What happened
On July 8, 2012, at approximately 16:56 JST, an Aero Asahi Corporation McDonnell Douglas MD900 (registration JA6911) departed from the rooftop landing field of the Japanese Red Cross Asahikawa Hospital in Hokkaido. The flight was an emergency medical mission intended to transport a patient from Kamifurano Town.
As the aircraft transitioned from a hover to forward flight and climbed roughly 15 feet, the crew heard a hollow boom from the rear left side of the aircraft. This was immediately followed by an "ENG OUT" warning. A passenger observed sparks and black smoke emanating from the rear left section of the airframe. Instrument readings indicated that the No. 1 engine's tachometer and torque had dropped to zero, while the No. 2 engine's torque had entered the red zone due to the increased load. To maintain altitude and clear obstacles, the pilot increased forward speed and eventually diverted to Asahikawa Airport, where the aircraft landed safely at 17:09.
The investigation
The JTSB investigation, involving representatives from the United States and Canada, focused on the internal condition of the No. 1 Pratt and Whitney Canada PW-207E engine. A teardown inspection conducted at the manufacturer's facility revealed significant internal destruction. Investigators examined the compressor turbine (CT) vane ring, turbine blades, and the turbine support case (TSC).
Findings
Technical analysis revealed that the CT vane ring had suffered severe thermal damage, specifically at the six o'clock position, where a portion of the ring had been burnt away. The investigation identified several contributing factors:
- Thermal fatigue cracks had developed in the outer wall of the CT vane ring due to repeated exposure to high temperatures during previous flights.
- As these cracks penetrated the outer wall, pressurized cooling air from the exterior leaked into the combustion gas passageway.
- This leakage disturbed the flow of combustion gases and caused a localized increase in temperature.
- The resulting extreme heat led to the fracturing of the CT blades and subsequent downstream damage to the PT blades.
Data from the aircraft's instrumentation confirmed that the exhaust gas temperature (EGT) for the No. 1 engine had spiked to 1,011°C, significantly exceeding the operating limit of 850°C.
Probable cause
The incident was caused by a severely damaged CT vane ring that created an extreme overtemperature condition within the engine's hot sections, resulting in the structural failure of the compressor turbine blades and power turbine blades.