What happened
On 24 May 2008, a Cessna 208 Caravan amphibious floatplane, registration G-MDJE, was conducting a non-revenue commercial positioning flight from the River Clyde to Loch Lomond. While cruising at 1,200 feet over the Partick area of Glasgow, the pilot felt a vibration lasting approximately 10 seconds. A visual inspection from the cockpit revealed that the left water rudder had detached from the aircraft.
The pilot subsequently diverted to Glasgow Airport, where the aircraft landed without further incident. The detached water rudder was later located in a garden in Partick. There were no injuries to the two crew members on board.
The investigation
Investigators examined the rudder attachment post and the steering system. The examination of the rudder mounting post revealed that the welds on the pivot tube had failed. The damage to the post was consistent with a significant force being applied to the rudder from behind while it was deployed. Specifically, the steering input bar was bent, and the mounting post was distorted, suggesting the rudder had been forced to the left.
Microscopic analysis of the failure surfaces showed characteristics of an overload rather than fatigue. The investigation also noted that the attachment post had been heavily coated with grease for corrosion prevention, which may have obscured damage during inspections. Furthermore, a discrepancy was identified between the manufacturer's installation drawing, which suggested a cable tension of 30 lb (plus or minus 5 lb), and the service manual, which specified 10 lb (plus or minus 5 lb). At the time of the incident, the steering cable tension was measured at 8 lb.
Findings
- The water rudder detached in flight due to the failure of the attachment post.
- The damage to the rudder post was consistent with the rudder being struck from behind with sufficient force to cause an overload failure of the welds.
- This impact likely occurred while the aircraft was maneuvering on the water, rather than the aircraft reversing into a submerged object.
- The heavy application of protective grease on the post may have prevented the detection of any pre-existing damage during inspections while the aircraft was out of the water.