A320 FLT CREW RPTS ACFT REVERTS TO ALTERNATE LAW AFTER AN ELAC FAILED DSNDING THROUGH 12000 FT. ACFT GOES TO DIRECT LAW AS GEAR IS LOWERED AND A 40 KT AIRSPD DISCREPANCY BTWN FO AND CAPT IS NOTED.

Narrative

ON ARR AT 12000 FT STATUS MESSAGE OF CATIII DUEL. WITHIN 1 MIN ECAM FLT/CTL ALTERNATE LAW (PROTECTIONS LOST); FO WAS FLYING AND I PERFORMED ECAM; SET UP FOR FLAPS 3 DEG APCH. AT ABOUT 5000 FT AND 10 MI OR LESS FROM MARKER. FLAPS 1 DEGS 210 KIAS FO ASKED FOR GEAR DOWN. WITH GEAR DOWN WENT TO DIRECT LAW MANUAL TRIM. AT THIS TIME THE FO AND I NOTICED AN AIRSPD DIFFERENCE OF 40+ KTS BTWN THE CAPT AND FO AIRSPD. CAPT AIRSPD WAS 40 KIAS LESS. AS PIC I DECIDED TO TAKE THE ACFT AND MAKE THE LNDG. FO XCHKED CAPT AND FO AIRSPD AGAINST STANDBY AIRSPD AND DETERMINED FO AIRSPD WAS CORRECT. AT THAT POINT I FLEW THE AIRPLANE USING THE STANDBY AIRSPD WITH FO BACKING ME UP. ACQUIRED THE RWY WITH AIRSPD STABILIZING AND CHKLIST COMPLETE. APCH AND LNDG WAS UNEVENTFUL WITH EVERYTHING NORMAL ON ROLLOUT AND TAXI IN. IN MY JUDGEMENT THE BIGGEST DANGER TO US ON THIS ARR WAS THE AIRSPD DIFFERENCE BTWN THE CAPT AND FO AIRSPD INDICATORS. SUPPLEMENTAL INFO FROM ACN 793805: WHILE DSNDING DURING SPORADIC ST ELMO'S FIRE; FIRST SIGN OF A PROB WAS ECAM: CATIII INOP. THIS WAS WHILE DSNDING FOR ABOUT 20 MINS THROUGH 'GREEN' RADAR RETURNS AND LIGHT CHOP. THE AUTOPLT AND AUTOTHROTTLES STARTED TO GET SLOPPY AND DO ABNORMAL THINGS; IE: OPEN DSCNT AT 280 KTS BUT PITCHED DOWN FOR 300 AND THROTTLES STARTED TO INCREASE. SO; I TURNED OFF THE AUTOPLT AND AUTOTHROTTLES FOR HAND FLYING. THEN ECAM FOR ALTERNATE LAW. CAPT ACCOMPLISHED. BY NOW STARTING TO INTERCEPT THE LOC. AT 210 KTS; FLAPS 1 DEG; ABOUT 5000 FT AND LESS THAN 10 MI FROM MARKER I CALLED FOR GEAR TO DSND. NOW IN DIRECT LAW. AT 170 KTS; FLAPS 3 DEGS WITH GEAR AND DSNDING TO INTERCEPT THE GS; THE CAPT NOTICED OUR AIRSPDS WERE DIFFERENT. HIS WAS 40 KTS LESS THAN MINE. HE THOUGHT AS CAPT HE SHOULD LAND THE ACFT AND WE XFERRED CTL. I THEN XCHKED BOTH PFD AIRSPDS WITH THE STANDBY AIRSPD INDICATOR. THE FO AND STANDBY WERE THE SAME AT ABOUT 170 KTS AND THE CAPT WAS ABOUT 140 KTS. I TOLD HIM TO FLY OFF THE STANDBY AIRSPD. AT ABOUT 2100 FT WE BROKE OUT OF THE BROKEN CLOUDS WITH THE RWY IN SIGHT; CONFIGURED; STABILIZED. FINAL APCH; LNDG; TAXI IN UNEVENTFUL. RECEIVED NO OTHER ECAM'S OR MASTER CAUTIONS. BOTH ENGS SHUT DOWN AT GATE. CALLBACK CONVERSATION WITH RPTR ON ACN 793804 REVEALED THE FOLLOWING INFO: MAINT REPLACED 1 ELAC TO RETURN THE ACFT TO OPERATIONAL STATUS; ALTHOUGH MORE THAN ONE MUST HAVE FAILED TO CAUSE THE ACFT TO REVERT TO ALTERNATE LAW. THE RPTR BELIEVES THAT THE AIRSPD INDICATION DISCREPANCY WITHOUT ANY WARNING WAS THE GREATEST THREAT TO SAFETY IN THIS INCIDENT.