ACR MLG TRACK DEVIATION.

Narrative

AFTER TKOF; FLYING THE LOOP DEP (SID) FROM SJC; THE F/O ANSWERED A CLRNC ISSUED TO THE ACFT THAT TOOK OFF IN FRONT OF US. THIS WAS CORRECTED AND WHILE THE CORRECTION WAS BEING SORTED OUT; WE RECEIVED OUR CLRNC WHICH WAS 'TURN TO A HDG OF 120 DEGS FOR RADAR VECTOR TO VOR; MAINTAIN 10000'.' THE 'RADAR VECTOR' PART DID NOT REGISTER WITH THE CREW. WE CONTINUED OUR CLB TOWARD THE ASSIGNED ALT AND AT 7000' STARTED OUR TURN TOWARD THE VOR. THE CTLR CAUGHT THE TURN AND STOPPED US WITH A NEW HEADING: 'MAINTAIN PRESENT HDG.' THE REMAINDER OF THE DEP WAS UNEVENTFUL; THERE WAS NO TFC CONFLICT. THIS INCIDENT WAS IN MY MIND CAUSED BY PLT FATIGUE. PAST EXPERIENCE HAS SHOWN THAT MISSING RADIO CALLS AND ERRORS IN FOLLOWING CLRNCS IS AN EARLY INDICATION OF PLT FATIGUE. RECOGNIZING THIS MY COPLT AND I TOOK STEPS TO INCREASE OUR AWARENESS. THIS INCLUDED DRINKING COFFEE; SEAT BACK UPRIGHT; COCKPIT LIGHTS UP AND TALKING TO EACH OTHER AND CLOSELY MONITORING THE PF. I HAVE ALSO NOTICED THAT AS FLT CREW (INCLUDING F/A'S) FATIGUE INCREASES THE CONVERSATION BECOMES HYPER TO THE POINT OF SILLINESS--ESPECIALLY AFTER LEAVING THE ACFT AT THE END OF THE TRIP.