AN A320 PILOT DECLARES EMERGENCY AFTER THE REMAINING BRAKE STEERING CONTROL UNIT CHANNEL FAULTED. THE BSCU UNIT HAD ALREADY BEEN REPLACED 10 DIFFERENT TIMES.

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ONE BSCU CHANNEL WAS ON MEL. WHEN EXTENDING THE GEAR FOR LNDG THE SECOND CHANNEL FAULTED WITH ASSOCIATED WARNINGS. MECH TOLD US AFTERWARD AT THE GATE THAT THE ACFT HAD TEN OF THESE BOXES BSCU IN THE PAST FEW MONTHS. ACFT WARNINGS DECLARED EMER; CONTACTED OPS AND MAINT CTL; FOLLOWED PROCS; LANDED WITHOUT EVENT. FIND OUT WHY THIS ACFT IS HAVING SO MANY PROBS WITH THE BSCU UNITS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT NOSE STEERING WAS NOT AVAILABLE AFTER THE BRAKE STEERING CONTROL UNIT (BSCU) REMAINING CHANNEL TWO HAD FAULTED WITH ASSOCIATED WARNINGS COMING UP. THE BSCU CHANNEL ONE WAS PREVIOUSLY DEFERRED PER MEL. NO OTHER PROBLEMS UNTIL HE EXTENDED THE LANDING GEAR AND THEN THE CHANNEL TWO FAILED. REPORTER ALSO STATED THEY DID A GO-AROUND TO VERIFY ACFT'S STATUS AFTER DECLARING AN EMERGENCY. THE STEERING LOSS AND THE WARNINGS WERE THE SAME AS WHEN THE NOSEWHEEL STEERING SWITCH IS MOVED TO THE OFF POSITION. THE GREEN SYSTEM PRESSURE WAS GOOD. SOME OF THE PREVIOUS BSCU WRITE-UPS HAD BEEN FOR THE CHANNEL ONE FAULTS.