The UK AAIB recommended
that ATR modify maintenance instructions to highlight the possibility that a key rudder component can be easily installed incorrectly, leaving the aircraft susceptible to uncommanded rudder deflections.
Investigators found that an improperly installed cam on an ATR72 rudder travel limitation unit (TLU) led to a March 15 incident in which an Air Contractors ATR72 experienced flight control problems soon after takeoff from Edinburgh. The crew (the plane's only occupants) declared and emergency and returned safely to Edinburgh, but only after battling significant control problems both in the air and on the runway. Reports AAIB:
During the maintenance input which preceded the incident flight, work was performed on the rudder system which required the disassembly of the TLU mechanism. During reassembly, the right hand cam was installed in the incorrect orientation. Neither an independent inspection nor an operational test of the TLU system was performed. The incorrect assembly was not identified until the TLU mechanism was inspected by the maintenance organisation after the incident.
AAIB noted that the ATR Aircraft Maintenance Manual (AMM) requires an operational test. (The flight crew performed a standard pre-flight check of the flight controls, but that wasn't enough to detect the problem.)
AAIB is urging ATR to warn operators immediately of the potential of installing a cam backwards and modify its AMM to reflect the possibility, and amend its AMM to require an extended post-maintenance operational check of the TLU to detect any anomalies.
AAIB is looking into the specific factors the led to the cam's incorrect installation.
ATR said it doesn't have any other reports of improperly installed TLU cams.
Read the full AAIB special bulletin here (.pdf)