AN ACR B757 FLC DSNDED BELOW THEIR ASSIGNED ALT WHEN THE FLC BECAME DISTRACTED.

Narrative

COPLT WAS HAND FLYING AND WE WERE CLRED TO 5000 FT. ALL WAS STABLE AT 5000 FT AND THEN I WAS HAVING TROUBLE ADJUSTING MY SEAT FOR THE APCH (MY HEAD WAS DOWN). THEN I HEARD THE ALT ALERT GO OFF AND I SAW WE WERE IN A VERY SHALLOW/SMOOTH DSCNT PASSING THROUGH 4700 FT MSL. THE DSCNT WAS CHKED AT 4650 FT MSL AND WE RETURNED TO 5000 FT. SHORTLY AFTER THE ALT ALERT APCH CTL CAME UP AND VERY MATTER OF FACTLY TOLD US TO MAINTAIN 5000 FT AT WHICH I REPLIED 'ROGER.' THAT DESCRIBES THE WHOLE EVENT. NO TFC CONFLICT AROSE AND APCH CTL DID NOT SEEM ALARMED. THE COPLT WAS VERY UPSET. HE SAID HE WAS LOOKING OUTSIDE FOR TFC (VERY CLR VFR DAY OVER THE OCEAN BTWN SANTA CATALINA ISLAND AND THE MAINLAND) AND HE HAD NOT NOTICED THE DSCNT. IT NEVER FAILS; THE MOMENT YOU GET DISTRACTED (MY SEAT) SOMETHING OCCURS. I KNOW THIS PLT TO BE TOP NOTCH AND VERY CONSCIENTIOUS AS WELL AS BEING VERY DISCIPLINED. TO PREVENT THIS FROM EVER OCCURRING AGAIN I BELIEVE USING THE AUTOPLT TO MAINTAIN ALT DURING TFC WATCH WOULD HAVE BEEN THE BEST COURSE OF ACTION.