Brit Air CRJ7 at Bilbao on Jul 12th 2010, three flights with fractured elevator control cable
Last Update: May 22, 2012 / 14:04:29 GMT/Zulu time
Maintenance subsequently found one of the elevator control cables fractured.
The French Bureau dÂ’Enquetes et dÂ’Analyses (BEA) released their final report in French concluding the probable causes of the serious incident were:
The cable connecting the pitch servo motor with the elevator fractured at the aft cable quadrant due to fatigue, which was the result of an incorrect positioning of the end of the cable in the mount of quadrant. This incorrect positioning most likely occurred during manufacture of the aircraft while assembling the pitch control surfaces.
The fracture did not have significant impact on the flight during which it occurred and had no critical consequences until the flare of the third flight following the fracture. However, the tighter controls experienced during the flare of the third flight could have occurred at any other time of the flights and there was a real risk of reduction of flight controls' authority.
The autopilot was significantly degradated. The autopilot degradation had been logged in the tech logs of the aircraft on two flights following the fracture. Following the first log entry maintenance failed to identify the problem of the flight controls. Only after the second log entry following the unusual control forces needed to flare the aircraft maintenance found the cause of the problem.
The unusual control efforts necessary probably were the result of the cable jamming in the pitch servo motor mechanics, which had the effect of limiting the travel of the aft cable quadrant.
The BEA reported that the crew was conducting the outbound leg from Bilbao to Paris as well. While climbing out of Bilbao the crew detected the autopilot failed to perform according to the selected vertical mode until a "AP PITCH TRIM" warning appeared. The crew actioned the relevant checklists, however, the autopilot was unable to perform any vertical modes. Following several attempts to engage the autopilot the crew decided to continue the flight employing manual control. The crew produced an according tech log entry.
Following landing in Paris maintenance performed several maintenance actions but found no fault. In particular maintenance verified the controls were free to move and were unrestricted and tested the servo motor to move three times. Maintenance subsequently released the aircraft to service.
During the next flight from Paris back to Bilbao the crew was again not able to any reasonable vertical control out of the autopilot and resumed the flight by applying manual control. During final approach to Bilbao the crew needed unusual effort to initiate the flare.
Following landing in Bilbao maintenance discovered a cable between servo motor and aft cable quadrant had fractured, the free end of the cable formed a loop blocking the actuator.
The BEA reported that analysis of the flight data recorder revealed the fracture of the cable had occurred during an ILS approach to Bilbao preceding the rotation to Paris. The crew performing that flight had the autopilot engaged until about 1150 feet AGL, when the aircraft began to deviate from the vertical profile. The crew disengaged the autopilot and completed the landing manually, but did not attach any importance to the occurrence and therefore did not mention the encounter in the tech logs of the aircraft and did not mention the encounter to the next crew accepting the aircraft, who then performed the incident rotation to Paris.
Examination of the fracture confirmed the fracture had occurred progressively as result of fatigue caused by incorrect installation/positioning of the metallic locking pin at the end of the cable, so that the crimp portion was not positioned inside the throat of the aft cable quadrant causing abnormal bending forces to the tip of the crimp portion of the cable.
After the cable fractured the autopilot was no longer able to operate the elevator. At this point the fractured cable did not block the mechanism so that manual control of the elevator was still possible. However, the movements of the servo motor would still move the cable causing it to sting into the aft cable quadrant, the relaxation of which could cause the cable to form a loop.
The investigation was not able to explain with certainty how the fractured elevator control cable and the pitch trim messages displayed were linked.
The investigation found it likely that during the autopilot pitch servo tests in Paris the already fractured control cable stinged into the aft cable quadrant producing elevator movements, which satisfied maintenance personnel. The investigation however determined that full travel of the control surface could not have been achieved that way.
The BEA annotated that the flight crew produced a video of the ap pitch trim message with their mobile phones. This video proved useful because it permitted to ascertain the occurrence of the AP PITCH TRIM message which was not recorded by the flight data recorder. The flight data recorder only showed that a fault was displayed but not which fault message occurred.
The aircraft manufacturer had already taken several safety actions as result of the occurrence including update of the aircraft maintenance manuals.
The BEA repeated their recommendation to videotape the flight deck as result of the investigation.
This article is published under license from Avherald.com. © of text by Avherald.com.
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