Introduction
On July 4, 2008, a Saudi Arabian Airlines Boeing 747-300 experienced an uncontained failure of its No. 1 General Electric CF6-50 engine during initial climb after takeoff from King Abdul-Aziz International Airport in Jeddah, Saudi Arabia. The non-commercial flight was repositioning the aircraft to Jakarta, Indonesia for maintenance, including work on the No. 1 engine due to degraded power.
Sequence of Events
After takeoff, the flight crew observed fluctuations in the No. 1 engine's exhaust gas temperature and fan speed (N1) indications and reduced thrust. The fluctuations persisted during the initial climb. About one minute later, at approximately 1,100 feet above ground level, the engine's low oil pressure warning illuminated and the oil quantity indicator read zero. The flight crew shut down the engine, dumped fuel, and returned to the airport, where an uneventful landing was performed. No injuries were reported.
Post-Inspection Findings
Post-flight inspection revealed that the aft end of the No. 1 engine was missing. The aircraft's left wing and flaps had sustained damage from impact penetrations. Liberated engine components were recovered about 2.5 miles from the departure end of the runway. The General Authority of Civil Aviation of Saudi Arabia (GACA) sent these components to General Electric (GE) for metallurgical evaluation. The low pressure turbine (LPT) stage 3 disk had separated at the forward spacer arm, and all components aft of the separation were liberated. Later, the forward spacer arm section was removed from the engine at Jeddah and sent to GE.
Materials Analysis
GE examined the LPT S3 disk fracture surfaces at their facility in Evandale, Ohio, with oversight from the National Transportation Safety Board. The examination found that the disk had separated circumferentially near the fillet between the spacer arm and the disk rim. The fracture surface was damaged by post-fracture smearing, but interpretable areas showed damage consistent with high amplitude/high cycle fatigue (HAF) from multiple initiation sites, primarily on the inner diameter. The features indicated that, once initiated, cracks propagated rapidly through the spacer arm thickness and joined to form a single circumferential crack, resulting in disk separation. The material met all specifications except for a small deviation in the as-large-as grain size requirement, which likely played no role in the fatigue fracture.
Tests and Research
According to GE, when a high level of high pressure turbine (HPT) rotor unbalance occurs in the CF6-50 engine, the resulting synchronous vibration forces can interact with the engine's LPT through a common bearing support and excite a bladed-disk vibration mode within the engine operating range. A post-incident video borescope inspection of the engine's HPT found that three HPT stage one blades over a nine-blade sector were missing airfoil material, equivalent to the loss of about 1.8 blade airfoils. According to GE, a localized HPT blade airfoil loss equivalent to 1.8 blade airfoils will result in HPT rotor unbalance large enough to excite the bladed-disk vibration mode in the LPT S3 disk. The investigation was initially conducted by the GACA and was delegated to the United States on April 11, 2009.