What happened
During the takeoff roll at V1, the number 1 engine of a DC-10-30, powered by a General Electric CF6-50C2, suffered a breach in its casing. The failure occurred in the turbine plane, specifically between the 3 o'clock and 9 o'clock positions. This breach was caused by the failure of the 2nd-stage low pressure turbine (LPT) anti-rotation nozzle locks.
The resulting debris from the engine breach caused extensive collateral damage to the aircraft, including damage to the number 2 and number 3 engines, the fuselage, and the left landing gear.
The investigation
Investigators examined the recovered components of the accident engine, specifically two 2nd-stage nozzle locks and a section of the casing with an attached nozzle lock stud. The examination revealed intergranular fracture features, grain boundary degradation, and surface oxidation damage. These findings are characteristic of stress rupture.
A comparison engine was also examined, where a cracked 2nd-stage nozzle lock showed similar oxidation and intergranular fracture patterns, also consistent with stress rupture.
The investigation also reviewed the history of the nozzle locks. In May 1993, the manufacturer issued a service bulletin to replace existing locks with versions featuring thicker posts and arms, which required LPT case modifications. In March 1994, service bulletin SB 72-1082 introduced a new design made from a different material that did not require case modification; these were the locks installed on the accident engine. Two prior failures of the SB 72-1082 locks had been identified during inspections: one instance involved all 2nd-stage nozzle locks breaking without a casing breach, and another involved two 4th-stage nozzle locks failing without collateral damage. The manufacturer noted that these intergranular failures suggested either stress rupture or sustained peak low cycle fatigue.