WARNING HORN DISTR; PF GETS 1 DOT UP ON GS AND 10 KTS FAST ON CAT III APCH. HUD TEST INDICATES OK.

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DURING PRACTICE CAT III APCH TO ANC RWY 6R USING HUD; WITH FAA OCCUPYING THE JUMPSEAT. AT 1000 FT AGL WITH GEAR DOWN; FLAPS 30 (FINAL FLAPS); WHEN ARMING SPD BRAKE FOR AUTO DEPLOY UPON LNDG; GOT WARNING HORN. STOWED SPD BRAKE (FULL FORWARD WITH SPD BRAKE AND LIGHT EXTINGUISHED); WARNING HORN STOPPED. TRIED ONCE MORE TO ARM SPD BRAKE WITH SAME RESULTS. BECAUSE OF DISTR WENT 1 DOT ABOVE ON GS AND PLUS 10 KTS FAST. BUT HUD STILL DID A SELF TEST AT 1000 FT AGL AND PASSED CAT III. AT 500 FT AGL HUD GOES MORE SENSITIVE AND I WAS ON GS BUT STILL 10 KTS FAST SO GOT 'APCH WARN' SYMBOL ON HUD. BY 400 FT AGL; ALL WAS WITHIN PARAMETERS AND CAT III ALL THE WAY TO TOUCHDOWN; ROLLOUT. I ELECTED TO CONTINUE CAT III WITH 'APCH WARN' AT 500 FT BECAUSE I HAD FIELD IN SIGHT; AND WANTED TO SEE IF I COULD GET BACK TO THE PERFECT APCH. ON TOUCHDOWN AT 1800 FT DOWN RWY; ON CTRLINE; EVERYTHING WAS OK WITH THRUST REVERSERS DEPLOYED (AT 1.4 - 1.5 EPR). THE ACFT JOGGED L ABRUPTLY AT 100 FT; REVERSERS STOWED AND ACFT HANDLED PROPERLY. REVERSERS ENGAGED 3 MORE TIMES TO TEST (I WASN'T SURE THEY WERE WHAT CAUSED THE DRIFT L) AND ACFT HANDLED NORMALLY! SO THE JOG/DRIFT WAS CAUSED BY ?? (ICE/WIND)? ON ARR AT GATE; SPD BRAKE WARNING HORN WAS WRITTEN UP IN MAINT LOG. FAA JUMPSEATER DEPLANE WITH 'NOTHING FOR US.'