What happened
On July 8, 2011, an All Nippon Airways Boeing 767-300, registration JA8674, was performing a scheduled flight from Tokyo International Airport to Toyama Airport. While cruising at an altitude of approximately 8,500 meters, the flight crew reported a loud noise and vibrations originating from the No. 1 engine. In response to the vibration, the crew shut down the engine and initiated an emergency return to Tokyo International Airport. The aircraft landed safely at 09:51 JST after receiving priority handling from air traffic control.
The investigation
The investigation focused on the internal damage sustained by the General Electric CF6-80C2B2F engine. Inspectors found that one blade from the second stage of the High Pressure Turbine (HPT) had separated from its shank. This separation caused significant secondary damage, with the liberated fragment impacting the Low Pressure Turbine (LPT) along its entire axial length.
Investigators measured the thickness of the remaining HPT blades and discovered that the fractured blade had a shank wall thickness of only 0.037 inches, significantly below the minimum design value of 0.055 inches. A second blade with similar thinning was also identified during the inspection of other engines in the fleet. Further examination of repair records revealed that both the failed blade and the similar blade had undergone grit blasting during their last overhaul at a certified repair facility.
Findings
- The primary cause of the incident was the separation of a second stage HPT blade due to fatigue.
- The fatigue was driven by increased stress levels resulting from a reduced shank wall thickness.
- This thinning was likely caused by excessive grit blasting during the repair process.
- The work order at the repair facility lacked specific instructions regarding pneumatic pressure for the blasting process, which allowed for inconsistent and potentially damaging application by technicians.