What happened
On 14 January 2005, a Reims Cessna F406, registration G-SFPB, was conducting a fisheries patrol flight approximately 40nm northwest of the Sumburgh VOR in the Shetland Islands. During the mission, the crew performed a low-level pass at 200 feet over a fishing vessel for photography, which required a 30-degree left bank.
As the crew attempted to return to straight and level flight, the First Officer encountered heavy resistance when attempting to correct the aircraft to the left. The commander took control and initiated a PAN call, eventually positioning the aircraft for an approach to Runway 15 at Sumburgh. Throughout the approach, the difficulty persisted, with significant force required for left-hand control inputs.
Upon landing and taxiing, the commander performed a control check. During this process, the First Officer experienced a sudden jolt, after which the resistance vanished and the controls returned to normal operation. The crew noted that the restriction appeared to occur whenever the control wheel was moved to the left, regardless of its starting position.
The investigation
An engineer traveled to Sumburgh to inspect the aircraft. An extensive examination of the cables, chains, linkages, and attachment points revealed no mechanical obstructions or defects. The autopilot was also tested and found to be functioning normally. A subsequent inspection in Inverness, which involved removing the cabin floor, also failed to uncover any loose objects or debris that could have caused the restriction.
As a precaution, all four aileron attachment bearings were replaced. Metallurgical analysis of the original bearings by the AAIB revealed that the grease in three of the units had dried out, leaving a powdery residue. The bearings also showed evidence of extensive corrosion on the rollers and raceways.
Findings
- The aileron bearings were in a corroded state and exhibited stiff operation.
- The aircraft's frequent low-level operations in a salt-laden maritime environment increased its exposure to corrosive elements.
- Corroded bearings were the only significant mechanical finding, though they were not found to be the primary cause of the specific unidirectional restriction reported.
Safety action
- The aircraft manufacturer intends to issue a Service Bulletin calling for periodic inspections of the aileron and rudder bearings.