Page:NTSB Report, Japan Air Lines Flight 813.pdf/19

- 16 - Since no rivet material was found and the rivet holes in the area of damage were not elongated, the Board concludes that the low pressure compressor torque ring designed to prevent rotation of the low pressure compressor stator assembly was not secured in place by riveting during overhaul of the engine. As a result, when the restraining effect of the tight fit of the ring and the three borescope inspection port plugs was ovarcome by the normal rotational forces within the compressor, the stator assembly and torque ring began to turn.

At high rotational speed the torque ring bearing on the low pressure compressor case literally ground through the case until the case was so weakened that it disintegrated with explosive force. This is clearly sustained by the extreme abrasive and. heat damage found on the recovered pieces of compressor case and stator vane assembly which came from the precise location where the torque ring would make contact.

Because no part of the torque ring was found and none of its metal could be identified in the damage area,the Board considered the possibility that the torque ring itself was not installed. This was dismissed on the overall ev1dence. Had the torque ring not been in position, the lugs on the outer diameter of the fourth stage stator shroud would have been exposed rather than protected by the mating slots of the torque ring. Assuming such a situation, the lugs would have received the same abrasive and heat damage sustained by the unexposed parts. They did not show this damage; therefore, they were protected and only the torque ring could have furnished the protection. It is also reasoned that metal from the ring was not identified in