NTSB investigators say pilots of an American Airlines Boeing 757-200 that overran the runway on landing at the Jackson Hole airport on 29 December did not fully deploy aircraft's thrust reversers until 18 seconds after touchdown.
The thrust reverser timeline, culled from the flight data recorder (FDR), is one of several findings released on 12 January by the agency as part of its investigation into the incident.
None of the 181 passengers or crew were injured when the aircraft ultimately came to a stop 107m (350ft) past the end of the runway in hard-packed snow.
Investigators have determined through operational testing that there were no discrepancies in the air/ground sensor and thrust reverser system, though an examination of the auto speed brake mechanism in the cockpit revealed an improperly installed component with a missing bushing.
According to maintenance logs, the actuator was replaced in January 2008.
"System operation with this condition present is being investigated," says the NTSB.
Investigators say the spoilers had been manually extended during the approach to landing, but then placed in the "armed" position until landing. After landing, the air/ground sensor indicated "ground" for one second then switched back to "air" for about 0.5 second before returning to "ground".
"During the time period when the air/ground parameter switched back to 'air', the speed brake handle position momentarily moved toward the down position and then returned to the armed position where it stayed for the remainder of the recording," says the NTSB.
Though the FDR does not include positions of the spoiler panels on the wing, investigators have a video taken by a passenger and an airport security video that are being used in the reconstruction.
Pilots typically deploy spoiler panels and thrust reversers rapidly after landing to slow the aircraft.