CREW HAD AIRFRAME VIBRATION AND REDUCED CTL EFFECTIVENESS.

Narrative

ON DEP FROM MSP DETECTED AIRFRAME VIBRATION SIMILAR TO BEING IN CONTINUOUS LIGHT CHOP. VIBRATION PERSISTED. CONTACTED DISPATCH AND MAINT. COORDINATED WITH MAINT AND TRIED VARIOUS AIRSPEED; PITCH; AND BANK COMBINATIONS. VIBRATION PERSISTED. I DECIDED TO RETURN TO MSP. ADVISED CABIN CREW; DISPATCH; AND ATC. APCH WAS NORMAL UNTIL LAST 200 FT. WHEN CORRECTING FOR GUSTS AND XWIND CONTROLS FELT VERY SLUGGISH; ROLL RESPONSE WAS VERY SLOW. LNDG WAS HARD; AILERONS REQUIRED FULL OPPOSITE CTL INPUT TO KEEP WINGS LEVEL. AFTER LNDG PAX GAVE ME A NOTE DESCRIBING A 'FLUTTERING' RIGHT AILERON. OUR MAINT PERSONNEL ALSO NOTICED RIGHT AILERON FLUTTERING AS WE WERE TAXIING IN. PROBLEMS: WE HAD A VIBRATION FROM AN UNKNOWN SOURCE. WITH FLY BY WIRE WE HAD NO CTL SERVICE FEEDBACK INDICATING A FLIGHT CTL PROBLEM. ACFT AUTO FLT SYSTEM 'MASKED' FLT CTL PROBLEM. WE DISCOVERED AILERON DEGRADATION DURING LAST 200 FT OF APCH. NOT A GOOD SIT. HERE IS A CASE OF AUTOMATION MASKING ROOT CAUSE OF A POTENTIALLY SERIOUS PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE CREW WAS FLYING AN A320 ACFT. THE CREW HAD NO EICAM FAILURE MODE ANNUNCIATION. ACR MAINT FOUND A FAILED ELAC (ELEVATOR AND AILERON COMPUTER). NONE OF THE SELF TEST DIAGNOSTIC ROUTINES DISCLOSED A PROB. ONLY DURING A MAINT SHOP TEST DID A PROB SURFACE. THE RIGHT AILERON WAS THE AFFECTED FLT CTL. PLT CTL EFFECTIVENESS WAS EITHER DOUBLED OR CUT IN HALF DEPENDING ON WHETHER THE PLT INPUT WAS IN PHASE OR OUT OF PHASE WITH THE FAULTY ELAC COMPUTER. THE CAPT SAID THAT THE ACFT HAS TWO OF THESE COMPUTERS AND WHEN ONE BECOMES FAULTY IT IS SUPPOSE TO DISENGAGE ITSELF. THIS DID NOT HAPPEN.