ACR MLG OVERSHOT ALT IN CLIMB.

Narrative

WE HAD BEEN CLRED TO 5000' AFTER TKOF AND IN SUCCESSIVE STEPS; UP TO 8000'; 10000'; 12000' AND THEN UP TO 14000'; WHICH MAINTAINING AN ASSIGNED 250 KTS AIRSPD. AT 11000' WE WERE CLRED TO 14000' AND TO RESUME NORMAL AIRSPD. I SET 14000' IN THE ALT MCP WINDOW AND IT WAS VERIFIED BY THE CAPT. I THEN BEGAN TO COMPLETE REQUIRED PAPERWORK; SUCH AS TIME LOGS AND THE ACFT LOG. WE WERE RELATIVELY LIGHT AND WERE CLBING AT A HIGH RATE OF CLB. WHEN THE ALT ALERTER SOUNDED; I LOOKED UP AND CALLED OUT; '1000 TO LEVEL OFF.' SHORTLY THEREAFTER; CENTER CALLED ASKING US WHAT ALT WE WERE AT AND WHAT WE HAD BEEN CLRED TO. WE WERE AT 14700' AND 15000' WAS IN THE MCP ALT WINDOW. SINCE 15000' WAS SHOWING AND IT WAS ALSO OUR FILED CRUISE ALT; WE RESPONDED 15000'. CENTER THEN INSTRUCTED US TO DSND TO 14000' AND TURN 90 DEGS TO THE RIGHT. I DO NOT SPECIFICALLY RECALL BEING CLRED TO 15000' AND NEITHER PLT RECALLS TOUCHING THE MCP ALT SETTING AFTER 14000' WAS SET. APPARENTLY IT HAD CHANGED; AND SINCE THE ACFT WAS BEING FLOWN ON THE AUTOPLT; IT CLBED TO THAT ALT. MANUFACTURERS HAVE RPTED MCP ALT DISPLAY CHANGES OCCURRING BY THEMSELVES; APPARENTLY CAUSED BY TRANSIENT ELECTRICAL SIGNALS. IT IS MY BELIEF THAT THIS IS WHAT HAPPENED TO US ALSO; PERHAPS MFR SHOULD INSTALL AN ALTERNATE SYS IN PLACE OF THE PRESENT ONE UNTIL THE PROB CAN BE CORRECTED IN THIS EXTREMELY IMPORTANT (SAFETY) DEVICE. SUBJECT: NONSELECTED MCP DISPLAY CHANGES. REASON: TO ADVISE FLT CREWS THAT NONSELECTED CHANGES IN ALT MCP WINDOW SETTINGS CAN OCCUR. BACKGROUND INFO: SEVERAL OPERATORS HAVE RPTED MCP ALT DISPLAY CHANGES OCCURRING W/O WARNING DURING CLB OR DSCNT. THE MAJORITY OF THE CHANGES WERE BTWN 1000 AND 2000'; HOWEVER; LARGER DEVIATIONS HAVE OCCURRED. THEY HAVE BEEN ATTRIBUTED TO TRANSIENT ELECTRICAL SIGNALS BEING XMITTED FROM THE MCP TO THE FCC. THE FCC INTERPRETS THE TRANSIENT SIGNALS AS SELECTED CHANGES AND STORES THEM IN MEMORY. THE FCC THEN XMITS THESE NONSELECTED CHANGES TO THE MCP WHICH DISPLAYS AND USES THE NONSELECTED VALUES AS THE TARGET ALT FOR AUTOPLT CLB OR DSCNT. UNDETECTED CHANGES TO THE MCP DISPLAY MAY CAUSE THE AUTOPLT TO FLY THE AIRPLANE TO AN UNINTENDED ALT. RECOMMENDED OPERATING PROCS: ARE INVESTIGATING CORRECTIVE ACTIONS. UNTIL A SOLUTION TO THIS PROB IS FOUND; FLT CREWS SHOULD BE AWARE OF THE FOLLOWING RECOMMENDED OPERATING PROCS: CHK MCP SETTINGS AFTER ANY ELECTRICAL PWR INTERRUPTIONS. FOLLOWING MCP ALT WINDOW CHANGES; MONITOR ALT DISPLAY TO ENSURE DESIRED ALT IS DISPLAYED. CLOSELY MONITOR THE ALTIMETER DURING ALL ALT CHANGES TO ENSURE THAT THE AUTOPLT ACQUIRES AND LEVELS OFF AT THE CORRECT ALT. STANDARD CALLOUTS; CREW COORD AND XCHKING OF MCP SETTINGS AND FLT INSTRUMENTS ARE RECOMMENDED TO DETECT ANY NONSELECTED MCP DISPLAY CHANGES.