A C550 flight crew deviated from their assigned altitude.

Attributes
ACN	792466
Time
Date	200806
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : zkc.artcc
State Reference	KS
Altitude	msl single value : 39000
Environment
Flight Conditions	Mixed
Weather Elements	Thunderstorm Turbulence
Light	Night
Aircraft 1
Controlling Facilities	artcc : zkc.artcc
Operator	general aviation : corporate
Make Model Name	Citation II S2/Bravo
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 65 flight time total : 9400 flight time type : 1000
ASRS Report	792466
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

While in level cruise at FL390 and deviating around thunderstorm; I (captain) noted a flicker on my EFIS display and then all was normal. I had been deviating around the storm using the heading bug. I first noted that the aircraft was not responding to my heading commands. Then I noticed the altitude was not holding and drifting up. I arrested the ascent and was able to limit it to 260 ft per my altimeter. Once back on altitude; we (the crew) looked for the cause. It seems when the first officer adjusted the dim control on the radar he relocated the symbol generator selector as well. (The symbol generator control knob is co-located with the radar dim control.) when the symbol generator is moved from the 'both' selection (as he did momentarily); it causes the autoplt to follow the selection and because the first officer's altitude hold was not selected; the aircraft had no altitude hold command from the autoplt. I feel that it is a poor design to co-locate these controls on the mfd control panel. But given that; the flight director controls of both #1 and #2 need to be operated in synchronized fashion to prevent discrepancies in the event the autoplt reverts to PNF side. No comment was received from ATC and the discrepancy was less than 300 ft; therefore; a height keeping error report has not been filed.

Original NASA ASRS Text

Title: A C550 FLT CREW DEVIATED FROM THEIR ASSIGNED ALT.

Narrative: WHILE IN LEVEL CRUISE AT FL390 AND DEVIATING AROUND TSTM; I (CAPT) NOTED A FLICKER ON MY EFIS DISPLAY AND THEN ALL WAS NORMAL. I HAD BEEN DEVIATING AROUND THE STORM USING THE HDG BUG. I FIRST NOTED THAT THE ACFT WAS NOT RESPONDING TO MY HDG COMMANDS. THEN I NOTICED THE ALT WAS NOT HOLDING AND DRIFTING UP. I ARRESTED THE ASCENT AND WAS ABLE TO LIMIT IT TO 260 FT PER MY ALTIMETER. ONCE BACK ON ALT; WE (THE CREW) LOOKED FOR THE CAUSE. IT SEEMS WHEN THE FO ADJUSTED THE DIM CTL ON THE RADAR HE RELOCATED THE SYMBOL GENERATOR SELECTOR AS WELL. (THE SYMBOL GENERATOR CTL KNOB IS CO-LOCATED WITH THE RADAR DIM CTL.) WHEN THE SYMBOL GENERATOR IS MOVED FROM THE 'BOTH' SELECTION (AS HE DID MOMENTARILY); IT CAUSES THE AUTOPLT TO FOLLOW THE SELECTION AND BECAUSE THE FO'S ALT HOLD WAS NOT SELECTED; THE ACFT HAD NO ALT HOLD COMMAND FROM THE AUTOPLT. I FEEL THAT IT IS A POOR DESIGN TO CO-LOCATE THESE CTLS ON THE MFD CTL PANEL. BUT GIVEN THAT; THE FLT DIRECTOR CTLS OF BOTH #1 AND #2 NEED TO BE OPERATED IN SYNCHRONIZED FASHION TO PREVENT DISCREPANCIES IN THE EVENT THE AUTOPLT REVERTS TO PNF SIDE. NO COMMENT WAS RECEIVED FROM ATC AND THE DISCREPANCY WAS LESS THAN 300 FT; THEREFORE; A HEIGHT KEEPING ERROR RPT HAS NOT BEEN FILED.

Data retrieved from NASA's ASRS site as of May 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.