CAPT OF AN LGT ACR ACFT TURNED TOO SOON ON A SID DEP AND OVERSHOT ASSIGNED ALT DUE TO ATTEMPTING TO HAVE THE AUTOMATIC FLT SYS CTL THE ACFT. FMC PROGRAMMING PROB; TURNED ACFT SOONER THAN ATC EXPECTED.

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DEPARTING SEA (CAPT FLYING) HE USED LNAV TO INTERCEPT RADIAL. ON THE SID THERE IS A 90 DEG TURN AT 8 NM. THE AUTOMATED SYS (LNAV) LEADS THE TURN SIGNIFICANTLY IN ORDER TO ROLL OUT ON THE NEW RADIAL/COURSE. WE WERE CLRED TO 9000 FT AND CLBING RAPIDLY AS WE STARTED THE TURN. DEP CTL CLRED US TO 15000 FT. HE THEN TOLD US WE WERE TURNING EARLY AND TO TURN BACK TO 340 DEG HDG AND TO MAINTAIN 9000 FT. WE ACKNOWLEDGED AND SHORTLY THEREAFTER HE GAVE US A HDG OF 070 DEGS AND 15000 FT AGAIN. ABOUT THE TIME WE WERE ROLLING OUT ON 070 DEGS AND TRYING TO FIGURE OUT WHAT WENT WRONG ON THE SID WE WERE AGAIN GIVEN 9000 FT. THE ACFT WAS NOW ON AUTOPLT AND RAPIDLY APCHING 9000 FT ON THE WAY TO 15000 FT. I SET 9000 FT BACK IN THE WINDOW AND THE CAPT DISCONNECTED THE AUTOPLT TO LEVEL AT 9000 FT BUT OVERSHOT TO ABOUT 9500 FT. THERE WAS NO TFC CONFLICT THAT WE WERE AWARE OF BUT A LOT OF CONFUSION IN A CRITICAL PHASE OF FLT. I SUGGEST USING RAW DATA IN SUCH A SIT TO MAKE SURE THE TURN DOESN'T BEGIN UNTIL 8 DME AND MAKE SURE THE AUTO-SYS/MAGIC DOESN'T CONFUSE OR GET YOU IN TROUBLE.