(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)