FLC OF A WDB ACR ACFT FORGOT TO RESET NEW COURSE IN THE ACFT AFDS SYS RESULTING IN NOT TURNING AT A DESIGNATED ASSIGNED POINT.

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ON CLBOUT TO FL310; WE WERE CLRED (FROM AN ASSIGNED RADAR VECTOR) DIRECT TO GRB. THE FO WAS FLYING. WE PROGRAMMED THE FMC TO ACCOMMODATE THE NEW CLRNC. NEITHER OF US REMEMBERED TO SELECTED LNAV FOR THE AFDS. THE HDG CHANGE WAS ABOUT 13 DEGS. 2 OR 3 MINS (10-15 MI) LATER WE NOTICED THAT WE HAD NOT DONE SO. WHY? 1) WITH SO FEW ROTATIONS AND AUGMENTED CREW OPS; I AM SPLITTING THE FLYING WITH MY FO (THEY TOO NEED TO FLY). I CAN LOG AN AVERAGE OF 1 1/2 TKOFS AND LNDGS PER MONTH. 2) MY FO IS DUAL QUALIFIED (CAPT ON THE LGT). THE BULK OF HIS PRACTICE; THEREFORE; BY FAR; IS ON AN ACFT WHEREIN ONCE ONE ENTERS 'DIRECT TO...' INTO THE FMC; IT IS AUTOMATICALLY ENTERED INTO THE AFDS AND ACTED UPON. SO FEW ROTATIONS AND A DUAL-QUALIFIED FO...A HIGHLY CONSCIENTIOUS AND DELIBERATE FO; I MIGHT ADD. NO ATC PROBS OR INQUIRIES RESULTED; TO THE BEST OF MY KNOWLEDGE. SUPPLEMENTAL INFO FROM ACN 253965: THIS WAS MY FIRST FLT BACK ON THE WDB AFTER FLYING THE LGT THE PREVIOUS MONTH. ON THE LGT THIS HABIT PATTERN (ARMING LNAV/MAN NAV) IS NOT REQUIRED. WHEN PROGRAMMING DIRECT IN THE LGT FMCG; MAN NAV (LNAV) ALWAYS AUTO-ENGAGES; ELIMINATING THIS ERROR. I DON'T THINK THIS ERROR REPRESENTS A SIGNIFICANT SAFETY PROB. THE HABIT PATTERN OF ARMING LNAV ON WDB ACFT IS QUICK TO LEARN AND THIS MISTAKE NOT LIKELY TO BE REPEATED TWICE BY SAME CREW!