Hawker 800XP flight crew has an altitude deviation.

Attributes
ACN	720501
Time
Date	200612
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zob.artcc
State Reference	OH
Altitude	msl single value : 23000
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Make Model Name	BAe 125 Series 800
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 80 flight time total : 12700 flight time type : 3000
ASRS Report	720501
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued advisory
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

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 postflt 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.

Original NASA ASRS Text

Title: HAWKER 800XP FLT CREW HAS AN ALTDEV.

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 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.

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.