A US National Transportation Safety Board (NTSB) update on the December 8 overrun of a Southwest Airlines Boeing 737-700 at Chicago Midway confirms that the flight crew experienced a problem with the thrust reversers during the landing.

The report also notes that for the runway conditions and brake usage, the crew needed about 800ft (240m) of runway more than was available.

NTSB staff have now completed their on-scene part of the investigation and are beginning to conduct fact-finding interviews and tests. A review of the cockpit voice and flight data recorders continue.

According to the report, the flight was uneventful through to the point of touchdown. The pilots, who have each been interviewed by the NTSB for over 3hr, say they were concerned about the weather, but that after four listens to the ATIS recorded weather update, a discussion with their dispatcher and a backup assessment from the on-board laptop computer, an approach path was planned for runway 31C.

The pilots say the computer also confirmed that the landing was within the operational parameters of the aircraft and Southwest’s procedures.

Using preliminary calculations, the 737 landed about 2,000ft down Midway’s 6,500ft runway, leaving the crew 4,500ft of asphalt to complete the landing. This is just within normal limits.

However, the aircraft traveled the remaining distance of the snow covered runway, crashed through a barrier and came to a stop on a highway.

A child in one of the two cars struck by the aircraft died in the incident.

The NTSB says that if the tailwind has been a headwind, the aircraft could have stopped about 200ft from the end of the runway. But it also notes: “The investigation has revealed that runway 31C was used because it contained lower landing minimums for aircraft using an instrument landing system approach. If runway 13C was used, the runway most aligned with the wind, pilots would have been unable to land because of insufficient landing minimums.”

In the NTSB update, the investigative body confirms that the crew had set the aircraft’s autobrakes to ‘max’ before landing, and that they activated after a ‘firm’ touchdown.

The report then notes: “The flying pilot (captain) stated that he could not get the reverse thrust levers out of the stowed position. The first officer, after several seconds, noticed that the thrust reversers were not deployed and activated the reversers without a problem.

“At some point, the captain noticed that the airplane was not decelerating normally and applied maximum braking manually. The first officer also became aware of the poor braking effectiveness, moved his seat farther forward, and also applied maximum braking.”

The report adds: “They [the flight crew] stated that they continued to apply maximum pressure to the brakes as the airplane went straight off the end of the runway and came to a stop.”

Documentation of aircraft performance from the scene has been completed, says the NTSB. “It was not possible to observe tire marks from much of the landing rollout due to the fact that the aircraft landed on a snow-covered runway and snow fell on the runway immediately following the accident,” says the safety agency.

Information from the flight data recorder shows that the autobrakes were active and providing high brake pressure, and that thrust reversers were activated 18sec after touchdown, or 14sec before the narrowbody passed through the blast fence.

These thrust reversers are still being examined.

The aircraft’s engines are also being tested, but already visual examination shows that despite some damage to the first stage compressor blades and evidence of debris from the fence all blades are intact.

A 60-day report shows no write-ups and that each engine had two thrust reverse sleeves, says the NTSB. All four sleeves were deployed until after the aircraft left the runway. There were no hydraulic leaks.

The brakes were in good condition with adequate wear and the main landing gear tires had “acceptable” tread and no flat spots.

Weather reports indicate that an enhanced snow band was in the area at the time of the landing. “This apparently is a somewhat unusual weather phenomenon, as the band swath was only 20mi to 30mi (30km to 50km) wide with snow accumulations of 10in (25cm) right over Midway airport,” says the NTSB.

Source: Flight International