Narrative:

Our flight had descended to FL280 with pilot's discretion to FL240. The captain was flying the leg. Both of us were tired, having flown 2 long legs with 3 to go. Captain chose to descend in vertical speed mode to FL280. Reaching FL280 the altitude alerter was reset to FL240. Altitude hold was confirmed as the active mode before resetting the alerter. After setting FL240 in the window altitude hold remains the active vertical mode, however vertical speed is now the armed mode and any movement of the vertical trim wheel will activate vertical speed mode. Additionally, with the altitude alerter set to FL240, no deviation warnings are generated if aircraft departs from FL280. My guess as to what caused the aircraft to deviate from FL280 was an inadvertent bump of the vertical trim wheel or some other unknown cause because the aircraft climbed, unbeknownst to either crew member, to FL298 before we both became aware of it as the aircraft began a slight pitching moment in response to what felt like mountain wave activity. The captain changed modes quickly and I never saw what vertical modes were active during the deviation. During the deviation I had been preoccupied with filing away approach plates and setting up for the arrival to lax. The captain was likewise preoccupied with similar tasks. A better way to make a cruise descent (where TOD is not a concern) would have been to set FL240 in the alerter first, FL280 on the crz page of the FMC and made a cruise descent to FL280. Lapses in attention are not uncommon when fatigue becomes a factor, however, this incident reinforces the extra vigilance required whenever any modes, vertical or lateral, are armed.

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Original NASA ASRS Text

Title: MLG ACFT IN CRUISE DEVIATED FROM ASSIGNED ALT WHEN VERT SPD MODE SHIFTED DUE TO INADVERTENT ACTUATION OF TRIM WHEEL.

Narrative: OUR FLT HAD DSNDED TO FL280 WITH PLT'S DISCRETION TO FL240. THE CAPT WAS FLYING THE LEG. BOTH OF US WERE TIRED, HAVING FLOWN 2 LONG LEGS WITH 3 TO GO. CAPT CHOSE TO DSND IN VERT SPD MODE TO FL280. REACHING FL280 THE ALT ALERTER WAS RESET TO FL240. ALT HOLD WAS CONFIRMED AS THE ACTIVE MODE BEFORE RESETTING THE ALERTER. AFTER SETTING FL240 IN THE WINDOW ALT HOLD REMAINS THE ACTIVE VERT MODE, HOWEVER VERT SPD IS NOW THE ARMED MODE AND ANY MOVEMENT OF THE VERT TRIM WHEEL WILL ACTIVATE VERT SPD MODE. ADDITIONALLY, WITH THE ALT ALERTER SET TO FL240, NO DEV WARNINGS ARE GENERATED IF ACFT DEPARTS FROM FL280. MY GUESS AS TO WHAT CAUSED THE ACFT TO DEVIATE FROM FL280 WAS AN INADVERTENT BUMP OF THE VERT TRIM WHEEL OR SOME OTHER UNKNOWN CAUSE BECAUSE THE ACFT CLBED, UNBEKNOWNST TO EITHER CREW MEMBER, TO FL298 BEFORE WE BOTH BECAME AWARE OF IT AS THE ACFT BEGAN A SLIGHT PITCHING MOMENT IN RESPONSE TO WHAT FELT LIKE MOUNTAIN WAVE ACTIVITY. THE CAPT CHANGED MODES QUICKLY AND I NEVER SAW WHAT VERT MODES WERE ACTIVE DURING THE DEV. DURING THE DEV I HAD BEEN PREOCCUPIED WITH FILING AWAY APCH PLATES AND SETTING UP FOR THE ARR TO LAX. THE CAPT WAS LIKEWISE PREOCCUPIED WITH SIMILAR TASKS. A BETTER WAY TO MAKE A CRUISE DSCNT (WHERE TOD IS NOT A CONCERN) WOULD HAVE BEEN TO SET FL240 IN THE ALERTER FIRST, FL280 ON THE CRZ PAGE OF THE FMC AND MADE A CRUISE DSCNT TO FL280. LAPSES IN ATTN ARE NOT UNCOMMON WHEN FATIGUE BECOMES A FACTOR, HOWEVER, THIS INCIDENT REINFORCES THE EXTRA VIGILANCE REQUIRED WHENEVER ANY MODES, VERT OR LATERAL, ARE ARMED.

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.