ALTDEV. DSCNT THROUGH ASSIGNED ALT.

Narrative

DSNDING TO 10000 AS CLRED ACFT WAS ON AUTOPLT AND DSNDING 3500 FPM OUT OF APPROX 11500 FT MSL. AT ABOUT 10700 FT MSL; CAPT ATTEMPTED TO SLOW DSCNT RATE AT EXACT SAME TIME AUTOPLT SWITCHED TO ALT CAPTURE MODE: WHICH TRIGGERED THE PITCH MODE DEFAULT. WE BOTH SAW THE PROB AND I SUGGESTED THE ALT HOLD MODE WHICH CAPT THEN ENGAGED. WHILE PASSING THROUGH 10000 FT CAPT CHANGED MORE AGAIN WHICH CAUSED DEFAULT TO PITCH MODE AGAIN. ACFT CONTINUED TO DSND AT APPROX 2000 FPM AND; WHILE PASSING 9800 FT; AUTOPLT WAS DISCONNECTED AND ALT TREND REVERSED AT 9500 FT MSL.THERE WAS NO CONFLICT WITH OTHER TFC AND THE CTLR JUST VERIFIED OUR ASSIGNED ALT. IN FACT HE CLRED US TO 9000 FT BEFORE ASKING. PROB AROSE WITH ATTEMPTED MODE CHANGE ON AUTOPLT; AND ALT DEV OCCURRED. CONTRIBUTING FACTORS: 1) DSCNT INTENTIONALLY STARTED LATE TO SAVE FUEL. 2) HIGH DSCNT RATE USED 3500 FPM AND 420 KIAS. 3) BOTH PLTS HORIZ DISTR. 4) BAD TIMING ON MODE CHANGE BY CAPT (BOTH TIMES). 5) UNCONVENTIONAL AUTOPLT -- (IN MY OPINION). 6) FATIGUE.