Incident Overview
Shortly after takeoff, the flight crew heard a noise from the right side of the aircraft that sounded like an engine compressor stall. The crew shut down the #3 engine and made an uneventful landing at the departure airport. Pieces of cowling from the right engine fell in a residential area but caused no damage.
Engine Teardown Findings
A teardown of the engine revealed that turbine blades on the first stage low pressure turbine wheel had broken off. The broken blades penetrated the engine case and core cowl doors. Further investigation showed that two stud bolts (saddle bolts) had pulled out from strut bosses 2 and 3 in the turbine mid frame (TMF). This placed an additional load on strut bosses 1 and 4. Eventually, the saddle bolts on bosses 1 and 4 pulled loose, allowing the nozzle support flange to shift rearward. This shift allowed the nozzle segments to come into contact with the low pressure turbine, resulting in the failure.
History of Rework
There was indication that unauthorized rework had been accomplished on the saddle bolts before the owner (United Airlines) had purchased the airplane from another airline (Laker).