(Safety Recommendation A-06-16)

The NTSB has urged the FAA to prohibit airlines from using credit for the use of thrust reversers when calculating stopping distances on contaminated runways.

The urgent safety recommendation is the result of information learned by the NTSB during its investigation into a fatal runway overrun in Chicago last month.

"We believe this recommendation needs the immediate attention of the FAA since we will be experiencing winter weather conditions in many areas of our nation for several more months to come," NTSB Acting Chairman Mark V. Rosenker (below, right) said.

On December 8, 2005, Southwest Airlines flight 1248, a Boeing 737-7H4, landed on runway 31C at Chicago Midway Airport during a snow storm. The aircraft failed to stop on the runway, rolling through a blast fence and perimeter fence and coming to rest on a roadway after striking two vehicles. A 6-year-old boy in one of the automobiles was killed.

While approaching Chicago on a flight from Baltimore, the pilots used an on-board laptop performance computer (OPC) to calculate expected landing performance.

Information entered into the computer included expected landing runway, wind speed and direction, airplane gross weight at touchdown, and reported runway braking action.
The OPC then calculated the stopping margin. Depending on whether WET-FAIR or WET-POOR conditions were input, the computer calculated remaining runway after stopping at either 560 feet or 30 feet.

Both calculations were based on taking a stopping credit assuming engine thrust reverser deployment at touchdown. Flight data recorder information revealed that the thrust reversers were not deployed until 18 seconds after touchdown, at which point there was only about 1,000 feet of usable runway remaining.

The FAA does not allow the use of the reverse thrust credit when determining dispatch landing distances; in fact, historically decreases in stopping distances due to thrust reverser deployment were used to offset other variables that could significantly degrade stopping performance. However, the FAA does permit thrust reverser credit for calculating en-route operational landing distances for some transport category aircraft, like the 737-700 series, but not for others, like the 737-300.

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

Although the recommendation would prohibit the thrust reverser credit on all runways, its practical effect would be felt on planned landings only on contaminated runways, which is when the credit is included in stopping distance calculations.

Therefore, the Board is recommending that the FAA:

• Immediately prohibit all 14 Code of Federal Regulations Part 121 operators from using the reverse thrust credit in landing performance calculations. (A-06-16)
  (Urgent)

FMI: www.ntsb.gov/recs/letters/2006/a06_16.pdf