What happened
On 22 April 2006, a replica SE5A, registration G-BMDB, was involved in an incident at Boscombe Down Airfield. During the event, the aircraft began to taxi despite the pilot having recently removed the wheel chocks. While the pilot initially provided a different explanation for the movement, subsequent findings clarified the mechanical nature of the incident.
The investigation
Following the initial accident report, a follow-up investigation was conducted during an annual maintenance inspection intended for the aircraft's Permit to Fly renewal. During this inspection, the pilot identified excessive backlash within the throttle operation.
Technical examination revealed that a plain nut and shake-proof washer on the throttle Bowden cable bracket had loosened by approximately 1/8th of an inch. This looseness allowed the carburettor butterfly arm, which is connected to the cable, to move independently. This mechanical play meant the throttle could partially open even when the cockpit lever was in the locked position.
Findings
- The primary cause of the unintended movement was the loosening of the nut and washer on the bracket holding the outer sheath of the carburettor end of the throttle Bowden cable.
- This loosening allowed for movement in the butterfly arm despite the cockpit lever being secured.
- To prevent recurrence, the plain nut has been replaced with a Kaylock type stiff nut.