A CPR DH6 FLC LOST CTL OF THE ACFT DURING AN ATTEMPTED LNDG IN A XWIND THAT WAS NEAR THE MAX DEMONSTRATED.

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IN DHC6 TWIN OTTER ON RETURN TO SAN JOSE; CA; ON APR/THU/98; AS PNF. ACFT WAS IN POS FOR LNDG ON RWY 12R WITH WIND RPTED 190 DEGS AT 15 KTS GUSTING 20 KTS. FLAPS 10 DEGS. THE LNDG WAS BEING MADE WITH NEW CAPT; RECENTLY HIRED AND TRAINED. BRIEFED APCH WITH XWIND LNDG AND ELECTED TO LAND RWY 12R FOR MORE SAFETY. UPON TOUCHDOWN WITH RWY AND ROLLOUT GUST OF WIND BLEW ACFT TO 45 DEG ANGLE AND WHILE BRAKES WERE STILL BEING APPLIED; ACFT WENT OFF RWY 12R INTO GRASS. NO DAMAGE OR INJURIES RESULTED TO ACFT; CREW; OR PAX. PROPER XWIND LNDG TECHNIQUES WERE USED AND BRIEFED PRIOR TO TOUCHDOWN. AS PNF; TOUCHDOWN WAS MADE AND I EXPECTED BRAKES AND REVERSE PROPS TO BE APPLIED IMMEDIATELY WHILE I ASSISTED WITH AILERONS INTO THE WIND. AFTER APPROX 2 SECONDS I WAS ABOUT TO CALL OUT BRAKES AND TO SLOW THE ACFT WHEN AIRPLANE WX VANED INTO WIND; AND OFF RWY AT APPROX 10-15 KTS. IF THERE HAD BEEN CORRECTIVE ACTION AT THIS TIME; ACFT COULD HAVE BECOME UNMANAGEABLE (INDUCED OSCILLATIONS). PLT APPLIED GOOD JUDGEMENT IN CTLING ACFT AS IT WENT OFF RWY 12R. TWR THEN ASKED IF EVERYTHING WAS OK AND MY RESPONSE WAS 'EVERYTHING WAS OK HERE AND WE WERE GETTING BACK ON RWY.' NO DAMAGE TO ACFT UPON INSPECTION. XWIND LIMITS HAVE BEEN DEMONSTRATED BY AFM AT 90 DEGS 20 KTS WITH FLAPS 37 1/2 DEGS.