What happened
During the climbout phase of flight, the crew of a Boeing 747 heard a loud "thump," which was immediately followed by changes in the instrument indications for the number three engine. Upon inspection, a crewmember identified a hole approximately the size of a fist in the side of the engine cowling.
In response to the event, the captain shut down the number three engine. The aircraft returned to its departure airport and landed without further incident. No injuries were reported among the occupants.
The investigation
A postlanding inspection of the aircraft revealed that the inboard aileron had sustained substantial damage. Subsequent disassembly of the number three engine uncovered significant internal damage, including eleven 2nd stage turbine blades that had fractured in the midspan area. Additionally, one other 2nd stage turbine blade was found to be elongated and necked down in the midspan area, with most of its shroud tip removed by abrasion.
The investigation also identified a 16 1/2-inch long hole in the turbine exhaust case, located between the case front flange and the front mount rail.
Review of engine service records showed that during a previous overhaul of the turbine exhaust case, the requirements of Airworthiness Directive (AD) 96-25-10 had been met through the incorporation of Chromalloy Anniston Division's Supplemental Type Certificate (STC) SE00047AT-D. This STC was intended to increase the wall thickness of the turbine exhaust case to provide enhanced containment during an internal engine failure.
A portion of the turbine exhaust case was analyzed at the NTSB Materials Laboratory. A metallurgist confirmed that the hardness and microstructure of the component were consistent with the design specifications required by the Chromalloy Anniston Division STC.