To: wagtail23
In the scenario you describe having an uncommanded flat/slat movement (which I have never heard of so would like to hear your story) the first thing would be an indication of the flaps/slats movement on the E/WD followed by a message stating that there is a flap/slat fault and there would be an attention getter sound also.
During the developmental testing of the slat system one of the hydraulic modules on the PCU suffered an external leak. They removed the PCU from the test rig and removed the defective module replacing it with another unit. They pressure cycled the new module until it reached 1800 cycles the point at which the original unit failed.
They then reinstalled the PCU into the test rig. The rig was configured for flight mode, which meant that the Command Sensor Unit was in the off position and the computer was turned on. They then applied hydraulic pressure and the slats started to move out at slightly less than half speed. Since the CSU was in the off position the computer was not receiving a command signal and therefore could not respond to the uncommanded movement and the wing tip brake could not turn on stopping the movement. This same basic PCU was also used on the Flap drive system and was subject to the same type of failure.
This problem was never made known to Airbus and was never considered in the writing of the training syllabus and in the Ops manuals.
The problem was traced to an internal leak on the other module (not the one that suffered the external leak), which bypassed the control solenoids in the PCU. The internal and external leaks it was determined were caused by hairline cracks that emanated from a faulty spark erosion manufacturing process. The corrective action in my opinion will only delay the reoccurrence of the problem.
The Flap Slat Computer was never fully analyzed for failure modes and their effects and therefore it was never determined what internal failure or defective system architecture enabled the uncommanded movement.
On the very first revenue flight from Frankfurt to Cairo the flaps could not be retracted during taxi to the terminal. They could not determine the origin of the fault because the “dolls eyes” on the Flap Slat Computer did not indicate a fault. They were forced to return to Frankfurt in a non-revenue status with the flaps extended. At Frankfurt they disconnected the Flap drive and hand cranked the Flaps back to the flight position and reconnected the system. They never did figure out why the computer never detected a fault.
Along the same lines an Air Canada A-320 suffered an uncommanded Flap retraction during takeoff. They were never able to determine what caused the problem. The same manufactures were involved with both programs.
All of this is covered in Wings of Wax. Thank you very much for your input.
