What happened
During an unscheduled domestic cargo flight, a Boeing 7MA-132 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 aircraft. Following the event, the crew diverted the flight to an alternate airport, where the aircraft landed without further incident. There were no fatalities or injuries.
The investigation
An examination of the engine identified an uncontained separation involving approximately a 180-degree arc of the second stage turbine disk rim. Investigators found that the missing section of the rim matched circumferential rub marks on the second stage turbine disk and the rear inner rails of the second stage turbine vanes. These rubbed areas did not show heat cracking, suggesting the contact was not a long-term issue but rather a sudden event.
The investigation focused on the second stage retaining bolts, which are manufactured with a baked-on FelPro C-200 anti-seize compound. Metallurgical examination of these bolts revealed cracking in the head-to-shank fillet radius. Additionally, silver was found on the bolts, consistent with the use of Silver Goop. While Silver Goop is authorized for use in the low-pressure compressor area, it is not authorized by the manufacturer for the high-pressure turbine area because its reaction with FelPro C-200 creates a corrosive mixture capable of causing bolt fractures.
Maintenance records indicated that the engine had been purchased as-is and had not been properly preserved after being removed from another aircraft five years prior. The high-pressure turbine (HPT) module had been transferred from another engine belonging to the operator before being installed on the accident engine. The operator's inspection of the HPT was limited to a visual check, which was insufficient to detect vane shifts or verify the integrity of the second stage turbine vane assembly.
Findings
- An uncontained separation of the second stage turbine disk rim occurred.
- The use of Silver Goop anti-seize on the retaining bolts created a corrosive environment when in contact with the manufacturer-specified FelPro C-200 compound.
- This chemical reaction contributed to cracks in the bolt fillet radius.
- The loads produced by this specific disk rim separation were estimated to be between 3.6 and 6.6 times greater than those caused by a fan blade separation.