CONFUSED BY NONSTANDARD AVIONICS DISPLAY AND OP; CAPT OF B747-200 FAILS TO COMPLY WITH RNAV SID FROM RKSI.

Narrative

JUST AFTER TKOF THE CAPT (PF) DEVIATED FROM THE DEP SID. THE RNAV SID STARTS WITH A RWY HDG TO THE FIRST FIX AT 7 DME WHERE THE FIRST L-HAND TURN IS. PART OF THE TROUBLE THE CAPT EXPERIENCED WAS THE DRAMATIC ACFT DIFFERENCES IN THE 'P10' FROM OUR OTHER ACFT. APPARENTLY THE CAPT HAD INADVERTENTLY TURNED THE FLT DIRECTOR OFF ON HIS SIDE AND WAS FOLLOWING A FULL L NEEDLE BIAS. NORMALLY THE FLT DIRECTOR IS BIASED OUT OF VIEW WHEN OFF; BUT APPARENTLY NOT IN THIS ACFT. I WAS THE FO (PNF) AND DID NOT NOTICE THE EARLY TURN AS WE HAD JUST TAKEN OFF AND WAS BUSY CHANGING THE VHF RADIO FREQ; RAISING THE GEAR AND SELECTING A SNAPSHOT OF THE ENG DISPLAY. ONCE I NOTICED WE WERE IN A TURN; WE WERE ALREADY 35 DEGS OFF TRACK AND DIVERGING. I IMMEDIATELY BEGAN TO TURN US BACK TOWARDS THE SID AND REQUESTED RADAR VECTORS BACK TO THE PROPER FIX WHILE WE EVALUATED THE CAPT'S NAV TROUBLE AND I FOUND HIS FLT DIRECTOR SWITCHED OFF. THE OLD CLASSIC INSTS WERE A CONTRIBUTING FACTOR AS ANY FMS OR MOVING MAP OR GPS DISPLAY WOULD HAVE PREVENTED THE EARLY TURN.