What happened
On 30 October 2007, a Cessna F172M Skyhawk, registration G-BEMB, was conducting a private flight from Goodwood to Lydd. While positioned south of Shoreham Airport at 3,100 feet, the pilot reported to Shoreham Approach that the ailerons were freezing and resisting movement. The pilot found it necessary to apply significant pressure to the yoke and use increased right rudder to prevent the aircraft from banking left.
Due to the severity of the control issues, the pilot declared a ‘MAYDAY’ to avoid flying over the town of Shoreham. Air traffic control provided an approach for Runway 20, which required a downwind landing. The aircraft landed without further incident. However, while taxiing to the parking area, the pilot observed that the ailerons moved freely to the left when the grip on the yoke was released. After the engine was shut down and electrical power was disconnected, the control issue disappeared.
The investigation
An engineering firm based at Shoreham inspected the aircraft following the event. The investigation focused on the aircraft's basic autopilot system, which features wing levelling and heading functions activated by rocker switches. The engineer was able to replicate the flight symptoms by engaging the wing levelling function. Because the integrity of the autopilot system could not be fully verified, the engineer pulled and gagged the autopilot circuit breaker. Following this action, the aircraft has continued to operate normally with the system inhibited.
Findings
- The pilot and passenger did not interact with the autopilot controls during the flight.
- The pilot had visually confirmed the autopilot was switched off during pre-flight checks.
- The autopilot wing levelling function became engaged during flight, causing the observed aileron control restrictions.