What happened
On 5 September 1999, a Piper PA-15 Vagabond, registration G-BOVB, was conducting a private local flight from a private airstrip at Twineham. While the aircraft was cruising at 1500 feet and 85 mph, the transparency located above the pilot and passenger suddenly detached and fell from the airframe. This event caused a significant increase in drag and a loss of lift, necessitating an immediate forced landing at Lulhams Farm, Ripe, in East Sussex. There were no injuries to the pilot or the passenger.
The investigation
An investigation conducted by three members of the operating group examined the failure of the approximately 800 x 700 mm acrylic component. The transparency had been positioned directly above the seats, with its forward edge meeting the top of the windshield and secured by cap strips. The investigation looked into the material properties of the acrylic, the method of attachment using self-tapping screws, and the environmental factors affecting the component.
Findings
Several contributing factors were identified regarding the detachment. The investigation established that UV exposure likely reduced the flexibility of the acrylic material, which is otherwise prone to stress fractures. Furthermore, the self-tapping screws were the same diameter as the pre-drilled holes, which prevented necessary thermal expansion and contraction. It was also noted that the fixings may have been overtightened, creating stress points around the holes.
Aerodynamic forces also played a role, as the area between the wings creates a low-pressure zone that causes the transparency to bow continuously. Finally, the use of cap strips made it difficult to detect potential cracks during routine pre-flight inspections.
Safety action
To prevent a recurrence, the operating group is replacing the acrylic transparency with a heavier-gauge polycarbonate (Lexan) material, chosen for its resistance to UV damage and stress fractures. The new installation will feature oversized holes for the fasteners to allow for thermal movement. Additionally, the group will implement a maintenance schedule involving the periodic removal of cap strips to ensure the integrity of the fixings and the surrounding material can be properly inspected.