Incident
A Boeing 747-132, operating an unscheduled domestic cargo flight under 14 CFR Part 121, was climbing through 15,000 feet mean sea level when the number one engine separated from its forward and aft engine mounts. The separation caused substantial damage to the airplane. The flight diverted to an alternate airport and landed without further incident.
Examination
Post-incident examination of the engine revealed an uncontained separation of an approximately 180-degree arc of the second stage turbine disk rim. The missing section of the rim coincided with circumferential rub marks on the second stage turbine disk that corresponded to rub marks on the rear inner rails of the second stage turbine vanes. The rubbed areas of the disk and vanes did not exhibit heat cracking consistent with a relatively long time period of rubbing relative to a sudden disk-to-vane contact. The fan rotor bearing did not exhibit rotational distress, and the engine showed no evidence of a blade separation, case rupture, or any other uncontainment.
Metallurgical examination of the second stage turbine retaining bolts revealed cracking in the head-to-shank fillet radius and the presence of silver consistent with Silver Goop. Silver Goop is an anti-seize compound not authorized by the engine manufacturer for use in the high-pressure turbine area but authorized in the low-pressure compressor area due to the catalytic properties of silver. The combination of silver with wet FelPro C-200, a baked-on antiseize compound applied during manufacture, resulted in a corrosive mixture that can contribute to bolt fractures.
The induced loads from the second stage disk rim separation were estimated to have been 3.6 to 6.6 times greater than those induced by a fan blade separation.
Maintenance History
The accident operator purchased the engine as-is. According to maintenance records, the engine was not preserved when it was removed from an airplane belonging to another operator about five years prior to the accident. The high-pressure turbine (HPT) module was removed from this engine and installed on one of the accident operator's other engines before being removed again and installed on the accident airplane engine. The accident operator's examination of the HPT was limited to a visual inspection that would not have detected a vane shift or ascertained the integrity of the second stage turbine vane assembly.