What happened
On 29 June 2008, a Cessna 560XL Citation XLS, registration G-OROO, was performing a ferry flight from Bournemouth, Dorset, to Biggin Hill, Kent, following recent maintenance. During the climb from 7,000 ft to 8,000 ft, the flight crew experienced a vibration in the control column accompanied by a thud and a rumbling sound originating from the rear of the aircraft. This vibration caused the autopilot to disconnect. The crew performed a check of the flight controls, but no further issues were detected. The aircraft continued its flight without further incident, completing a standard approach and landing.
The investigation
Upon inspection of the aircraft following shutdown, investigators discovered that roughly 7 and a half percent of the left engine upper cowling had detached. This separation caused damage to the leading edge of the fin and the left elevator. A detailed examination of the hardware revealed that several leading edge cowling fasteners had been pulled through the cowl structure. While the trailing edge and some outboard leading edge fasteners remained attached to the nacelle, the primary cause was identified as unsecured leading edge cowling fasteners.
Further investigation into the maintenance history revealed that the mechanic responsible for reinstalling the upper left engine cowling had been interrupted for several minutes during the process. The individual descended from the engine and did not recall returning to secure the inboard fasteners. Additionally, the maintenance organisation's internal panel re-fitment and post-maintenance safety checks failed to detect the error.
Findings
- The separation of the engine cowling was caused by several leading edge fasteners not being properly secured during maintenance.
- The mechanic's work was interrupted, leading to the omission of the inboard fastener installation.
- Existing maintenance inspection protocols failed to identify the unsecured components.