FDR data showed that one of the pilots (likely the captain, based on post-incident statements and CVR data) briefly moved the reverse thrust levers to the stowed position and then back to the interlock position about 10 sec. after touchdown. The data further showed that the reverse thrust levers were moved forward of their interlock position allowing the full deployment of the thrust reversers about 18 sec. after touchdown.
During post-incident interviews, both incident pilots indicated that they were unaware of a circumstance in which the thrust reversers could be locked in transit and were unaware of the actions needed to correct the situation. (Further, American Airlines personnel in general, including the company's 757/767 fleet manager, were unaware of this rare event or its resolution.) It is likely that, during their post-landing manipulations of the reverse thrust levers, the pilots moved the levers forward enough to deactivate the system because when the levers returned to their interlock position, the system was properly configured, and the thrust reversers deployed normally.
So, unlikely failure number two was a glitch in the reverser operation logic that was unknown to the crew.
The Safety Board concluded “although the momentary interruption of the air/ground system's 'ground' signal after touchdown would not normally adversely affect the deployment of thrust reversers, in this case it coincided almost precisely with the initial deployment of the thrust reversers and resulted in the thrust reversers locking in transit instead of continuing to deploy. [Talk about Murphy's Law!]”
Immediately after touchdown, the pilot monitoring made calls indicating that the speed brakes and thrust reversers had deployed when, in fact, they had not. The Safety Board evaluated possible explanations for the captain's “erroneous and premature speed-brake and thrust-reverser callouts and his failure to monitor and notice that the speed brakes had not automatically deployed as expected.”
According to the Board, the only positive indication available to the captain to verify extension of the speed brakes would have been the aft position of the handle, which was visible from and within reach of both pilots' seating positions. FDR data showed that the speed-brake handle was in the armed position for landing and began to move within 1 sec. of landing but did not continue to move to the aft (extended) position as expected. The Safety Board concluded that the captain's speed brakes “deployed” callout was likely made in anticipation (not in confirmation) of speed-brake deployment after he observed the speed-brake handle's initial movement. Both pilots likely presumed that the reliable automatic speed-brakes were functioning normally and focused on the thrust-reverser problem after the “deployed” callout was made.
About the same time that the speed-brake handle started to move, the amber annunciation lights on the EICAS display would have provided the captain with a cue that the thrust reversers were in transition. Although the captain called out “two in reverse,” this callout was not based on the illumination of the green annunciation since the air/ground sensing system cycling to “air” mode prevented the reversers from deploying at that time. (Immediately following the captain's “two in reverse” callout, the CVR recorded the first officer stating, “No reverse” in a voice that sounded strained.) Given the typical reliability of the thrust-reverser system, it is likely that the captain made the callout because he expected normal thrust-reverser activation.
The Safety Board determined that the probable cause of this incident “was a manufacturing defect in a clutch mechanism that prevented the speed brakes from automatically deploying after touchdown and the captain's failure to monitor and extend the speed brakes manually. Also causal was the failure of the thrust reversers to deploy when initially commanded. Contributing to the incident,” said the Board, “was the captain's failure to confirm speed-brake extension before announcing their deployment and his distraction caused by the thrust reversers' failure to initially deploy after landing.”
Based on those findings, the Safety Board recommended the FAA:
Require all operators of existing speed-brake-equipped, transport category airplanes to develop and incorporate training to specifically address recognition of a situation in which the speed brakes do not deploy as expected after landing.