ACR LGT FLT TRACK HDG DEV.

Narrative

AFTER XING A FLT PLANNED FIX; I TURNED THE ACFT TO THE OUTBOUND HDG FOR DIRECT NAV TO THE NEXT VOR. THE CAPT ADVISED HE WAS GOING TO THE BACK TO USE THE FACILITY. AT THIS TIME I REFERRED TO MY HIGH ALT CHART TO REF OUR FLT PLANNED RTE. BEFORE THE CAPT RETURNED; CENTER QUESTIONED ME ABOUT MY HDG. I LOOKED UP AND NOTICED THAT THE ACFT WAS IN A SLIGHT TURN AND WAS NOT HOLDING IN THE HDG MODE OF THE AUTOPLT. THE ACFT HAD TURNED A SIZEABLE DEG OFF THE ORIGINAL HDG AND A CORRECTION WAS MADE BACK TO THE ORIGINAL HDG. BEFORE TURNING THE ACFT I CHKED TO MAKE SURE THAT THE TURN KNOB ON THE AUTOPLT WAS CENTERED. IT WAS IN FACT AND THE ACFT SHOULD NOT HAVE TURNED AT ALL; BUT IT DID. A BIG LESSON WAS LEARNED FROM THIS EVENT. FIRST OF ALL WHEN A CREW MEMBER LEAVES HIS POS ON THE FLT DECK; CONSTANT VIGILANCE MUST BE MAINTAINED IN HIS ABSENCE. THIS IS NOT THE TIME TO BE REFERRING TO CHARTS ETC. EVEN THOUGH I HAD BEEN GLANCING UP PERIODICALLY TO CHK MY ALT FOR DEV IT NEVER OCCURRED TO ME THAT THE ACFT WAS TURNING OFF HDG. YOU JUST CAN'T MAINTAIN THE HIGH LEVEL OF CONCENTRATION NEEDED UNLESS YOU FOCUS YOURSELF SOLELY ON THE OP OF THE ACFT. THE ABSENCE OF A CREW MEMBER ON THE FLT DECK DEMANDS A CONSTANT AND UNDIVIDED ATTN FOR THE SAFE OP OF THE ACFT.