speed caused a 100' climb. Conversely if you climbed at high speed and then decelerated (usually when you switched on the anti ice
) you would descend slightly.
It is one of the best handling airliners and it had switches which you could actually see in the dark and geared turbo fans, which ain't anything new Messers Pratt and Whitney.