What happened
On September 21, 2008, an Air India Boeing 747-337 (Combi), registration VT-EPW, departed Narita International Airport for Indira Gandhi International Airport. During the climb, at an altitude of approximately 15,700 feet, the crew heard a loud "clunking" noise originating from the No. 3 engine. Simultaneously, flight instruments indicated a sharp drop in the engine's N1 speed from climbing power to approximately 79%.
Following the mechanical disturbance, the flight crew leveled the aircraft at 20,000 feet to assess the situation. After attempting to increase thrust without success, the crew decided to shut down the No. 3 engine. To prepare for an emergency return, the aircraft performed a controlled fuel jettison. The flight eventually returned to Narita International Airport and landed safely on Runway 16R. There were no injuries among the 182 people on board.
The investigation
The Japan Transport Safety Board (JTSB) examined the aircraft's digital flight data and cockpit voice recorders, alongside a detailed teardown of the No. 3 engine. Investigators analyzed the fracture surfaces of the low-pressure turbine (LPT) section and inspected the engine's maintenance history. The investigation also looked into the damage sustained by the aircraft's right wing and the impact of falling engine fragments on ground objects located beneath the flight path.
Findings
Technical analysis revealed that the No. 3 engine suffered extensive internal damage. The primary cause was the fracture of nozzle guide vanes (NGVs) in the second stage of the low-pressure turbine. This fracture was driven by intergranular corrosion on the inner wall surfaces of the NGVs, which lacked the oxidation-resistant coating present on the outer surfaces.
Once these vanes fractured and became liberated, they struck downstream turbine blades and vanes, creating a chain reaction of secondary breakage throughout the LPT stages. Some of these high-speed fragments were ejected through the engine exhaust, striking the aircraft's right wing access door and inner engine panels. Furthermore, fragments of the turbine blades and vanes fell from the sky, striking and damaging three passenger cars parked directly under the aircraft's flight path.
While a manufacturer service bulletin had previously identified the susceptibility of these components to corrosion, the operator had not yet replaced the NGVs with the recommended coated versions during scheduled maintenance.