Incident Overview
On December 4, 2020, Japan Airlines flight 904, a Boeing 777-200 registered as JA8978, was climbing after takeoff from Naha Airport en route to Tokyo International Airport. At an altitude of FL170 over the sea approximately 50 km north of Naha Airport, the crew experienced an abnormal sound accompanied by aircraft shaking. The left engine (No. 1) showed anomalies on the instruments. The captain shut down the engine, declared an emergency, and landed back at Naha Airport. The aircraft carried 189 people (captain, 10 crew, 178 passengers). No injuries were reported.
Damage
Post-flight inspection revealed two fan blades of the left engine had fractured. The fan cowl door and other fragments from the nacelle separated from the aircraft. Impact from fragments damaged the fuselage and horizontal stabilizer. No damage to property other than the aircraft was reported.
Investigation Findings
The Japan Transport Safety Board (JTSB) conducted a detailed investigation. The fan blades were made of a hollow structure. The fracture of fan blade No. 15 initiated at a nodule—a small lump of granular grains—that bonded to the internal surface during the polishing process of manufacturing. This nodule caused a crack to form. The aircraft continued to fly without detecting the crack in subsequent regular inspections, leading to fatigue fracture. Fan blade No. 16 also fractured as a consequence of the initial failure.
The investigation determined that the inspection method (Thermal Acoustic Image, TAI) and intervals used during regular maintenance were insufficient to detect defects in the fillet region of the fan blades. The probability of detection was low for cracks of the size that existed.
Probable Cause
The JTSB concluded that the serious incident was certainly caused by the fan blades of the left engine fracturing during takeoff climb, resulting in parts and cowlings departing and airframe damage. It is highly probable that the fracture initiated from a nodule introduced during manufacturing, and the crack went undetected during subsequent inspections, leading to fatigue fracture. It is probable that the insufficient detection capability of the inspection method and intervals contributed to the failure to detect the crack.