What happened
On March 25, 2009, a Japan Air Commuter Bombardier DHC-8-402, registration JA847C, departed Tanegashima Airport for Kagoshima Airport. Shortly after takeoff, while the aircraft was climbing through approximately 3,800 feet, the crew heard a loud bang and experienced a sudden nose deviation to the left. Simultaneously, the No. 1 engine's oil pressure warning light illuminated, and the engine's low-pressure compressor speed dropped sharply.
The flight crew promptly shut down the No. 1 engine. However, they discovered that the propeller was windmilling at approximately 500 rpm and could not be feathered, despite attempts to use the alternate feathering button. This lack of feathering created significant drag, necessitating an emergency landing. The crew requested an emergency landing at Kagoshima Airport and, due to the high drag from the unfeathered propeller, requested fire services to be on standby. The aircraft landed safely at Kagoshima Airport at 10:26, with no injuries reported among the 42 people on board.
The investigation
The Japan Transport Safety Board (JTSB) conducted an investigation involving authorities from Canada and the United Kingdom. The investigation focused on the internal components of the No. 1 engine's reduction gear box. Through teardown examinations, investigators identified that the 1st stage input gearshaft and the flange coupling shaft had each suffered fractures at three different locations. Furthermore, the helical input gearshaft was found to be fractured.
Findings
Technical analysis revealed that the fatigue fracture of the helical input gearshaft was the primary driver of the incident. Metallurgical examination determined that the fracture originated from an impurity inclusion within the metal stock used to manufacture the shaft. This impurity acted as a stress concentration point, leading to the development of fatigue cracks under repetitive operational stress. The resulting failure of the shaft caused fragments to displace, damaging the engine casing and destroying various turbine blades and vanes. Additionally, the engine failure caused a blockage in the oil passage and damaged the feathering pump motor, which prevented the propeller from feathering.