US investigators have determined that the speedbrakes on a Southwest Airlines Boeing 737-700 were not armed before the twinjet slid off the runway at Chicago Midway.
The crew did not deploy the thrust reversers until 16s after touchdown on runway 13C, which was damp as a result of rain showers.
In an update to the inquiry the National Transportation Safety Board says the braking action was reported as "fair" by a preceding Southwest 737 crew.
But it adds that flight recorder information shows the pilots of the incident flight did not arm the speedbrakes during preparations for arrival.
The crew had created extra workload by initially uploading and briefing the wrong approach procedure, and not realising the error until receiving clearance to leave the holding pattern and begin the approach to Midway.
Having reprogrammed the flight-management system for the correct approach, a recalculation indicated sufficient landing distance available. Runway 13C is 6,522ft (1,988m) long.
The crew correctly set the autobrake but the NTSB says the pilots experienced "additional operational distractions" during the final minutes of the approach, including a momentary flap overspeed.
This flap issue occurred at about the time that the before-landing checklist would normally have been performed - a checklist which includes arming of the speedbrakes.
But the NTSB says "no mention" of the checklist or the speedbrakes was found on the cockpit-voice recorder, and the flight-data recorder shows that the speedbrakes were not armed.
After touchdown the speedbrakes did not deploy and the thrust reversers were not activated. The captain "quickly" applied full manual braking after realising that the aircraft was not slowing as expected, says the NTSB, and reverse thrust was engaged with about 1,500ft of runway remaining - an action which automatically deployed the speedbrakes.
"As the airplane neared the end of the pavement, the captain attempted to turn onto the connecting taxiway but was unable," it states. The 737 hit a taxiway light and rolled 200ft into grass.
Without the speedbrakes' interruption of lift, the deceleration capability is "severely degraded", the NTSB says, because the braking effectiveness on the type is reduced by as much as 60%. Delay in the selection of reverse thrust also contributed to the amount of runway used.
Simulations determined that, had the speedbrakes deployed at touchdown, the 737 would have stopped with 900ft of runway to spare - and as much as 1,950ft if the thrust reversers had been activated at the same time.
None of the 139 passengers and crew members was injured in the 26 April 2011 incident.