US National Transportation Safety Board (NTSB) officials have ruled that pilots’ failure to use thrust reversers "in a timely manner" was the primary cause of the December 8 2005 overrun of a Southwest Airlines Boeing 737-700 at Chicago’s Midway Airport in a night-time snowstorm.

The accident killed one person on the ground and injured 22, including 18 of the 103 passengers and crew on board Flight 1248 inbound from Baltimore.

The aircraft traveled approximately 500ft (150m) past the end of the runway, coming to rest in a roadway where it crushed an automobile.

An additional probable cause was the pilot’s lack of familiarity with the aircraft’s automatic braking system. Southwest had recently published guidance for pilots to begin using the aircraft’s automatic braking system on landings, though the airline had decided to hold off on the implementation.

The pilot had not used the automatic system in practice before the accident flight.

Post-crash analysis revealed that immediate application of full reverse thrust would have stopped the aircraft on the available runway. Investigators proposed that the distraction caused by using the autobrake system delayed the pilot in selecting reverse thrust.

Information from the flight data recorders showed that the pilot did not fully deploy the thrust reversers until 14s after touchdown, 10s later than the four previous Southwest 737s that landed successfully in the half-hour preceding the accident.

Flight 1248 also landed with an 8kt quartering tailwind component with "fair to poor" braking action as reported by a Gulfstream business jet that landed just prior to the 737’s arrival.

Post-crash analysis showed that braking effectiveness steadily decreased between the time the runway was plowed 27min earlier and the arrival of the accident airplane.

Southwest’s standard operating procedures (SOPs) call for pilots to choose the worst case, most conservative braking performance in "mixed" braking action reports.

Southwest SOPs also prohibits landing with more than 5kt tailwind when braking action is reported as "poor". Runway condition reports, made by pilots, are subjective and no technologies currently exist to standardize the reports.

Contributing to the accident, according to the board, was Southwest’s failure to provide pilots with guidance and training on landing distance computations as well as its inadequate presentation of onboard performance computer information, the airline’s version of an electronic flight bag (EFB).

Also contributing to the accident was the pilot’s decision not to divert to another airport given that conditions exceeded Southwest’s SOPs.

Recommendations to the FAA as a result of the accident include requiring all Part 121 and Part 135 operators to perform landing distance computations with the most recent arrival conditions including a 15% margin of safety to take into account uncertainties in foul weather conditions.

In the Midway accident, the Southwest pilots had assumed the landing distance results they generated on their EFB before landing did not include reverse thrust, leading them to believe they would have additional margin.

The computer program for that particular aircraft already included reverse thrust in the computation, however.

The software on the EFB also limited the tailwind component used in the calculations to 5kt, providing misleading information to the pilots regarding landing distance.

Immediately after the accident the NTSB asked the FAA to prohibit airlines from using thrust reversers in landing distance computations. The FAA instead opted to mandate additional landing distance margins requiring changes to airline operations specifications.

Pushback from the airline industry however caused the agency to reconsider, according to the NTSB.

The FAA instead decided to pursue a longer term rulemaking for landing distance margin while publishing a safety alert asking operators to voluntarily institute the change.

According to an FAA survey presented by the NTSB, of the 65 airlines that responded to a questionnaire, 27 had adopted the 15% safety margin in full and 22 had adopted the changes in part. Those 49 airlines carry 92% of all passengers.

In today’s final hearing, the board also asked the FAA to demonstrate technologies and operational feasibility of equipping transport aircraft to automatically calculate and transmit braking effectiveness using information from flight data recorders, a process being studies in the US and abroad.