HAWKER 800XP FLT CREW HAS AN ALTDEV.

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WE WERE QUESTIONED UPON CONTACTING A DIFFERENT ZOB CTLR WHAT ALT OUR ACFT WAS CLRED TO ON A STEP-DOWN DSCNT. WE CHKED IN AT FL230. (THE ALT IN THE ALT SELECTOR.) THE CTLR SAID HE WAS EXPECTING US TO BE AT FL240. THE CREW WAS FOLLOWING OUR COMPANY PROCS OF THE PNF DIALING IN THE CLRED ALT INTO THE SELECTOR AND THE PF VERIFYING THE ALT PRIOR TO EXECUTION. THE CTLR SAID THERE WAS NOT A PROB AND TO CONTINUE TO MAINTAIN FL230. AT THE POSTFLT BRIEF; WE THINK THAT THE DESIGN OF THE COLLINS PROLINE 21 ALT SELECTOR MAY HAVE BEEN A FACTOR ALONG WITH A POSSIBLE MALFUNCTION OF THE UNIT. THE SELECTION KNOB HAS VERY FINE TUNING CLICKS FOR EACH 1000 FT PER CLICK. THE KNOB ALSO ALLOWS THE USER TO STOP BTWN CLICKS. WE BELIEVE WHAT MAY HAVE OCCURRED WAS THE SELECTOR WAS ACTUALLY SET BTWN FL240 AND FL230; WITH THE PFD'S DISPLAYING FL240 AT THE TIME THE CLRNC FROM FL270 WAS GIVEN. SOMETIME AFTER; THE KNOB ADVANCED A FULL CLICK TO FL230 DUE TO TURB OR ACFT VIBRATION; WITHOUT THE CREW NOTICING OR WITHOUT ANY PLT INPUT.