What happened
On 12 October 2006, a Lockheed L188C Electra, registration G-FIJV, was performing a cargo flight from Nottingham East Midlands Airport to Cork. During the taxi phase prior to departure, the cowlings for the number 3 engine detached from the aircraft. The loss of these components caused impact damage to the aircraft's fuselage and the number 4 propeller.
The flight proceeded to its destination without further incident. The crew was unaware of the missing cowlings until the aircraft arrived in Cork, where ground personnel noticed the missing parts on a taxiway at the departure airfield. There were no injuries to the two crew members on board.
The investigation
The AAIB investigation focused on the mechanical failure of the number 3 engine's components. Examination of the aircraft revealed that the latch attachment structure for the cowlings had failed due to an overload. While the latches themselves were found to be in good working order, the hinges showed that the doors had been pulled backwards and upwards before being released.
Investigators examined the air starter motor, which is mounted on the propeller reduction gearbox. Findings indicated that the casing of the air starter motor's reduction gearbox had broken up. Although some fragments were missing, the recovered pieces showed that the failure of the casing released the rotating clutch assembly into the nacelle. This event caused the left cowling to deform, allowing propeller wash to enter the nacelle and increase the aerodynamic loads on the cowling attachments.
Findings
- The primary cause of the incident was the failure of the air starter motor gearbox casing on the number 3 engine during taxi.
- The failure of the casing released the clutch unit into the nacelle, which deformed the left cowling.
- This deformation allowed propeller wash to enter the nacelle, creating an overpressure that overloaded the cowling latch attachment structure.
- The loss of the cowlings subsequently led to the number 4 propeller striking the right cowling.
- It is suspected that a crack may have developed in the gearbox casing, which went undetected because it was located in a region not easily visible through oil leaks or inspections.
Safety action
Following the incident, the operator implemented a new regime of regular inspections for the air starter units on their Lockheed Electra fleet to check for any signs of defects or cracks.