Accident Overview
On Friday, June 16, 2023, at about 11:32 JST, a Cessna T303 (registration JA5309) was on the apron at Naha Airport undergoing an engine function check in preparation for departure. White smoke was observed coming from around the No. 1 (left) engine, and a warning light indicating a possible engine fire illuminated. The captain shut down both engines. Only the captain was on board; no injuries were reported. Evidence of a fire was found in the engine cowl housing No. 1 engine.
Sequence of Events
The captain started engine No. 1 and then engine No. 2 on the apron. After starting, the instruments displayed normal. While increasing power on engine No. 1 for a functional check, the captain smelled something burning and saw white smoke from the vicinity of engine No. 1. The "L ENG FIRE" warning light was illuminated. The captain shut down both engines. After ensuring propellers stopped, the captain took a portable fire extinguisher and exited the aircraft. Noticing increasing white smoke, the captain informed an Air Traffic Services Flight Information Officer at Naha Airport Office by telephone. The captain discharged the extinguisher toward the left engine cowling, but the fire was not extinguished. Approximately seven minutes later, fire trucks arrived and applied water to the left engine cowling, extinguishing the fire.
Damage to Aircraft
Examination revealed the left engine cowling was partially burnt. Inside the cowling, the turbine part of the exhaust turbocharger was detached from the exhaust tailpipe at their connections. The coupling joining the connections was damaged. Additionally, the wastegate valve, which bypasses exhaust gas flow to control boost pressure, was detached from the exhaust tailpipe at the connections. The heat shield near the exhaust tailpipe and two vacuum system hoses were burnt out.
Coupling Damage Findings
The coupling is a single structure with an outer band and v-retainer segments attached via spot welds. After removal, corrosion was found throughout the coupling. Fractures in the outer band and cracking in the v-retainer segment were found around spot weld areas. The coupling bore a number that differed from the part number in the aircraft manufacturer's parts list.
Maintenance and Inspection History
The aircraft was purchased second-hand by the captain in July 2022. A periodic inspection occurred in November 2022. The aircraft’s latest maintenance manual (revised September 2011) requires a 50-hour or six-month inspection of the engine exhaust system, including visual check of connections, cracks, corrosion, and weld damage. The maintenance company inspected the coupling per the manual and found no anomalies. The captain did not confirm the latest manual revision and was unaware of the six-month inspection requirement; thus, no inspection had been conducted since November 2022, over seven months prior. The aircraft’s logbook did not record coupling installation details. On June 12, 2023, the captain found foreign objects resembling melted plastic on the left engine cowling. The next day, a black discoloration was seen on the access door surface; inspection inside the cowling via the access door revealed no abnormalities. However, the coupling is in a position not visible even with the access door open.
JTSB Conclusions
The Japan Transport Safety Board concluded that the fire was most likely caused by damage to the coupling connecting the turbine part in the left engine cowling to the exhaust tailpipe. This damage led to separation at the connections, allowing hot exhaust gases to escape and ignite the heat shield, hoses, and part of the engine cowling. Cracking at or near a spot weld likely progressed, causing fracture of the outer band and cracking of the v-retainer segment. The black discoloration on June 13 may have indicated prior coupling damage. The JTSB stated that inspection within six months of the last periodic inspection might have detected the damage. It also noted that a detailed inspection following the discovery of melted foreign objects or black discoloration could have identified the coupling damage.