What happened
On October 7, 2012, a Zenair Stol CH 701 hydro-ultralight, registration 971-BS, departed from the Le Gosier hydrobase in Guadeloupe for a training flight. Shortly after takeoff, at an altitude of approximately 200 feet, the instructor took control of the aircraft, reduced power, and notified the student of a problem, intending to return to the base.
During this maneuver, the aircraft entered a right-hand bank. The instructor was unable to recover control, and the aircraft struck the sea surface in a steep nose-down attitude while banking right. The aircraft sank and settled at a depth of 3 meters about 300 meters from the shore. The student pilot managed to exit the cabin and reach the surface, but the instructor was killed. The aircraft was destroyed.
The investigation
Investigators examined underwater photographs of the wreckage, which confirmed the aircraft hit the water with a significant nose-down pitch and a right-hand bank. The examination revealed a failure of the rear right wing strut. While other struts remained attached to the fuselage and wings, all struts exhibited paint blistering caused by corrosion.
Detailed analysis of the failed rear right strut showed that the breakage occurred prior to the impact with the water. Severe corrosion inside the tube had significantly reduced its cross-sectional area, leading to its structural failure. No other mechanical anomalies capable of causing the loss of control were found in the wreckage.
Findings
It is probable that during takeoff, the rear right strut failed due to the combined stresses of aerodynamic lift and the repeated impacts of the floats against the water surface. This structural failure caused two critical effects:
- A change in the incidence of the right wing, which reduced lift and induced the right-hand bank.
- A loss of tension in the aileron control cables, rendering aileron control ineffective.
The primary cause of the accident was the pilot's failure to account for the impact of corrosion on the strength of the strut.