Mcdonnell Douglas Helicopter 369E N62PJ accident near Burns, Oregon

Probable cause

The total loss of engine power during cruise flight due to the separation of one of the 4th-stage turbine wheel blades due to low-cycle fatigue cracking that had progressed to high-cycle fatigue cracking, which ultimately led to the overload failure of the blade.

NTSB analysis

The commercial pilot was conducting a local public flight. He reported that, during cruise flight about 400 ft above ground level over densely wooded mountainous terrain, he heard a "loud bang" emanate from the engine, followed by a left yaw and a "severe medium frequency vibration." The pilot subsequently performed a 180-degree, left-turn autorotation to a nearby forest service logging road. During the landing, the main rotor blades struck trees adjacent to the road, and the left skid slid into a ditch, which resulted in the subsequent separation of the left skid.

Postaccident examination of the airframe and engine revealed that one 4th-stage turbine wheel blade had fractured at the blade root and that several additional turbine blades exhibited damage. Metallurgical examination of the fracture surface area revealed that it exhibited signatures consistent with low-cycle fatigue cracking that had progressed to high-cycle fatigue cracking, which led to the failure of the blade due to overload and resulted in the subsequent loss of engine power. Fluorescent penetrant inspection of the 4th-stage turbine wheel did not reveal any other cracks. In addition, the microstructure, hardness, and composition of the 4th-stage turbine wheel conformed to the engineering drawing requirements.

A review of the maintenance logbooks revealed that the 4th-stage turbine wheel was part of a post-service bulletin (SB) commercial engine bulletin (CEB)-1365 or "enhanced" power turbine assembly design. According to the engine manufacturer, the pattern and appearance of the fracture on the accident wheel was similar to fractures on the wheels involved in eight other failures of the post-SB CEB-1365 power turbine assembly design. Representatives from the engine manufacturer reported that the low-cycle fatigue crack was likely initiated by a combination of two factors: (1) a high, positive thermal gradient in the airfoil trailing edge material near the hub during the engine starting process; and (2) the subsequent high, negative thermal gradient and high-speed stress into the blade during transient operation, such as autorotation.

Text from the NTSB. Open the NTSB docket for WPR13GA374 for the full record.