Duncan Aviation technicians recently worked two Hawker aircraft with the same discrepancy, but exhibiting different symptoms. On both, the flaps would stop moving in various positions and in one case could not be retracted.
While troubleshooting, we observed the two cable pulleys just aft of the transmitters were not moving freely. When cable tension was released, both pulleys were extremely hard to rotate. After installing new pulleys and proper rigging, the system functioned as normal.
Because these pulleys live in a hostile environment, they, and all remaining pulleys in the flap sync system, need to be included in the overall maintenance of the flap system.
For the flap synchronizer system to operate properly, not only must it be rigged correctly with minimal wear on the transmitters and receiver, but no excess friction can be present in the cable sync system.
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