Incident Details
On 14 January 2005, at approximately 0946 UTC, a Reims Cessna F406 (registration G-SFPB) was conducting a fisheries patrol flight 40 nm northwest of Sumburgh VOR, Shetland Islands. The aircraft had just completed a low-level pass at 200 ft over a fishing vessel, involving a 30° banked left turn. After the pass, the crew initiated a right turn to return to straight and level flight. When attempting a further left correction, the handling pilot, the First Officer, encountered strong resistance. The commander was alerted, and together they found that excessive force was required to maintain straight flight. Pitch control remained normal, and the aircraft climbed to 1,000 ft. A gentle right turn required normal control force, but returning to wings level required excessive effort when the control yoke was 3° to 5° left of center.
Flight Crew Actions
The commander assumed control and made a 'PAN' call, positioning the aircraft for a straight-in approach to Runway 15 at Sumburgh. Control difficulties persisted during the approach, with left corrections requiring considerable effort. The aircraft landed without incident. While taxiing, the commander performed a full and free check of the flight controls and found the resistance to left aileron input identical to that in flight. The First Officer attempted the same check and felt a jolt, after which the restriction disappeared, allowing normal movement. The commander later noted that the restriction seemed to occur whenever the control wheel was turned left, regardless of position, and described it as feeling "electrical" in origin, though the autopilot was disengaged.
Post-Flight Inspection
An engineer from the operator's base in Inverness flew to Sumburgh later that day. By then, the control restriction was no longer present. An extensive inspection of all cables, chains, linkages, and attachment points, including removal of access panels and shrouds, revealed no evidence of the restriction. The autopilot was checked and found normal. The aircraft was cleared for a test flight two days later and flown to Inverness without further incident. Upon arrival, the cabin floor was removed, and a repeat inspection of the control system found no defects or loose articles. As a precaution, all four aileron attachment bearings, noted to be stiff in operation, were replaced. The aircraft returned to service, and the problem did not recur.
Bearing Examination
The removed aileron bearings were sent to the AAIB for metallurgical examination. Disassembly revealed that the grease in three bearings had dried out, leaving a powdery deposit. The fourth bearing (left hand inboard) was from a different manufacturer, had a single row of convex rollers and no cage, and contained a relatively large amount of grease. The other three were dual race bearings with concave rollers and a cage. All dual race bearings showed extensive corrosion on roller and raceway surfaces; the single race component had less extensive corrosion. The corrosion caused stiffness but no signs of seizure.
Discussion
The AAIB noted that the restriction was unidirectional (left aileron only) and occurred at an outside air temperature of +6°C, which excluded ice in the bearings or system as a potential cause. The unidirectional nature also tended to discount an autopilot malfunction, supported by satisfactory operation after the aircraft returned to service. The corroded bearings were considered to cause nothing worse than slightly elevated aileron forces throughout the range, not the specific restriction. The crew report suggested a small object, such as a nut or rivet, might have caused a temporary restriction in a bellcrank, lever, or cable quadrant, but exhaustive examination found no such object. The aircraft manufacturer did not believe the bearing condition was responsible but proposed issuing a Service Bulletin for periodic inspection of aileron and rudder bearings, with the Aircraft Maintenance Manual to be amended accordingly.