Incident Description
During climbout, the crew of a Boeing 747 heard a loud 'thump' and subsequently noted a change in the number three engine instrument indications. One crewmember observed a fist-sized hole in the side of the engine cowling. The captain shut down the number three engine, returned to the departure airport, and landed without further incident.
Inspection Findings
A post-landing inspection revealed that the inboard aileron sustained substantial damage. Disassembly of the engine disclosed that eleven 2nd stage turbine blades were fractured in the midspan area, and one other 2nd stage turbine blade was elongated and necked down in the midspan area with most of the shroud tip rubbed off. The turbine exhaust case had a 16 1/2-inch long hole between the case front flange and the front mount rail.
Engine Service Records
Engine service records indicated that when the turbine exhaust case was overhauled, Airworthiness Directive (AD) 96-25-10 was complied with by incorporating Chromalloy Anniston Division's Supplemental Type Certificate (STC) SE00047AT-D. The intent of STC SE00047AT-D is to comply with FAA Airworthiness Directive (AD) 96-25-10 by increasing the wall thickness of the turbine exhaust case, thereby providing enhanced containment capabilities in the event of an internal engine failure.
Material Examination
A portion of the turbine exhaust case was sent to the NTSB Materials Laboratory for examination. A Safety Board metallurgist confirmed that both the hardness and microstructure were consistent with design specifications stipulated in Chromalloy Anniston Division's STC.