NTSB Wants Restriction on Engine Reverser Stopping Credit

The U.S. National Transportation Safety Board (NTSB) wants the FAA to prohibit airlines from taking credit for the use of thrust reversers when calculating stopping distances on contaminated runways.

The recommendation, which the NTSB described as "urgent," is the result of information learned by the Board during its investigation into a fatal runway overrun on Runway 31C by a Southwest Airlines' B-737-700 at Chicago's Midway airport last month, during a snow storm.

The aircraft failed to stop on the runway, rolling through a blast fence and perimeter fence, coming to rest on a roadway after striking two vehicles.  A six-year-old boy in one of the automobiles was killed.

Before the approach, the pilots used an on-board laptop performance computer to calculate expected landing performance on the 6,520 feet (1,988 meter)-long runway.  Along with information on the expected landing runway, wind, and landing gross weight, they selected a reported runway braking action. Depending on whether WET-FAIR or WET-POOR conditions were inputted, the computer calculated the remaining runway after stopping in this instance to be either 560 feet or 30 feet respectively.

Both calculations were based on taking a stopping credit that assumed normal engine thrust reverser deployment.  But in this case, post-crash flight data recorder information revealed that the thrust reversers didn't deploy until 18 seconds after touchdown, at which point there was only about 1,000 feet of usable runway remaining.  (Investigators are examining the thrust-reverser system on the subject Southwest airplane.  The captain told them the system did not appear to operate correctly after touchdown.)

The FAA doesn't allow the use of the reverse thrust credit when determining landing distances for dispatch purposes; however, the FAA does permit thrust reverser credit for calculating en-route operational landing distances for some transport category aircraft, such as the 737-700 series.

If the thrust reverser credit had not been allowed in calculating the stopping distance for flight 1248, the pilot's performance computer would have indicated that a safe landing on runway 31C wasn’t possible, the Board said.  "As a result, a single event, the delayed deployment of the thrust reversers, can lead to an unsafe condition, as it did in this accident," the Board wrote in its recommendation letter.

Although the recommendation would prohibit the use of thrust reverser credit when calculating needed landing distance, only planned landings on contaminated runways would be affected, when the credit is included in stopping distance calculations.  01-28-2006.

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