BOEING B747 near Jeddah — 2008-07-05

Probable cause

Failure of the low pressure turbine stage 3 disk due to a design that is vulnerable to high pressure turbine unbalance-induced synchronous vibration that cannot be detected in flight, and the subsequent uncontained engine failure.

NTSB narrative

The airplane made an air turnback and successful landing following an uncontained No. 1 engine failure during initial climb after takeoff. There was minor impact damage to the airplane's left wing and flaps. The No. 1 engine low pressure turbine (LPT) case had separated circumferentially in plane with the stage 3 (S3) LPT turbine, and the engine components aft of the separation were liberated. A high pressure turbine (HPT) borescope inspection found localized airfoil damage sufficient to result in HPT rotor unbalance. HPT rotor unbalance-induced synchronous vibration forces interacted with the engine LPT rotor system through a common bearing support and excited a bladed-disk mode response in the LPT S3 disk. The resonant amplitude of the alternating stress experienced by the disk resulted in bending loads exceeding the material endurance limit, and high cycle/high amplitude fatigue cracks initiated along the LPT S3 disk forward spacer arm rim diameter. Once initiated, the cracks propagated rapidly through the spacer arm thickness and the individual cracks joined to form a single circumferential crack, resulting in the separation of the disk at the forward spacer arm. The freed aft portion of the LPT rotor accelerated and penetrated the engine case, releasing high-energy debris as disk fragments and all of the engine components aft of the LPT S3 nozzles were liberated. NTSB Safety Recommendations A-10-98 and A-10-99 (Urgent); A-10-100, and A-10-101 were published in connection with this investigation. For further information about these recommendations, see NTSB case number ENG09IA004.

NTSB coding

• Uncontained engine failure · Initial climb

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