Narrative:

We had been cleared to 5000' after takeoff and in successive steps, up to 8000', 10000', 12000' and then up to 14000', which maintaining an assigned 250 KTS airspeed. At 11000' we were cleared to 14000' and to resume normal airspeed. I set 14000' in the altitude MCP window and it was verified by the captain. I then began to complete required paperwork, such as time logs and the aircraft log. We were relatively light and were climbing at a high rate of climb. When the altitude alerter sounded, I looked up and called out, '1000 to level off.' shortly thereafter, center called asking us what altitude we were at and what we had been cleared to. We were at 14700' and 15000' was in the MCP altitude window. Since 15000' was showing and it was also our filed cruise altitude, we responded 15000'. Center then instructed us to descend to 14000' and turn 90 degrees to the right. I do not specifically recall being cleared to 15000' and neither pilot recalls touching the MCP altitude setting after 14000' was set. Apparently it had changed, and since the aircraft was being flown on the autoplt, it climbed to that altitude. Manufacturers have reported MCP altitude 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 system in place of the present one until the problem can be corrected in this extremely important (safety) device. Subject: nonselected MCP display changes. Reason: to advise flight crews that nonselected changes in altitude MCP window settings can occur. Background information: several operators have reported MCP altitude display changes occurring west/O warning during climb or descent. The majority of the changes were between 1000 and 2000', however, larger deviations have occurred. They have been attributed to transient electrical signals being transmitted from the MCP to the FCC. The FCC interprets the transient signals as selected changes and stores them in memory. The FCC then transmits these nonselected changes to the MCP which displays and uses the nonselected values as the target altitude for autoplt climb or descent. Undetected changes to the MCP display may cause the autoplt to fly the airplane to an unintended altitude. Recommended operating procedures: are investigating corrective actions. Until a solution to this problem is found, flight crews should be aware of the following recommended operating procedures: check MCP settings after any electrical power interruptions. Following MCP altitude window changes, monitor altitude display to ensure desired altitude is displayed. Closely monitor the altimeter during all altitude changes to ensure that the autoplt acquires and levels off at the correct altitude. Standard callouts, crew coordination and xchking of MCP settings and flight instruments are recommended to detect any nonselected MCP display changes.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of August 2007 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.