What happened
During the landing roll, the crew attempted to deploy the thrust reversers. While the #2 and #3 reversers unlocked and deployed successfully (indicated by green lights), the #1 reverser experienced binding and only reached the unlocked position (indicated by an amber light).
The captain attempted to check the #1 lever, at which point the aircraft yawed to the right. Despite immediate corrective actions by the pilot, the aircraft lost directional control due to the asymmetric thrust caused by the inoperative reverser. The aircraft continued off the runway, crossed two roadways, and the nose gear collapsed.
Following the stop, an evacuation was ordered. While the evacuation slides deployed normally, use of the slides at doors 4L and 4R had to be discontinued because the aircraft's position on its nose created an excessive vertical angle for the slides.
The investigation
An examination of the engine performance following touchdown revealed that while engines #2 and #3 provided reverse thrust, the #1 engine accelerated with forward thrust. This imbalance caused the aircraft's nose to lift to a 4-degree attitude, contributing to the excursion.
An inspection of the throttle quadrant identified that the S1-847 switch was curled and twisted. Additionally, two support shafts holding the switches were found to be bent, which caused the intermittent operation of the #1 thrust reverser.