Safety board says calculations for stopping distance should exclude use of engines
Airlines should be banned from assuming thrust-reverser deployment when calculating runway stopping distances, according to the US National Transportation Safety Board (NTSB).
In a letter to the US Federal Aviation Administration, the NTSB argues that any advantage gained in practice from thrust-reverser deployment at touchdown should provide an additional safety margin, and not be automatically factored into stopping distance calculations because the operation of all systems as predicted cannot be guaranteed.
The NTSB recommends the FAA should change its regulations to disallow the assumption of a fully functioning reverse-thrust system following the 8 December runway overrun of a Southwest Airlines Boeing 737-700 at Chicago Midway airport.
Thrust reversers could not be deployed on touchdown, according to the captain, but were operated 18s into the landing run when the first officer noticed they were not deployed and activated them. The NTSB says the Southwest pilots used on-board laptop performance computers during the flight to calculate the runway stopping distance. Their entries into the computer assumed engine thrust- reverser deployment at landing, which is standard Southwest procedure for its 737-700s, but not for its -300s and -500s, the NTSB notes.
The FAA’s regulations prohibit usage of thrust-reverser credit when determining landing distances before dispatch, but on some aircraft, including the 737-700, it is permitted for calculating operational landing distances en route using updated runway friction co-efficients and meteorological data. In this case the crew determined that, with all systems operating as planned, their aircraft would stop with 170m (560ft) of runway remaining, the NTSB reports.
In the event, flight data recorder information notes the thrust reversers were not deployed until 18s after landing, when only about 305m of usable runway remained, according to the NTSB. The aircraft went over the runway end at 50kt (93km/h). The safety board concludes that if thrust-reverser credit was barred, the computer would have shown that a safe landing on runway 31C at Midway was impossible. “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,” says the NTSB recommendation.
DAVID LEARMOUNT / LONDON & JOE SINGLETON / WASHINGTON DC
Source: Flight International
