Investigators reviewed the flight data recorder (FDR) and cockpit voice recorder (CVR) data and confirmed that the pilots positioned the speed-brake lever to its “armed” detent about 7 min. before landing at JAC. FDR data further indicated that, after the airplane touched down, the speed-brake lever moved briefly from its armed position but then returned to it and remained for the duration of the landing. This movement of the speed-brake lever coincided with the air/ground sensing system cycling from “ground” mode to “air” mode and then back to “ground” mode. The speed-brake lever's movement from its “armed” position indicated that the automatic speed-brake actuator had partially extended upon initial touchdown. Normally, when the air/ground signal indicated “ground” a second time, the automatic speed-brake system would have driven the speed-brake lever beyond its “armed” position to fully deploy the speed brakes.
In this case, however, the speed brakes failed to automatically deploy even though the pilots had armed the system. Initial examination and testing of the incident airplane's automatic speed-brake system and its components revealed no evidence of a malfunction that would have prevented normal operation.
However, the aircraft was returned to service and on March 31, 2011, the automatic speed-brake system failed again. At that point, the automatic speed-brake mechanism was removed and examined. What the examination uncovered was a latent assembly defect in the no-back clutch mechanism that intermittently prevented the speed-brake actuator from automatically driving the speed-brake lever beyond its armed detent to extend the speed brakes.
(Because the effects of this defect were intermittent and the defect's visual detection would require disassembly of the no-back clutch mechanism [a function usually performed by the manufacturer or another external facility, not the operator], an operator would not likely have detected the defect during normal maintenance testing. When this assembly defect in the no-back clutch was identified, the manufacturer of the no-back clutch told NTSB investigators that the company would clarify its documentation to ensure proper assembly of the units. Boeing told the Safety Board that it is “currently writing a Fleet Team Digest article that will contain the information concerning the no-back clutch and its possible intermittent anomaly.”)
So — bottom line — unlikely failure number one was a manufacturing defect in the airplane's speed-brake no-back clutch mechanism that prevented the speed brakes from automatically deploying during the incident landing.
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.