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

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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.