What happened
During the final approach to runway 04, the flight crew confirmed that hydraulic and emergency air pressures were stable and that relevant circuit breakers were engaged. The first officer indicated that the reverser systems were in the armed position. The aircraft type touched down approximately 1,000 to 1,500 feet before the runway threshold on the 7,003-foot runway. During the initial landing roll, the captain employed aerodynamic braking.
Three seconds after touchdown, the first officer noted that the thrust reversers had not deployed and remained only in the armed state. Shortly thereafter, an increase in engine RPM was recorded, followed by verbal distress from the captain. The captain requested manual braking and eventually commanded the use of the emergency brake. Approximately four seconds after this command, a mechanical sound was heard, followed by a drop in engine RPM. The captain then stated the aircraft was leaving the runway. The plane exited the runway end, descended through a localizer tower, and eventually slid across a highway.
Findings
Post-accident investigations determined that the thrust reversers were in an unstowed but undeployed state, and the drag chute had also failed to deploy. While the brake calipers and pads were found to be functional and within operational limits, the failure of the thrust reversers to deploy was a critical factor. The aircraft's conversion included a system where reverser deployment was electrically controlled and hydraulically actuated, featuring an accumulator for use during hydraulic failures. Safety interlocks were designed to prevent deployment unless the throttle was at idle, the arm switch was active, and the aircraft was on the ground via squat switches.