ACFT SLIDES OFF RWY ON LNDG ROLL.

Narrative

BASED ON THE NATURE OF THE WX; IT WAS DISCUSSED AND DECIDED THAT A CAT II APCH WAS IN ORDER. ALL CAT II OPS ARE MANDATORY AUTOLAND APCH PER PLT OPERATING HANDBOOK AND COMPANY POLICY. THE APCH WAS FLOWN AS SUCH; AND TOUCHDOWN WAS NORMAL ALTHOUGH SLIGHTLY LONGER THAN IF THE CAPT HAD HAND FLOWN IT. UPON INITIATING REVERSE THRUST; THE L ENG SPIKED TO APPROX 1.62 AND THE RIGHT CAME TO APPROX 1.24. THE AC WAS TRACKING DOWN THE RWY. SHORTLY AFTER THIS; THE ACFT DRIFTED SLIGHTLY LEFT OF CTRLINE AND THE ROLLOUT MODE OF THE AUTOPLT SEEMED TO SLIDE TO THE L BEHIND US. THE CAPT WAS USING REVERSE THRUST TO TRY AND HELP MAINTAIN DIRECTIONAL CTL; BUT WAS NOT HAVING MUCH SUCCESS. HE TRIED MANY COMBINATIONS OF REV THRUST; BRAKING; AND AERODYNAMIC CTL INPUTS TO CTL THE ACFT. ALL OF THIS SOON LED TO THE ACFT EXISTING THE RWY; WHEN WE LEFT THE PAVEMENT; WE WERE WELL SLOWED; APPROX 60 KTS; AND THE RIDE INTO THE GRASS WAS SMOOTH AND SHORT LIVED. ONCE STOPPED I IMMEDIATELY NOTIFIED THE TWR OF OUR CIRCUMSTANCES AND LOC; AND REQUESTED GND SUPPORT ASSISTANCE. TOUCHDOWN OCCURRED AT XA55 AND THE CABIN DOOR OPENED AT XB15 TO BEGIN REMOVING PAX VIA STAIRS ON A TRUCK. BUSES WERE WAITING NEARBY TO TAKE THE PEOPLE TO THE TERMINAL. ABSOLUTELY NO ONE WAS INJURED. ALSO THE ACFT WAS TAXIED OUT WITH NO DAMAGE HAVING OCCURRED TO ANY PART OF THE ACFT. AS TO THE REASON THIS HAPPENED; MY FEELING IS THAT THERE WAS NOT AN ACCURATE WAY TO DETERMINE THE EFFECT THE REVERSE THRUST HAD ON THE AUTOPLT INPUTS; AND THE CAPT; BECAUSE OF THIS HAD TREMENDOUS DIFFICULTY CTLING THE ACFT ON A RWY THAT WAS VERY SLIPPERY. I BELIEVE CAT II OPS ON A WET OR SLIPPERY RWY IN FACT BE A VERY BAD IDEA.