LEAR LR-60 FLT CREW HAS AN ALTDEV.

Narrative

WE WERE CLBING THROUGH FL230 ON THE WAY UP TO FL250. I HAD SET THE ALT ALERTER TO FL250 AND BECAME BUSY WITH PNF DUTIES AND CONVERSING WITH THE PAX. I DID NOTICE THAT WE WERE CLBING AT APPROX 4500 FPM AND WAS ANTICIPATING THE AUTOPLT TO CAPTURE THE ALT SET IN THE ALT ALERTER NEARLY 1000 FT PRIOR TO THE ALT THAT WAS SET. THE ALTDEV TONE WAS HEARD AND THAT IS WHEN THE PF DISENGAGED THE AUTOPLT AND TOOK MANUAL CTL OF THE ACFT. THE AUTOPLT IS EXTREMELY SLOW IN REVERSING A CLB TO A DSCNT; ESPECIALLY WHEN THE ACFT IS CLBING WITH SUCH A HIGH VERT RATE. FOR THIS REASON THE PF DISENGAGED THE AUTOPLT TO CORRECT FOR THE ALTDEV AS QUICKLY AS POSSIBLE. BY THE TIME THE ACFT WAS ABLE TO BEGIN A DSCNT BACK TO THE ASSIGNED ALT; DUE TO THE MOMENTUM OF CLBING AT 4500 FPM; AN UNACCEPTABLE ALTDEV HAD OCCURRED. WE WERE PRO-ACTIVE IN NOTIFYING ATC TO LET THEM KNOW THAT WE WERE MAKING THE CORRECTIVE ACTION BACK TO THE ASSIGNED FLT LEVEL OF FL250. A COMMON ERROR FOR MANY LR60 PLTS IS TO TRIM THE ACFT AFTER THE AUTOPLT HAS CAPTURED THE PRE-SELECTED ALT. WHEN THIS IS DONE; THE ALT PRE-SELECT IS DISENGAGED AND THE AUTOPLT WILL NOT CAPTURE THE ALT SET IN THE ALERTER UNLESS THE ALT PRE-SELECT IS RE-ENGAGED. THIS IS LIKELY WHAT HAPPENED ON THIS OCCURRENCE -- THE PF BEGAN TO TRIM THE ACFT TO REDUCE THE HIGH RATE OF CLB UPON HEARING THE ALT ALERTER 1000 FT PRIOR TO REACHING THE ALT THAT WAS SET IN THE ALT ALERTER. IN SO DOING; THE ALT PRE-SELECT WAS DISENGAGED AND THE ACFT WENT INTO NORMAL PITCH MODE AND; THEREFORE; DID NOT CAPTURE THE ALT THAT WAS SET.