An expert crew experiences simultaneous system anomalies after touching down on a short runway.
Obviously, the crew expected two things to happen immediately after touchdown: the speed brakes to deploy automatically, and the reversers to deploy after being commanded to do so. However, the speed brakes never deployed automatically, although they could have been operated manually, and the reversers were extended late probably because of the rapidity with which the PF attempted to get the levers over the hump. So, the Safety Board took a close look at these systems and the flight crew interactions with them. The airplane's automatic speed-brake system consists of six panels on the upper surface of each wing that can be activated automatically or manually at touchdown to disrupt the airflow over the wings, maximizing the weight on the landing gear and increasing wheel brake effectiveness.
Although automatic speed brakes are not generally required for landings (because pilots can manually deploy them at any time), use of the automatic speed brakes can (or at least, should) ensure their prompt deployment after touchdown, thus optimizing the airplane's deceleration during the landing roll.
To deploy the speed brakes automatically, the pilots move the speedbrake lever to its “armed” detent before touchdown. By design, when the lever is so positioned, the actuator automatically drives it to its full aft position after touchdown (indicated by the air/ground sensing system signal's transition from “air” mode to “ground” mode). This normally results in the extension of the speed-brake panels to their fully deployed position.
However, if the air/ground sensing system reverts back to “air” mode after the automatic speed-brake actuator has begun to extend (during a bounce, for example), the actuator will retract automatically and retract the speed-brake lever. If the air/ground sensing system signal subsequently transitions back to “ground” mode, the speed-brake system is designed to again drive the speed-brake lever to extend the speed brakes.
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 toldinvestigators that the company would clarify its documentation to ensure proper assembly of the units. 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.