CHICAGO_Federal investigators on Friday recommended banning a landing calculation used by the pilots of a jetliner that skidded off a runway and into a city street, where it killed a 6-year-old boy.
The National Transportation Safety Board said the pilots should not have factored in the plane's thrust reversers - which help slow the plane - when they estimated how long it would take to stop during a December snowstorm at Chicago's Midway Airport.
The agency said the Southwest Airlines jet touched down with about 4,500 feet of runway remaining, but snowy conditions and other factors meant the plane needed about 5,300 feet of runway to stop.
According to flight recorder data, the thrust reversers did not deploy until 18 seconds after landing, the report also said. That's more than 10 seconds beyond normal deployment, according to aviation experts.
The jet skidded through a fence and crushed a car, killing the boy.
Though thrust reversers are a critical element for all jet landings, continuing to use the calculation to figure out how much space a plane has to land could lead to more accidents, the NTSB said.
Federal Aviation Administration spokesman Greg Martin said only a small number of pilots include the thrust reverser factor now.
"It's not a widespread practice," he said.
Only late-model Boeing 737s, like the one in the Midway crash, have the option of using the thrust reverser credit in landing calculations, FAA spokeswoman Alison Duquette said.
Friday's report was the latest indication of what went wrong, but the NTSB is trying to determine what steps the pilots could have taken to avoid the accident, agency spokesman Keith Holloway said.
"We just know that this didn't help the situation," he said.
Midway, hemmed in by residential and business streets, lacks the FAA's recommended 1,000-foot buffer zone at the end of its runways. Only 82 feet separated the end of the runway and the fence.
Nearly 300 commercial airports in the U.S. don't have adequate runway buffers, according to the FAA. A recently passed federal law requires the airfields to extend runway barriers by 2015 or build beds of crushable concrete that can help an airplane rapidly decelerate.
Thrust reversers typically deploy automatically - six seconds or less after a plane's wheels touch the runway, said Paul Czysz, professor emeritus of aerospace engineering at Saint Louis University.
In this case, a computer used to calculate landing specifications "assumed they would go on immediately," he said.
Investigators haven't determined whether the pilots tried to deploy the thrust reversers manually, NTSB spokesman Keith Holloway said.
The flight's captain told investigators he did try immediately upon landing but could not. The first officer said he deployed the reverse thrusters after he noticed they weren't working, the report said.
Associated Press writer Leslie Miller in Washington contributed to this report.