Incident Sequence
During the landing rollout, the captain deployed the thrust reversers. The number 2 and 3 reversers unlocked and deployed, indicated by green lights. However, the number 1 reverser was binding and only moved to the unlocked position, indicated by an amber light. The captain was not alerted to this condition, although the second officer was expected to monitor the indicator lights. The captain reported reaching to check the number 1 lever. Around that time, the aircraft yawed to the right. Despite immediate corrective action by the captain, directional control was lost due to the yawing caused by the inoperative thrust reverser.
The aircraft continued off the runway, crossed two taxiways, and the nose gear collapsed. After the aircraft stopped, an evacuation was ordered. The evacuation slides deployed normally, but because the aircraft came to rest on its nose, the slides at doors 4L and 4R were not used due to their excessive vertical angle.
Findings
Investigation revealed that after touchdown, the number 1 engine accelerated with forward thrust while the number 2 and 3 engines provided reverse thrust. At about the same time, the aircraft's nose lifted off the runway to a 4-degree pitch attitude, and the aircraft departed the runway.
System Examination
Examination of the throttle quadrant found that the S1-847 switch was curled or twisted, and two support shafts holding the switches were bent. These defects caused intermittent opening of the number 1 reverser.