Narrative:

Medium large transport with FMC-EFIS descending into sfo from iah. Captain flying, first officer performing all other PNF duties. Aircraft just leveled at 240 after descent from FL280. First officer 'off the air' giving final PA announcement to passenger. Upon returning to the frequency, first officer heard captain acknowledge ATC transmission for clearance to 11000 ft. Autoplt was engaged throughout entire flight with navigation and LNAV modes engaged. Upon clearance to 11000 ft, captain points to altitude selector window and first officer selects 11000. Then followed a brief discussion as to fact that FL240 should be maintained until passing a fix about 3 mi in front of aircraft at which point first officer selects FL240 on altitude selector although momentarily overshooting selected altitude to FL250. During this time, aircraft had begun descent from FL240 to about FL236 at which time airspeed dropped abruptly from 280 KIAS to 210 KIAS and nose pitched sharply up to 15 degree. Aircraft began rapid climb of about 2500-3000 FPM and reached 24800 ft, by the time captain disconnected the autoplt to level aircraft and begin descent to appropriate altitude. The cause of this uncommanded climb was never determined by crew and did not result in any traffic conflict to our knowledge. Taking into account the complexity of the medium large transport FMC and its ability to revert automatically from one mode to another as well as the high cockpit workload at this point, one has no time to try and diagnose the reason behind an unwanted autoplt action and disconnection is the only prudent action.

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Original NASA ASRS Text

Title: ACR MLG ALTDEV EXCURSION FROM CLRNC ALT THEN ALT OVERSHOOT WHEN RETURNING TO CLRNC ALT. ALL WITH THE 'HELP' OF FMC AND AUTOPLT.

Narrative: MLG WITH FMC-EFIS DSNDING INTO SFO FROM IAH. CAPT FLYING, FO PERFORMING ALL OTHER PNF DUTIES. ACFT JUST LEVELED AT 240 AFTER DSCNT FROM FL280. FO 'OFF THE AIR' GIVING FINAL PA ANNOUNCEMENT TO PAX. UPON RETURNING TO THE FREQ, FO HEARD CAPT ACKNOWLEDGE ATC XMISSION FOR CLRNC TO 11000 FT. AUTOPLT WAS ENGAGED THROUGHOUT ENTIRE FLT WITH NAV AND LNAV MODES ENGAGED. UPON CLRNC TO 11000 FT, CAPT POINTS TO ALT SELECTOR WINDOW AND FO SELECTS 11000. THEN FOLLOWED A BRIEF DISCUSSION AS TO FACT THAT FL240 SHOULD BE MAINTAINED UNTIL PASSING A FIX ABOUT 3 MI IN FRONT OF ACFT AT WHICH POINT FO SELECTS FL240 ON ALT SELECTOR ALTHOUGH MOMENTARILY OVERSHOOTING SELECTED ALT TO FL250. DURING THIS TIME, ACFT HAD BEGUN DSCNT FROM FL240 TO ABOUT FL236 AT WHICH TIME AIRSPD DROPPED ABRUPTLY FROM 280 KIAS TO 210 KIAS AND NOSE PITCHED SHARPLY UP TO 15 DEG. ACFT BEGAN RAPID CLB OF ABOUT 2500-3000 FPM AND REACHED 24800 FT, BY THE TIME CAPT DISCONNECTED THE AUTOPLT TO LEVEL ACFT AND BEGIN DSCNT TO APPROPRIATE ALT. THE CAUSE OF THIS UNCOMMANDED CLB WAS NEVER DETERMINED BY CREW AND DID NOT RESULT IN ANY TFC CONFLICT TO OUR KNOWLEDGE. TAKING INTO ACCOUNT THE COMPLEXITY OF THE MLG FMC AND ITS ABILITY TO REVERT AUTOMATICALLY FROM ONE MODE TO ANOTHER AS WELL AS THE HIGH COCKPIT WORKLOAD AT THIS POINT, ONE HAS NO TIME TO TRY AND DIAGNOSE THE REASON BEHIND AN UNWANTED AUTOPLT ACTION AND DISCONNECTION IS THE ONLY PRUDENT ACTION.

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.