What was found
APU FAILURE WHILE AT START UP SPEED PRIOR TO DISPATCH. APU GEARBOX ACCESSORY UNIT SEPARATED FROM THE COMPRESSOR ON THE BOTTOM SIDE. COMBUSTION SECTION HAS A HOLE ON THE BOTTOM SIDE. *S/D* THE APU EXPERIENCED OVERSPEED AND SHUTDOWN AT START-UP. UPON RECEIPT, IT WAS DISCOVERED THAT THE TURBINE WHEEL HAD SPLIT INTO TWO OR MORE PIECES. THE FAILURE WAS OF A CONTAINED CLASSIFICATION (SHRAPNEL OR MATERIALS NOT EXITING THE APU SHROUD). THIS APU HAD NO MAJOR WRITE-UPS SINCE INSTALLATION. IT IS OUR POSITION THAT THE FAILURE WAS CAUSED BY AN INCLUSION TYPE CASTING FLAW.