AN A320 LNDG AT IAD; HAS LNDG EXTENSION PROBS CAUSED BY A FAULTY LNDG GEAR COMPUTER UNIT. PLT SWITCHES TO ALTERNATE LNDG GEAR COMPUTER UNIT AND LANDS SAFELY.

Narrative

FO FLYING SFO-IAD. 3 HRS LATE DEP DUE TO INBOUND ACFT WITH MECHANICAL PROB WITH A FLT COMPUTER. DURING FLT; SAME PROB SURFACES WITH THE COMPUTER 'SLAT; FLAP CTL COMPUTER.' WORKED PROB WITH MAINT ENRTE AND IT CLRED. ON APCH; FO (VERY GOOD SOP AND BOOK KNOWLEDGE) HIGH AND FAST ON GS. DUE TO STABILIZED APCH REQUIREMENTS; INSIDE OM; A GAR WAS EXECUTED. GEAR AND FLAPS HAD BEEN EXTENDED NORMALLY. GEAR RETRACT NORMAL. DID CHKLIST AND ADVISED PAX OF GAR. FO NOW READY FOR SECOND APCH -- SPD 180 KTS. FLAPS 2 DEGS AS SOP. APCHING OM; GEAR DOWN AND FINAL CHKLIST. TIME TO EXTEND AND SOUNDS WERE NOT NORMAL. AS I REACHED FOR GEAR HANDLE TO 'JIGGLE IT' -- IE; TO SEE IF IT WAS IN PROPER POS -- AN ECAM WARNING ANNUNCIATED. GEAR NOT DOWN WAS MESSAGE; AND FIX WAS TO GRAVITY EXTEND GEAR. HOWEVER; GEAR NOT DOWN WARNING WAS PARTIAL. MESSAGE AND WARNING INCOMPLETE ALSO. THIS WARNING COMES ON AT 750 FT RA; NOT 1500 FT RA. ENGS/THROTTLES WERE NOT IDLE DUE TO COGNITIVE BELIEF WARNING WAS IN ERROR AND 'SLAT; FLAP; CTL COMPUTER' PERHAPS INVOLVED. TIME/SEQUENCE OF GEAR NOT NORMAL. VERBAL DISCUSSIONS WITH INSTRUCTORS AT OUR TRAINING FACILITY DURING TRAINING TO 'CYCLE GEAR TO CHANGE GEAR COMPUTER #1 TO #2.' I CHOSE TO CYCLE GEAR. WARNING CLRED; GEAR COMPUTER CHANGED FROM #1 TO #2 AND GEAR EXTENDED NORMALLY. GEAR COMPUTER; KNOWN AS LNDG GEAR CTL INTERFACE UNIT #1; FOUND TO HAVE INTERNAL FAULT NOT IDENTED ON WARNING SCREEN. SLAT; FLAP; CTL COMPUTER HAS A LINK TO LNDG GEAR CTL INTERFACE UNIT #1. SUBSEQUENT HISTORY ON ACFT -- BOTH LNDG GEAR CTL INTERFACE UNITS REPLACED. I BELIEVE COGNITIVE THOUGHT PROCESS WAS INSTRUMENTAL IN ASSESSMENT AND ACTIONS INVOLVED. GRAVITY EXTEND OF GEAR ELIMINATES NOSEWHEEL STEERING ON SLIPPERY RWYS AND WOULD REQUIRE TUG TO GATE. DID NOT BLINDLY FOLLOW ACFT ALERT. I HAD CONCERN OF SECOND MISSED APCH TO PAX FEAR. CONCERN ON NIGHT LNDG WITH NOSEWHEEL STEERING INOP DUE GRAVITY EXTENSION. ALTHOUGH ECAM MESSAGE SAID GRAVITY GEAR EXTENSION; I HAD STRONG COGNITIVE THOUGHTS THAT SLAT; FLAP; CTL COMPUTER PROB WAS INVOLVED. SEQUENCE TIME TO RETRACT IS ADDRESSED IN FLT MANUAL TO RECYCLE; BUT NOT ADDRESSED ON LNDG FAULT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: FLC; FO FLYING; MADE A MISSED APCH BECAUSE OF HIGH AIRSPD ON FINAL. WHEN THE ACFT WAS LINED UP AND STABILIZED FOR THE NEXT APCH; THE FLC LOWERED THE LNDG GEAR BUT THE SOUND OF THE LNDG GEAR EXTENDING WAS DIFFERENT TO THE RPTR. THIS GAVE A MESSAGE TO GRAVITY FALL THE LNDG GEAR. THE RPTR CAPT DIDN'T WANT TO FREE FALL THE LNDG BECAUSE IT WOULD TAKE LONGER TO DO AND FORCE ANOTHER MISSED APCH. HE DIDN'T WANT THE PAX TO BE CONCERNED OVER ANOTHER MISSED APCH EVENT. ALSO; IF THE LNDG GEAR SHOULD EXTEND BY GRAVITY; THERE WOULD BE NO NOSEWHEEL STEERING AVAILABLE FORCING THE ACFT TO BE TOWED TO THE GATE BY A TUG. THE RPTR THEN DECIDED TO RAISE THE LNDG GEAR AND EXTEND IT ONE MORE TIME. BY DOING THIS; ANOTHER LNDG GEAR CTL INTERFACE UNIT WOULD BE ACTIVATED TO CTL LNDG GEAR EXTENSION. WITH EACH RETRACTION OF THE LNDG GEAR; THE ALTERNATE LNDG GEAR CTL INTERFACE UNIT IS ACTIVATED FOR THE NEXT GEAR EXTENSION. THIS WORKED SUCCESSFULLY EXCEPT IT DID NOT SHOW THE R MAIN GEAR DOOR AS BEING IN THE PROPER POS FOR LNDG. THE LNDG GEAR WAS DOWN AND LOCKED SO THE RPTR DECIDED TO LAND AND TAXI TO THE GATE. A LNDG GEAR UNSAFE WARNING DID APPEAR ON THE ECAM; BUT OTHER INDICATIONS SHOWED THE LNDG GEAR TO BE SAFELY EXTENDED. FLC DECIDED TO LAND AND TAXI TO THE GATE; WHICH WAS DONE SUCCESSFULLY. IT TURNED OUT THAT THE LNDG GEAR CTL INTERFACE UNIT #1 HAD A BROKEN WIRE WHICH WAS CAUSING FAULTY WARNINGS. BY SIMPLY RAISING AND LOWERING THE LNDG GEAR; THE LNDG GEAR CTL INTERFACE UNIT WAS CHANGED TO ONE THAT WAS WORKING PROPERLY FOR A GOOD GEAR EXTENSION.