An expert crew experiences simultaneous system anomalies after touching down on a short runway.
What about the reversers?
Although use of thrust reversers is not required during landing, reversers help reduce the airplane's stopping distance when they are deployed early in the landing roll. To initiate thrust-reverser extension, the airplane must detect that it is on the ground, and the pilot flying must lift the reverse thrust levers up and rearward to their interlock position. At that point, the thrust reversers would begin to deploy, and, after they reach their mid-travel positions, the pilot flying must move the levers farther aft to apply reverse thrust, increasing engine power as required to help stop the airplane.
Each engine has its own thrust-reverser control system that hydraulically deploys the thrust reversers based on electrical and mechanical commands it receives from several sources including: pilot inputs; the air/ground sensing system; the thrust-reverser auto restow system; and multiple thrust-reverser system sensors, relays and feedback signals. The thrust-reverser systems function independently except for the common signal they receive from the air/ground system. Because a thrust-reverser extension command is a function of several system inputs, an intermittent loss of any one of these inputs could briefly interrupt continuous deployment.
During the incident landing, a momentary interruption in the “ground” signal from the air/ground sensing system occurred almost immediately after the thrust reversers began to extend. Such interruptions in the “ground” signal are not unusual (commonly occurring during bounced landings, for example). Under normal circumstances, such interruptions are benign and go undetected by pilots because the thrust reversers continue to deploy automatically when the air/ground “ground” signal resumes with no further pilot action required. However, during the incident landing, the thrust reversers locked in transit and did not continue to deploy. The pilots made multiple attempts to deploy the reversers after the air/ground sensing system returned to “ground” mode; however, the thrust reversers did not deploy until about 18 sec. after touchdown.
Post-incident testing of the thrust-reverser control system verified that each engine's thrust-reverser system was fully operational and that each engine's thrust-reverser translating sleeve extended and retracted per the specified maintenance requirements.
However, a detailed review of the thrust-reverser control system design identified one potential scenario in which the momentary change from “ground” mode to “air” mode could cause each engine's thrust-reverser sync-lock mechanism to lock in transit. Such a lockout could only occur if a momentary change from the “ground” mode to the “air” mode occurs in the instant immediately after the thrust reversers begin to extend after touchdown, and in the split second before the thrust reverser's auto restow system is activated. This lockout would prevent movement of the thrust reversers until about 5 sec. after a pilot moves the reverse thrust levers back to their stowed position, allowing the thrust-reverser system to deactivate and begin deployment again when commanded.
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,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!]”