American B752 at Jackson on Dec 29th 2010, overran runway
Last Update: July 3, 2012 / 15:02:37 GMT/Zulu time
The National Transportation Safety Board determines 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 captainÂ‘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 captainÂ‘s failure to confirm speedbrake extension before announcing their deployment and his distraction caused by the thrust reversersÂ‘ failure to initially deploy after landing.
The first officer (11,800 hours total, 3,582 hours on type) was pilot flying, the captain (19,645 hours total, 10,779 hours on type) was pilot monitoring, both pilots were familiar with the challenging conditions at Jackson Hole Airport. The runway braking action had been reported good for the first two thirds of the runway and poor for the last third of the runway. After reviewing the American Airlines landing charts for Jackson Hole the crew determined they were legal and safe to land on Jackson Hole's runway 19. The crew discussed landing within the first 1000 feet of runway and then make use of the wheel brakes and thrust reversers to slow the aircraft. The speedbrakes were armed and the automatic brakes set the maximum.
The aircraft touched down about 600 feet past the runway threshold with 5700 feet of runway remaining, the first officer attempted to deploy the thrust reversers promptly but the thrust reversers did not deploy. Following several attempts by the first officer to deploy the thrust reversers the captain attempted to deploy the thrust reversers and eventually succeeded when 2100 feet of runway were remaining, the aircraft however went past the runway end and runway end safety area and came to a stop in deep snow off the paved surface.
The captain assessed the situation and decided to not evacuate, ground personnell then helped passenger to disembark normally.
The NTSB analysed that both pilots had taken into account all necessary and available information and had appropriately decided landing at Jackson Hole was in accordance with company and performance guidelines. All necessary before landing actions had been taken appropriately.
The NTSB analysed that the first officer had very quickly attempted to deploy the thrust reversers being aware of the runway and landing conditions, however, the air/ground switches had temporarily transitioned back to "air" about 1 second after touchdown before returned to "ground" for the remainder of the landing roll, precisely at the moment when the first officer attempted to deploy the thrust reversers. Because of this precise timing however "a rare mechanical/hydraulic interaction occurred in the thrust reverser system, and the thrust reversers were locked in transit instead of continuing to deploy." The thrust reversers thus did not begin to re-deploy until 18 seconds after touchdown.
The NTSB further analysed that without speedbrakes deployed braking efficiency of the aircraft may be degraded as much as 60%. The captain had announced 2.8 seconds after touchdown "deployed" although in fact the speedbrakes had not deployed, the captain should have called "no spoilers" instead. Had either of the pilots noticed the speedbrakes had not deployed and manually corrected that condition the stopping distance would have been greatly reduced. Data off the flight data recorder confirmed the speedbrake had been armed and began to deploy after touchdown but stopped its movement when the air/ground sensor temporarily transitioned to air after touchdown. Normally, as soon as ground is sensed again the automatic speedbrake system would continue to drive the speedbrake lever into its fully deployed position, however, this time the "automatic speedbrakes failed to automatically deploy as designed after touchdown".
After the speedbrake system was removed from the aircraft for detailed examination a "a latent assembly defect in the no-back clutch mechanism that intermittently prevented the speedbrake actuator from automatically driving the speedbrake lever beyond its armed detent to extend the speedbrakes" was identified.
The NTSB identified as a safety issue: "the captainÂ‘s expectation that the speedbrake and thrust reverser systems would function reliably and routinely as the pilots had observed them function during multiple previous landings might have led to less vigilant monitoring of those systems. Research has shown that reliable automated systems can lead pilots to have trust and confidence that the system will function as designed. In some cases, this expectation of proper system functioning can lead to poor system monitoring and failure to detect automation malfunctions." and continued: "Further, pilots are often required to divide their attention between multiple tasks in routine flight operations, and the challenges involved in managing multiple tasks is heightened during unexpected or abnormal situations. In this incident, the pilots encountered an abnormal situation when both the speedbrake and thrust reverser systems did not deploy as expected." concluding:
"Although pilots are typically trained to handle a single emergency, the reality is that pilots are sometimes faced with situations that include multiple abnormalities or emergencies. Training for multiple emergency and abnormal situations would provide pilots the opportunity to practice the processes and skills needed to handle such events so that the pilots would be better prepared to handle them in flight. Trained pilots would have better situational awareness and would be better equipped to adapt a learned process to the specific circumstances and time constraints of the event they encounter. This increased situational awareness is even more important in time-critical situations when pilots must respond quickly."
The NTSB also analysed: "Because of the rare mechanical/hydraulic interaction that locked the thrust reversers in transit during the incident landing, the pilots needed to stow the reverse thrust lever to unlock the system before attempting to redeploy the thrust reversers. However, postincident interviews with American Airlines pilots indicated that company pilots were not aware of this technique, and moving the reverse thrust levers to the stowed position during the landing roll would not be an intuitive action."
Aircraft Registration Data
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This article is published under license from Avherald.com. © of text by Avherald.com.
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