Background
On 17 December 2009, a Boeing 747-200F freighter, registration 9V-JEB, operated by Jett8 Airlines Cargo, departed from Singapore Changi Airport for Hong Kong. The aircraft was equipped with four General Electric CF6-50E2 engines. The flight crew consisted of a Pilot-in-Command, a First Officer, and a Flight Engineer. The First Officer was the pilot flying.
Incident Description
At approximately 1130 hours Singapore time, the aircraft took off from Runway 02C. About five minutes later, while climbing through 7,000 feet, the crew heard a muffled bang and detected the smell of burning oil in the cockpit. No fire warning was observed. The Flight Engineer noted that the oil pressure and quantity of the No.4 engine were decreasing rapidly following the bang, and both low-pressure rotor speed (N1) and high-pressure rotor speed (N2) dropped significantly but did not reach zero. Exhaust gas temperature remained within limits. The crew shut down the No.4 engine and requested a return to Changi Airport. After performing the engine failure checklist, the aircraft landed uneventfully on Runway 02C. There were no injuries.
Damage to Aircraft
Post-flight inspection revealed that the No.4 engine had suffered an uncontained failure. The stage 3 low-pressure turbine (LPT) rotor and all blade sections aft of it were missing, along with the exhaust core nozzle and exhaust centre body. The fan module, including stage 1 fan blades, shroud, and outer guide vanes, showed no damage. Multiple debris fragments were recovered from the engine cowls. The No.4 pylon structure sustained multiple damages, and there were punctures on the bulk cargo door and the right side of the vertical stabiliser. The lower surface of the right wing had minor dents and punctures; an under-wing fuel tank access panel was punctured, resulting in some fuel leakage. No evidence of fire was found.
Further examination of the high-pressure compressor (HPC) revealed that one stage 9 blade had lost a large portion of its airfoil, with metallurgical analysis indicating a fracture consistent with high-cycle fatigue (HCF). Two stage 10 blades also had half of their airfoil missing. Damage to stages 10 through 14 was attributed to impact from the debris of the broken stage 9 blade. In the high-pressure turbine (HPT), two stage 1 blades had more than 50% airfoil loss, and thirteen others showed nicks, oxidation, or cracks. Seventy-nine stage 2 HPT blades were broken with impact damage on leading edges.
Investigation Findings
The Air Accident Investigation Bureau of Singapore conducted the investigation. Examination of the engine components determined that the uncontained failure originated from a circumferential fracture of the stage 3 LPT disk forward spacer arm at the intersection of the spacer arm and the disk forward flange. The stage 3 LPT failure was consistent with the engine having sustained vibration due to high-pressure rotor imbalance. The S9 HPC blade fracture was identified as high-cycle fatigue, but the origin of the fatigue could not be determined due to damage at the fracture area.
Probable Cause
The investigation concluded that the uncontained engine failure resulted from a circumferential fracture of the stage 3 LPT disk forward spacer arm, and the stage 3 LPT failure was consistent with vibration from high-pressure rotor imbalance.