The U.K. AAIB published a Special Bulletin including three safety recommendations regarding control difficulties that were encountered by the crew of an ATR-72 during a post-maintance flight.
The aircraft had undergone routine maintenance at an engineering facility at Edinburgh Airport immediately prior to the incident flight.
Everything appeared normal during the crew’s pre-flight checks, which included a full-and-free check of the flying controls.
The aircraft took off at 21:22 from runway 24 at Edinburgh, with the co-pilot acting as the handling pilot.
After carrying out a standard instrument departure the crew climbed the aircraft to FL 230 at a speed of 170 kt with the autopilot engaged. As the aircraft levelled and accelerated through about 185 kt, the crew felt the aircraft roll to the left by about 5 to 10° and they noticed that the slip ball and rudder trim were both indicating fully right. The co-pilot disengaged the autopilot and applied right rudder in an attempt to correct the sideslip and applied aileron to correct the roll. He reported that the rudder felt unusually “spongy” and that the aircraft did not respond to his rudder inputs. Approximately 15° to 20° of right bank was required to hold a constant heading with the speed stabilised above 185 kt and a limited amount of aileron trim was applied to assist. Shortly after regaining directional control a FTL CTL caption appeared on the Crew Alert Panel (CAP) and the FLT CTL fault light illuminated on the overhead panel, indicating a fault with the rudder Travel Limitation Unit (TLU). The commander requested radar vectors from ATC for a return to Edinburgh, later declaring a PAN.
The crew carried out the required procedure from the Quick Reference Handbook (QRH). As part of the procedure they established that both Air Data Computers (ADC) were operating, before manually selecting the TLU switch to the LO SPD position. The aircraft had at this point temporarily slowed to below 180 kt. The co-pilot reported that on selection of LO SPD more roll control input was required to maintain heading and that roll authority to the right was further reduced. The commander therefore decided to return the TLU switch to AUTO and the required roll control input reduced. The green LO SPD indicator light did not illuminate.
An approach was made to runway 24, the aircraft was established on the ILS and was normally configured for a full flap landing. The crew added 10 kt to their approach speed, in accordance with the QRH. The co-pilot had to operate the control wheel with both hands in order to maintain directional control; the commander operated the power levers in the latter stages of the final approach. The co-pilot reported that the aircraft became slightly more difficult to control as the speed reduced, but remained controllable.
The aircraft landed just to the left of the runway centreline, whereupon the commander assumed control of the aircraft and applied reverse thrust. Despite the application of full right rudder pedal during the rollout, the aircraft diverged towards the left side of the runway. The commander re-established directional control using the steering wheel tiller. The aircraft was taxied clear of the runway and back to the engineering facility for inspection.
Th subsequent investigation and testing demonstrated that it is possible to incorrectly install the cams on the rear rudder quadrant shaft during maintenance. In this incident, the right hand cam was installed in the incorrect orientation and neither an independent inspection nor an operational test of the TLU system was performed. The incorrectly installed right hand cam was not detected prior to releasing the aircraft to service. When the TLU system automatically activated as the aircraft accelerated through 185 kt, the right hand roller encountered resistance as it came into contact with the upper lobe of the incorrectly installed cam, rather than slotting into the vee groove. This caused an uncommanded rudder input and associated control difficulties.
Three safety recommendations were made to the manufacturer, ATR.
The investigation is ongoing.