What was found
(CAN) ON FIRST CREW FIRST WALK AROUND AT 2300 HRS. CREW OBSERVED NR 4 BRAKE DISC WAS NOT STRAIGHT. FURTHER INVESTIGATION FOUND THAT THE NR 4 BRAKE DISC WAS SHEARED AROUND FLANGE AND SITTING CRACKED. MAINTENANCE WAS CALLED. A SERVICEABLE WHEEL ASSEMBLY WAS INSTALLED AND AIRCRAFT RETURNED TO SERVICE.AN INSPECTION WAS DONE ON THE OTHER 3 WHEEL ASSEMBLY AND NO DISCREPANCIES WERE OBSERVED. THE OPERATOR CHECKED ALL THEIR OTHER BN2AS AND NO DISCREPANCIES WERE OBSERVED. DEFECT WAS PUT DOWN TO OLD DISC (STILL WITHIN WEAR LIMITS).