Incident Overview
A Piper PA-15 Vagabond (registration G-BOVB) was conducting a local private flight from a strip at Twineham, East Sussex, on 5 September 1999. At an altitude of 1,500 feet and an indicated airspeed of 85 mph, the transparency overhead the pilot and passenger detached and fell from the aircraft. The pilot subsequently executed a forced landing at Lulhams Farm, Ripe, East Sussex. Both occupants were uninjured.
Aircraft and Damage
The aircraft, manufactured in 1948, was powered by a single Lycoming O-145-B2 piston engine. The transparency, measuring approximately 800 x 700 mm, was fixed directly above the seats. Its forward edge butted against the top edge of the windshield, and attachment fixings on all sides incorporated cap strips. The detachment of the transparency constituted the reported damage.
Pilot Information
The commander, aged 30, held a Private Pilot's Licence and had accumulated 284 flying hours, of which 26 were on the PA-15 type. In the preceding 90 days, he had flown 21 hours, with 6 hours in the last 28 days.
Investigation Findings
An investigation conducted by three members of the operating group concluded that several factors had contributed to the failure:
1. Ultraviolet (UV) exposure had likely reduced the flexibility of the transparency. 2. The material was acrylic, which has been found to suffer from stress fractures. 3. The self-tapping screws were exactly the same size as the pre-drilled holes, leaving no allowance for thermal expansion or contraction. 4. The fixings may have been overtightened, causing stress fractures around the pre-drilled holes. 5. The area above the seats, between the wings, is a low-pressure area, causing continual but slight outward and inward bowing of the transparency. 6. The cap strips obscured areas most prone to cracking, making them not obvious during pre-flight inspections.
Corrective Actions
The operating group is replacing the transparency with a heavier gauge polycarbonate material, 'Lexan', which reportedly does not suffer from UV damage or stress fractures. Additionally, they are drilling oversize holes for the fasteners and planning periodic removal of the cap strips to allow inspection of the fasteners and surrounding transparency.