Boeing 757-200 accident at Jackson Hole, Wyoming, 29 Dec 2010 (N668AA)

Accident narrative

On December 29, 2010, about 1138 mountain standard time, American Airlines Flight 2253 ran off the departure end of runway 19 after landing at Jackson Hole Airport in Jackson Hole, Wyoming. The Boeing 757-200, registration N668AA, came to a stop in deep snow about 730 feet past the end of the runway. None of the 179 passengers, two pilots, and four flight attendants were injured, and the airplane sustained minor damage.

### The flight

Flight 2253 originated from Chicago O’Hare International Airport about 0941 central standard time. The first officer was the pilot flying and the captain was the pilot monitoring. Both pilots were experienced and familiar with winter operations at Jackson Hole; the captain had 19,645 hours of total flight time, including 10,779 hours in the 757, and the first officer had about 11,800 total hours, with 3,582 in the 757. The Board found no evidence of fatigue or medical conditions affecting their performance.

En route, the crew gathered weather and runway information. Instrument meteorological conditions prevailed at Jackson Hole, with light snow, a broken cloud layer at 400 feet, and an overcast layer at 1,000 feet. A corporate jet that landed an hour earlier reported "good" braking action on the first two-thirds of the runway and "poor" action on the last third. Friction testing provided MU values of 0.43, 0.43, and 0.39 for the three sections of the runway.

The pilots reviewed this data alongside the airplane’s performance capabilities and determined they could land safely. They planned to touch down in the first 1,000 feet of the 6,300-foot runway and maximize braking effectiveness early. In preparation, they armed the speedbrakes for automatic deployment and set the automatic wheel brakes to the "MAX AUTO" setting.

### The landing

The approach was normal, and the airplane touched down firmly about 600 feet beyond the approach threshold at 1137:43.5. Flight data recorder (FDR) data showed the air/ground sensing system transitioned to "ground" mode, but about one second later, it briefly transitioned back to "air" mode for 0.5 seconds before returning to "ground" mode for the rest of the landing roll.

Immediately after touchdown, the first officer attempted to deploy the thrust reversers. About 2.8 seconds after touchdown, the captain called out "deployed," likely referring to the speedbrakes, and 1.2 seconds later called "two in reverse." However, neither system had actually deployed.

At 1137:48.0, the first officer stated, "no reverse" in a strained voice. The captain responded, "I got it," taking over the thrust reverser controls and telling the first officer to steer. The pilots made multiple attempts to deploy the thrust reversers, but they remained locked in transit. The thrust reversers finally began to deploy about 18 seconds after touchdown, reaching full reverse power about 10 seconds later.

Meanwhile, the speedbrakes, which the crew had armed before landing, never automatically deployed. Because both pilots were focused on resolving the thrust reverser problem, neither noticed the speedbrake failure. The airplane continued off the paved surface and stopped in the snow.

### What the investigation found

The Board’s investigation examined the air/ground sensing system, the thrust reversers, and the automatic speedbrakes.

The air/ground sensing system was found to be fully functional. The Board concluded the brief interruption of the "ground" signal most likely resulted from a momentary unloading of the main landing gear just after touchdown.

The thrust reverser system was also fully operational. However, the Board found that the precise timing of the first officer’s initial deployment of the thrust reversers coincided almost precisely with the momentary interruption of the "ground" signal. This created a rare mechanical/hydraulic interaction that caused the thrust reverser sync-lock mechanism to lock in transit. To unlock the system, the reverse thrust levers needed to be moved back to the stowed position for about 5 seconds. FDR data showed the captain briefly stowed the levers about 10 seconds after touchdown, which deactivated the system and allowed the reversers to deploy normally when commanded again at 18 seconds. The pilots stated they were unaware of this lockout scenario, and the Board noted a lack of guidance from Boeing regarding the unintended lockout.

Testing of the automatic speedbrake system revealed a latent manufacturing defect in the no-back clutch mechanism. An improperly secured braking pin intermittently prevented the actuator from driving the speedbrake lever beyond its armed detent. This defect prevented the automatic deployment during the incident landing, though the Board noted it would not have prevented the pilots from manually deploying the speedbrakes.

A Boeing performance analysis determined that with the delayed thrust reverser deployment but prompt speedbrake deployment, the airplane would have stopped about 4,500 feet down the runway. Without the speedbrakes, the calculated stopping distance was about 6,800 feet, which exceeded the available runway length.

The Board evaluated the crew’s performance and concluded that the captain’s erroneous callouts were likely made in anticipation of the systems functioning normally. Once the thrust reversers failed to deploy, both pilots tunneled their attention on that problem. By taking control of the reverse thrust levers, the captain deviated from his pilot monitoring duties. The Board concluded that if the captain had adhered to his monitoring responsibilities, he likely would have recognized the speedbrake nondeployment and manually extended them, which would have greatly decreased the stopping distance.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this incident was a manufacturing defect in a clutch mechanism that prevented the speedbrakes from automatically deploying after touchdown and the captain’s failure to monitor and extend the speedbrakes manually. Also causal was the failure of the thrust reversers to deploy when initially commanded. Contributing to the incident was the captain’s failure to confirm speedbrake extension before announcing their deployment and his distraction caused by the thrust reversers’ failure to initially deploy after landing.