What was found
(CAN) THE AUXILIARY POWER UNIT 3RD STAGE TURBINE WHEEL HAD A STRUCTURAL FAILURE, WITH TWO BLADES SEPARATING FROM THE INTEGRAL WHEEL. THE BLADE SEPARATIONS INITIATE FROM CRACKS ADJACENT TO COOLING RIVET HOLES. CRACKS ARE CAUSED BY HIGH TEMPERATURE GRADIENT ACROSS THE WHEEL, A RESULT OF STATIONARY FELT-METAL-SEAL DETERIORATION. SERVICE BULLETINS 49-6536R1, 49-5883, AND 49-5884 WERE DESIGNED TO REDUCE THIS PREMATURE FAILURE PROBLEM. THE BLADE SEPARATION ON THIS TURBINE (S/N NR 3-18040-500) IS AN EXAMPLE OF PRE-MOD TURBINE FAILURES NOT INDICATIVE OF POST-MOD SERVICE. PART TC: 15,464.