What happened
On 3 August 2007, a Slingsby T67M260 Firefly, registration G-BYOB, was performing a training flight at Stapleford Tawney Airport. The flight involved an instructor and a student. During the initial taxi for departure, the student performed standard rudder functional checks. While applying full right pedal, the student heard a loud bang, after which both left pedals became disconnected. The flight was subsequently cancelled.
The investigation
An investigation by the AAIB established that the left wheelbrake master cylinder had detached from the No 1 pedal mechanism. Maintenance inspections further revealed that the composite floor panel had suffered delamination where the left rudder bar support bracket was attached, causing the bracket to tilt.
Investigators determined that these failures were consistent with an overload caused by exceeding the maximum allowable nosewheel steering angle during ground towing. When the aircraft is towed using a towbar without a load-limiting feature, exceeding the permitted steering angle transmits excessive force through the steering control rod to the rudder/brake pedal mechanism. In this instance, the excessive rotation of the right rudder bar caused the left rudder bar to move rearward, applying an upward load to the left brake master cylinder.
Findings
- The left wheelbrake cylinder detached from the pedal mechanism.
- The aircraft's composite floor panel experienced delamination at the bracket attachment point.
- The damage was caused by excessive loads applied to the pedal mechanism during ground towing.
- The steering angle exceeded the limits marked on the engine cowling during a previous towing operation.