UK Air Accidents Investigation Branch inspectors determined that the twin-engined aircraft had undergone routine maintenance on its rudder immediately before the 15 March flight.
As the ATR 72-200 reached flight level 230, travelling through 185kt, it rolled 5-10° left while the rudder trim indicated fully-right. The co-pilot, flying, disengaged the autopilot and applied aileron and right rudder in a bid to correct the roll and side-slip.
Some 15-20° of right bank was necessary to hold the heading constant. The crew requested vectoring to return to Edinburgh.
"The co-pilot had to operate the control wheel with both hands in order to maintain directional control," says the AAIB. "The commander operated the power levers in the latter stages of the final approach."
Neither of the two pilots, the only occupants of the ATR, was injured. The aircraft involved was a 21-year old airframe, serial number 183.
The AAIB found that the maintenance at Edinburgh had involved disassembling the rudder's travel limitation unit, a system which reduces rudder deflection at speeds above 185kt.
Two cams form part of the engaging mechanism to reduce rudder authority. While the left-hand cam had been correctly installed, the right-hand cam had been transposed through 180°.
"Neither an independent inspection nor an operational test of the [limitation unit] was performed," says the AAIB, even though a test of the unit after removal or installation is required by the aircraft maintenance manual.
The incorrect assembly was only discovered after the incident and the AAIB says that the maintenance manual does not indicate that the cam can be installed wrongly, which can lead to uncommanded rudder input in flight.
It has urged ATR to inform operators of the potential problem and to revise testing procedures in the maintenance manual. The AAIB states that ATR "intends to take the necessary actions" in response to the investigation.
(David Kaminski-Morrow - ATWOnline News)
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