What happened
On 23 November 2006, a Slingsby T67M260, registration G-EFSM, was conducting a dual aerobatic training flight near Cambridge Airport. The flight, intended to teach spin recognition and recovery, involved an instructor and a student pilot. During a demonstration spin to the left, the instructor attempted to initiate recovery by applying full anti-spin rudder. However, the right rudder pedal became stuck in the fully deflected left position.
After two unsuccessful attempts to move the controls, the instructor applied significant force to the right pedal, which released with a loud cracking noise. While the pilot was able to successfully recover from the spin, the mechanical obstruction delayed the recovery process by approximately two and a 1/2 turns, resulting in a descent of roughly 700 feet.
The investigation
An investigation by the AAIB, involving the aircraft manufacturer and maintenance specialists, focused on the rudder control system. The examination revealed that the No 3 rudder pedal could contact a fixed bracket supporting the engine control cables when positioned fully forward. This contact was made possible by excessive end-float in the left rudder bar and a lateral displacement of the mechanism.
Further inspection of the aircraft structure revealed cracking in the composite floor beneath the rudder bar support bracket. While it could not be determined if this damage preceded the incident or was caused by the high forces applied during the recovery attempt, the investigators noted that such structural degradation could contribute to the tilting of the bracket and subsequent pedal interference.
Findings
- The rudder pedal restriction was caused by the No 3 pedal making contact with an engine control cable bracket.
- This interference was facilitated by excessive lateral movement in the rudder bar mechanism.
- The investigation identified that the support bracket was susceptible to deformation under lateral loads.
- The aircraft's floor structure showed signs of cracking beneath the support bracket, which may have contributed to the mechanism's instability.
Safety action
Following the investigation, two Safety Recommendations were issued:
- The European Aviation Safety Authority (EASA) was advised to review the engine control cable bracket design on Slingsby T67 aircraft to increase clearance from the rudder pedals.
- A recommendation was made to improve the lateral stiffness and strength of the rudder bar support brackets and their attachments, alongside considering methods to limit loads applied to the rudder system during aircraft towing.