DC-10-30 N526MD incident near Bogota, CO

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 analysis

The airplane experienced an uncontained No. 2 engine failure during climb out after takeoff, and an uneventful landing was accomplished. A post-incident airplane inspection found that the No. 2 engine low pressure turbine (LPT) case had separated circumferentially in plane with the LPT stage 3 (S3) turbine and that all components aft of the separation were liberated. The engine's LPT S3 disk front spacer arm was fractured circumferentially at the spacer to disk rim fillet. Metallurgical examination of the fracture surfaces revealed high cycle, high-amplitude fatigue (HAF) cracks initiating from sites around 90 percent of the forward spacer arm inner diameter. Inspection of the high pressure turbine (HPT) found that one stage 1 (S1) blade was missing approximately 85 percent of its airfoil material. The investigation found that the HPT blade damage resulted in synchronous vibration forces that 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 that exceeded the material endurance limit and caused high cycle/high amplitude fatigue cracks to initiate 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.

Text from the NTSB. Open the NTSB docket for ENG09IA004 for the full record.