History of Flight
The aircraft, a BAe 146-300 registration G-OINV, was operating a scheduled public transport flight from Inverness Airport to London Gatwick. While cruising at FL250 with the autopilot engaged, air traffic control instructed a right turn of 10°. The heading bug was set accordingly, and the flight director bars indicated a right turn. The autopilot responded but achieved only 2° to 3° angle of bank, failing to reach the required heading. An "AIL" caption illuminated on the Master Warning Panel. The crew applied aileron trim to clear the caption, but it persisted. The commander disengaged the autopilot and attempted a manual right turn, finding lateral control restricted; maximum angle of bank attainable was 10° to 15°. Both control columns were similarly restricted.
The Aileron Disconnect Handle was pulled as a memory item. The crew then referred to the 'AILERON JAM OR UNCOMMANDED ROLL' checklist. The actions did not appreciably change the handling. The commander, in consultation with company operations, diverted to Manchester Airport. During descent, a low-speed handling check with landing gear extended and flap 24° was performed between 8,000 and 7,000 feet. The approach and landing with flap 24° were uneventful; the crew noted that lateral control forces gradually returned to normal during this period.
Crew Actions
The crew carried out the memory items for the 'AILERON JAM OR UNCOMMANDED ROLL' checklist. They pulled the Aileron Disconnect Handle. However, they did not apply a breakout force to the control wheel to achieve disconnection, as described in Note 1 of Card 32A. The crew did not consider Note 1 applicable because an immediate aircraft response was not required. They continued with Card 32B and completed a safe landing.
The commander had 3,200 hours on type; the first officer was recently qualified. The commander had practiced the breakout procedure in simulator training three times, but each instance involved a definite roll jam rather than the control restriction encountered.
Lateral Flight Control System
Lateral flight control is provided by aerodynamically and mass balanced ailerons, each operated by a servo tab, in conjunction with roll spoilers powered by the yellow hydraulic system. Geared trimming tabs are fitted to each aileron. Normally, left and right circuits are coupled. A disconnect device allows separate operation in case of a jam. Two cable and rod circuits, one connected to each pilot's hand wheel, control the respective aileron servo tab and roll spoiler. An interconnect cable links both ailerons, and a break-out detent strut links both hand wheels. To complete separation of the circuits, a breakout force must be applied to the control column, as described in the checklist.
Engineering Investigation
Examination at the operator's maintenance facility revealed that a section of insulation on the rear wing spar at Frame 29 on both left and right sides had become entwined with the aileron cables where they passed through pressure bungs. The insulation bags had become dislodged from retaining buttons. It was not known how long this condition had existed, and the zone had not been disturbed during recent maintenance. Inspections of other aircraft in the fleet found loose bags, but none touching cables. The manufacturer stated intent to revise the Maintenance Manual for more comprehensive inspection and securing information in this area.
Previous Occurrence
A previous aileron control restriction on a BAe 146-300 was reported in AAIB Bulletin 7/2001, with a different cause but similar lack of checklist understanding. Safety Recommendation 2001-27 was made to BAE Systems to review aileron disconnect procedures in the Manufacturer's Operating Manuals. BAE Systems accepted and conducted a review, deeming the manuals and checklists satisfactory.
Analysis
The pilots correctly actioned memory items when the restriction was discovered. The restriction did not lead to loss of immediate control. The Aileron Disconnect Handle was pulled, but the crew did not apply the breakout force as per Note 1. To achieve disconnection, a breakout force is necessary, but the checklist did not make this sufficiently clear. If all disconnect actions had been completed, the two sides would have operated independently; the insulation might have prevented one side from moving, but the other would have had sufficient authority to counteract rolling moment.
The recurrence of checklist misunderstanding indicates inadequate clarity. Safety Recommendation 2004-13 was made: the CAA should ensure BAE Systems revise manuals to clarify system operation and associated procedures.