Narrative:

Flight was climbing to FL270 along J149 (30 DME west of aml) en route to ord. The autoplt was engaged and FL270 had been 'set and armed' in the flight management annunciator when the clearance to FL270 had been received by ZDC. However, FL270 was exceeded, the aural warning was heard and I immediately disengaged the autoplt and descended the aircraft back to FL270. The maximum altitude reached was FL274. Obviously neither the first officer or myself had monitored the level off closely enough, but why had the 'arm' function of the FMA been lost? There was perhaps one explanation. Shortly before this excursion, ZDC had requested that we 'keep the speed up for sequencing to ord.' I responded that we could pick up the mach to mach .78. The pitch mode of the FMA had been in 'IAS,' so I pushed the 'IAS/mach' selector to obtain the mach mode. If the FGS had already entered the 'altitude capture' phase, the selecting on 'mach' on the pitch control would have eliminated the 'altitude capture' phase and put the autothrottles into the 'clamp mode, which is where they were when the altitude excursion occurred. The excursion points out again that one must be aware of their phase of flight (by x-chking the flight management annunciator) before selecting a different mode.

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Original NASA ASRS Text

Title: MLG FLT CREW CHANGING FLT MANAGEMENT COMPUTER INADVERTENTLY TOOK OUT OF AUTOPLT FUNCTION. ALT DEV.

Narrative: FLT WAS CLBING TO FL270 ALONG J149 (30 DME W OF AML) ENRTE TO ORD. THE AUTOPLT WAS ENGAGED AND FL270 HAD BEEN 'SET AND ARMED' IN THE FLT MGMNT ANNUNCIATOR WHEN THE CLRNC TO FL270 HAD BEEN RECEIVED BY ZDC. HOWEVER, FL270 WAS EXCEEDED, THE AURAL WARNING WAS HEARD AND I IMMEDIATELY DISENGAGED THE AUTOPLT AND DSNDED THE ACFT BACK TO FL270. THE MAX ALT REACHED WAS FL274. OBVIOUSLY NEITHER THE F/O OR MYSELF HAD MONITORED THE LEVEL OFF CLOSELY ENOUGH, BUT WHY HAD THE 'ARM' FUNCTION OF THE FMA BEEN LOST? THERE WAS PERHAPS ONE EXPLANATION. SHORTLY BEFORE THIS EXCURSION, ZDC HAD REQUESTED THAT WE 'KEEP THE SPD UP FOR SEQUENCING TO ORD.' I RESPONDED THAT WE COULD PICK UP THE MACH TO MACH .78. THE PITCH MODE OF THE FMA HAD BEEN IN 'IAS,' SO I PUSHED THE 'IAS/MACH' SELECTOR TO OBTAIN THE MACH MODE. IF THE FGS HAD ALREADY ENTERED THE 'ALT CAPTURE' PHASE, THE SELECTING ON 'MACH' ON THE PITCH CTL WOULD HAVE ELIMINATED THE 'ALT CAPTURE' PHASE AND PUT THE AUTOTHROTTLES INTO THE 'CLAMP MODE, WHICH IS WHERE THEY WERE WHEN THE ALT EXCURSION OCCURRED. THE EXCURSION POINTS OUT AGAIN THAT ONE MUST BE AWARE OF THEIR PHASE OF FLT (BY X-CHKING THE FLT MGMNT ANNUNCIATOR) BEFORE SELECTING A DIFFERENT MODE.

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