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NTSB says flawed use of thrust reversers key to Midway 737-700 overrun

Delayed deployment cited in Southwest 737-700 accident

The US National Transportation Safety Board has ruled that the pilots' failure to use thrust reversers "in a timely manner" was the primary cause of the overrun of a Southwest Airlines Boeing 737-700 at Chicago's Midway airport in a night-time snowstorm in 2005.

The accident on 5 December 2005 resulted in the death of one person on the ground, and injuries to 22 people, including 18 of the 103 passengers and crew on board Flight 1248 inbound from Baltimore.

The aircraft travelled roughly 150m (500ft) 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, although 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 until 14s after touchdown.

The accident flight also landed with an 8kt (15km/h) quartering tailwind component with "fair to poor" braking action as reported by a Gulfstream business jet that landed just before the 737's arrival.

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

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 about landing distance.

In last week's final hearing, the board asked the Federal Aviation Administration 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 USA and abroad.

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