What happened
On 5 November 2011, an Aeroprakt A22L Foxbat, registration G-CEWR, departed from Otherton Airfield in Staffordshire for a private flight. Shortly after takeoff, at an altitude of approximately 200 feet and an airspeed of 60 knots, the pilot encountered intense vibrations. This period of instability lasted for roughly 30 seconds. Following the vibration, the pilot observed that the rudder had become stuck in a neutral position. The pilot managed to maintain aircraft control using the ailerons and elevator, performing a circuit pattern before executing an emergency landing on the departure runway, Runway 34. There were no injuries to the pilot or the passenger.
The investigation
An engineer from the Light Aircraft Association (LAA) inspected the aircraft to determine the cause of the mechanical failure. The examination revealed that the spring hinge for the rudder anti-balance tab had seized. This seizure eliminated the necessary spring return force, allowing the tab to flap uncontrollably from side to side. The resulting high loads caused the control horn, which connects the cable to the tab, to bend downwards. This deformation caused the horn to strike the elevator, leading to damage on the upper surface of the elevator and physically restricting the rudder's movement. Additionally, the investigation found that the bolt securing the cable had been installed with the head facing downwards, which reduced the available clearance.
Findings
- The primary cause of the incident was a rudder flutter event triggered by the seizure of the anti-balance tab's spring hinge.
- The loss of spring tension allowed the tab to move freely, creating a resonant frequency that induced severe vibration.
- The mechanical stress from the flutter caused the control horn to bend and impinging upon the elevator, which jammed the rudder.
- Incorrect installation of the cable bolt contributed to insufficient clearance in the control mechanism.