What happened
On December 4, 2020, a Japan Airlines Boeing 777-20 and registered JA8978 was climbing toward Tokyo International Airport after departing Naha Airport. At an altitude of approximately 17,000 feet over the sea, the aircraft experienced intense vibrations and an abnormal noise accompanied by engine instrument anomalies in the left engine. The captain immediately declared an emergency and shut down the No. 1 engine. The aircraft successfully returned to Naha Airport, carrying 189 people, including 178 passengers and 11 crew members. There were no injuries during the incident.
Post-flight inspections revealed extensive damage. Two fan blades in the left engine had fractured, causing engine components, including the fan cowl door and parts of the nacelle, to detach and fly away from the aircraft. The impact of these departing fragments caused structural damage to both the fuselage and the horizontal stabilizer.
The investigation
The JTSB investigation focused on the structural integrity of the engine's fan blades and the failure of the engine casing to contain the fragments. Investigators performed a teardown of the left engine, which was a Pratt & Whitney PW4074. The analysis examined the fracture surfaces of the blades, specifically looking at blades No. 15 and No. 16.
Technical examinations utilized field emission scanning electron microscopy to inspect the blades. The investigation also reviewed the maintenance history of the fan blades, which had accumulated 43,060 hours and 33,520 cycles since new. Furthermore, the investigators evaluated the effectiveness of the Thermal Acoustic Image (TAI) inspection method and the frequency of scheduled inspections in detecting subsurface defects.
Findings
The investigation determined that the primary cause of the serious incident was the fracture of two fan blades in the left engine during the climb phase. This failure led to the separation of engine cowlings and fragments that struck the aircraft's airframe.
Detailed analysis suggested that the fracture likely originated from a small nodule—a granular lump—that had become embedded in the internal hollow structure of the blade during the manufacturing polishing process. This defect likely initiated a crack that went undetected during subsequent routine inspections. The investigation concluded that the existing inspection methods and intervals were insufficient to identify such defects in the fillet region of the blades, allowing the cracks to progress into fatigue fractures over time.