ACR ON ARR FAILS TO USE PROPER RADIAL AFTER XING VOR.

Narrative

I HAD INSTRUCTED THE CAPT THAT I WAS GOING TO GET THE ATIS FOR LNDG IN SJC AND HE ACKNOWLEDGED THAT HE HAD ATC COM AND THAT I WAS LEAVING THE 'LOOP'. WHEN I RETURNED WE HAD JUST CROSSED EL NIDO VOR IN A DSCNT AND IN A R TURN TO A HDG OF ABOUT 330 DEGS. THE EL NIDO 3 ARR INDICATES TO TRACK THE 'EL NIDO' 240 DEG RADIAL TO 'TORCH' INTXN. I ASKED THE CAPT IF THIS IS A VECTOR; WHERE WE WERE GOING AND WHAT IS OUR ASSIGNED ALT. AT THAT MOMENT BAY APCH ASKED US WHAT WE WERE DOING AND WHAT OUR HDG WAS. I GAVE BAY OUR HDG AND TURNED TO THE CAPT. (AT THIS POINT I HAD NO IDEA WHAT HE WAS DOING EITHER.) THE CAPT GOT ON THE RADIO TO BAY THAT HE WAS THE CAPT AND THAT HE MADE A MISTAKE; AND WAS RETURNING TO JOIN THE 240 DEG RADIAL. IN THE PAST 10 YRS AS AN AIRLINE PLT I HAVE SEEN THIS TYPE OF PROB MANY TIMES. IT IS MY OPINION THAT THE CAPT'S AGE (APPROX 55) AND LACK OF MENTAL AWARENESS LED TO THIS. THIS HAD BEEN ONE OF SEVERAL SUCH EVENTS ON THIS SAME TRIP; AND WHILE HE COULD PASS A FIRST CLASS MEDICAL WITH FLYING COLORS HE WAS HAVING MANY PROBS IN THE AIRPLANE. WHILE MANY PLTS COULD FLY TO THE END OF THEIR LIFE; MANY COULD NOT MENTALLY.