What happened
During an approach cleared for the ILS Runway 19 and a circle-to-land maneuver for Runway 24, the aircraft's autopilot and autothrottle systems were manually deactivated at approximately 800 feet above ground level. However, the autothrottle system unexpectedly reengaged when the aircraft was roughly 35 feet above the asphalt surface. The flightcrew was unaware that the system had re-activated.
As the aircraft touched down, the speed dropped below the pre-set target of 138 knots. In response, the autothrottle system began increasing power to maintain the target airspeed. This increase in power prevented the automatic deployment of the thrust reversers and ground spoilers, which require the power levers to be in the idle position. While the crew attempted to deploy the thrust reversers, they may not have applied sufficient force to the power levers to manually disconnect the autothrottle.
The autothrottle remained engaged for 16 seconds after the aircraft transitioned to ground mode. As the aircraft reached the end of the 6,013-foot runway, the pilot applied the emergency brake, but the aircraft ultimately departed the right side of the runway. At the time of the incident, wind conditions were recorded at 290 degrees with 16-knot sustained speeds and gusts up to 25 knots.
Findings
Investigation into the incident revealed that the autothrottle Engage/Disengage paddle switches lacked protective guards, making accidental activation possible. Furthermore, while the system produces an audible alert when the autothrottle is disengaged, it provides no acoustic notification when the system is engaged. The unexpected reengagement of the autothrottle prevented the necessary deployment of thrust reversers and spoilers, hindering the aircraft's ability to decelerate safely.