Narrative:

We were climbing through FL230 on the way up to FL250. I had set the altitude alerter to FL250 and became busy with PNF duties and conversing with the passenger. I did notice that we were climbing at approximately 4500 FPM and was anticipating the autoplt to capture the altitude set in the altitude alerter nearly 1000 ft prior to the altitude that was set. The altitude deviation tone was heard and that is when the PF disengaged the autoplt and took manual control of the aircraft. The autoplt is extremely slow in reversing a climb to a descent; especially when the aircraft is climbing with such a high vertical rate. For this reason the PF disengaged the autoplt to correct for the altitude deviation as quickly as possible. By the time the aircraft was able to begin a descent back to the assigned altitude; due to the momentum of climbing at 4500 FPM; an unacceptable altitude deviation had occurred. We were pro-active in notifying ATC to let them know that we were making the corrective action back to the assigned flight level of FL250. A common error for many LR60 pilots is to trim the aircraft after the autoplt has captured the pre-selected altitude. When this is done; the altitude pre-select is disengaged and the autoplt will not capture the altitude set in the alerter unless the altitude pre-select is re-engaged. This is likely what happened on this occurrence -- the PF began to trim the aircraft to reduce the high rate of climb upon hearing the altitude alerter 1000 ft prior to reaching the altitude that was set in the altitude alerter. In so doing; the altitude pre-select was disengaged and the aircraft went into normal pitch mode and; therefore; did not capture the altitude that was set.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.