A B757 FLT CREW INADVERTENTLY SELECTED WRONG BUTTON ON THE MODE CTL SELECTOR; RESULTING IN AN OVERSHOOT OF THEIR ASSIGNED ALT.

Narrative

THIS TRIP WAS A LINE OPERATING EXPERIENCE (LOE) FOR A NEW CAPT (L SEAT; PF) BEING ADMINISTERED BY ANOTHER CAPT ACTING AS FO (R SEAT; PNF). THE PARADISE ARR TO LAX WAS BEING FLOWN WITH THE FMS AS A DEMONSTRATION WITH BOTH VNAV AND LNAV ENGAGED TO 250 KTS. ALL VERT AND LATERAL CTLS WERE BEING COMMANDED BY THE FMS AND AUTOFLT SYS. WHEN A SPD REDUCTION WAS DIRECTED BY SOCAL APCH THE SPD SELECT BUTTON WAS PUSHED (IN ERROR) INSTEAD OF SELECTING SPD INTERVENTION ON THE AUTOFLT CTL PANEL. THE LINE CHK PLT DID NOT NOTICE THE ERROR UNTIL A XCHK AT THE PDZ VOR -- MINIMUM ALT IS 12000 FT; ACFT WAS APCHING 11000 FT MSL. WE IMMEDIATELY STOPPED THE DSCNT AND THEN CONTINUED WITHOUT INCIDENT. WE WERE TASK SATURATED -- GO DOWN; SLOW DOWN; RADIO CALLS; CHKLISTS; AND TRYING TO EXPLAIN THE APCH AND HOW TO BEST HANDLE THE SLOW DOWN/GO DOWN MANEUVER. I SAW HIM HIT THE BUTTON (RIGHT NEXT TO THE CORRECT BUTTON) SAW THE SPD WINDOW OPEN FOR THE SLOW DOWN; BUT DID NOT NOTICE WE CAME OUT OF VNAV AND WERE IN VERT SPD. WE FLEW RIGHT THROUGH THE ALT.