History of the Flight
The aircraft, a Jetstream 4100 registered G-MAJD, was on a post-maintenance test flight following a wing splice replacement in accordance with Service Bulletin J41-57-021. The crew consisted of three pilots. During pre-take-off checks, the aileron trim was tested for full and free movement. Shortly after takeoff, the pilot attempted to correct a roll bias using trim, but the initial input worsened the situation. Further trim application aggravated the roll, leading the pilot to realize the trim system was operating in the opposite sense. He reversed the trim input, regained balanced flight, and returned to the airfield for an uneventful landing.
Aileron Trim System
Roll trim on the Jetstream 41 is achieved through a tab on the left aileron, operated via a cable-driven manual system. The cables exit the pressure hull at station 360.50, descend to a pair of co-axial pulleys at the wing root, and then run along the aft face of the rear wing spar. The cables route through common fairleads and attach to upper and lower end fittings of the trim screwjack chain. The cable nipples have different shank lengths, and the chain end blocks are sized to prevent incorrect insertion.
Pre-flight Engineering Work
The work was managed by a project leader, a CAA licensed engineer, overseeing three teams working consecutive shifts. The aileron trim cables had been removed during wing removal and were later reinstalled by a subcontracted fitter. After re-attachment of the wing, the fitter did not find the labels he had applied earlier. He routed the cables based on what he considered their natural path, but incorrectly selected which cable went around which pulley: the upper cable went to the forward pulley and the lower cable to the aft pulley. He then connected the cables to the chain ends such that they did not appear to cross along the wing spar. The fitter was unaware that the cable nipples and chain fittings were sized to prevent mis-assembly and had no difficulty connecting them. He tensioned the system and rechecked tension after 24 hours.
Initial and duplicate inspections were carried out on 24 March by both licensed engineers. They visually examined the system and checked cable tension but did not identify the incorrect routing. On 27 March, the project leader performed functional checks alone, using a clinometer to verify control surface range, but did not recognize that the aileron trim tab movement was reversed. The contract engineer, who had accepted that the functional checks were completed, later overlooked the requirement to perform his own duplicate functional check.
Maintenance Documentation
The Service Bulletin was transcribed onto work cards with columns for 'AO' (Approved Operator) and 'DUP/FUNC' (Duplicate/Functional). The work card for reconnecting controls omitted the specific AMM reference for the aileron trim system (Chapter 27-15-00). The AMM section for aileron trim did not contain installation instructions; it included a schematic diagram with an unrelated error. The project leader wrote that work was completed satisfactorily, and the contract engineer stamped for duplicate inspection but did not perform a duplicate functional check.