Narrative:

We were questioned upon contacting a different ZOB controller what altitude our aircraft was cleared to on a step-down descent. We checked in at FL230. (The altitude in the altitude selector.) the controller said he was expecting us to be at FL240. The crew was following our company procedures of the PNF dialing in the cleared altitude into the selector and the PF verifying the altitude prior to execution. The controller said there was not a problem and to continue to maintain FL230. At the post-flight brief; we think that the design of the collins proline 21 altitude 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 between clicks. We believe what may have occurred was the selector was actually set between FL240 and FL230; with the pfd's displaying FL240 at the time the clearance from FL270 was given. Sometime after; the knob advanced a full click to FL230 due to turbulence or aircraft vibration; without the crew noticing or without any pilot input. A write-up was made in the flight log and our maintenance department was briefed on the problem. The hawker 800XP is 3 yrs old and the selector was replaced once before because of a similar issue. We are advising all company personnel regarding this event. We also contacted our local FSDO office to advise them what had happened. Collins will be addressed as well.

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Original NASA ASRS Text

Title: H125 CREW RPTS ALTDEV THOUGHT TO HAVE BEEN CAUSED BY DESIGN FLAW IN ALT ALERT WINDOW WHICH IS TOO SENSITIVE TO ADJUST.

Narrative: 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 POST-FLT 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. A WRITE-UP WAS MADE IN THE FLT LOG AND OUR MAINT DEPT WAS BRIEFED ON THE PROB. THE HAWKER 800XP IS 3 YRS OLD AND THE SELECTOR WAS REPLACED ONCE BEFORE BECAUSE OF A SIMILAR ISSUE. WE ARE ADVISING ALL COMPANY PERSONNEL REGARDING THIS EVENT. WE ALSO CONTACTED OUR LCL FSDO OFFICE TO ADVISE THEM WHAT HAD HAPPENED. COLLINS WILL BE ADDRESSED AS WELL.

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.