Northwest Airlines Flight 18 had departed from Hong Kong for a flight to New York-JFK with an intermediate stop at Tokyo-Narita, Japan. The flight, touchdown, and initial landing rollout at Narita, were routine. Engine thrust reversing was normal on all four engines until the flight crew moved the engine power levers out of reverse thrust at about 90 knots. During the rollout, the No. 1 engine and pylon rotated downward about the midspar pylon-to-wing fittings into a position in which the lower forward part of the engine nose cowl contacted the runway. The airplane was subsequently stopped on a taxiway, with the front of the No. 1 engine still contacting the ground. The lower forward engine nose cowl had been ground away as it slid along the runway. A fire near the No. 1 engine was rapidly extinguished by local fire fighters, and all passengers remained aboard. They were subsequently deplaned via portable boarding stairs about 30 minutes after the airplane was brought to a stop. There were no injuries. Maintenance and inspection personnel who worked on the airplane were not adequately trained and qualified to perform the required maintenance and inspection functions. In addition, the work environment for the heavy maintenance of the airplane was inadequate and contributed to an effort producing situation for the workers. PROBABLE CAUSE: “When C check of the aircraft was conducted, the retainer for the diagonal brace aft fuse pin of No.1 nacelle strut was removed and the aircraft was returned to service without reinstalling the retainer. In the course of the flights after this, the fuse pin moved, disconnecting the diagonal brace at the aft joint. It is estimated that when the thrust reverser was deployed during the landing roll under such circumstances, the forward fuse pin of the upper link was fractured due to a load in excess of the design rupture strength, dangling the forward portion of No.1 engine and a fire on leaked oil and fuel broke out in the aft portion of the nacelle strut.”