What happened
On June 11, 2016, an ATEC Zephyr 2000 was conducting a local flight near Büllingen, Belgium. While cruising at approximately 3,000 feet, the pilot experienced a sudden noise and a severe rightward banking tendency. The upper polyester fabric covering the right wing had detached from the aircraft's structure.
Despite the significant loss of lift and increased drag, the pilot maintained control by applying full power, using left stick input, and utilizing rudder corrections. The pilot elected to return to the home base rather than attempting a forced landing on unknown terrain. During the approach to the runway, the aircraft''s asymmetrical drag caused it to overshoot the runway, eventually coming to a stop in the grass. There were no injuries to the pilot or passenger.
The investigation
Investigators examined the aircraft's wing structure and the adhesive used for the fabric. The inspection revealed that while the fabric itself remained intact, the glue had failed across most of the bonded surfaces. Notably, the investigation found that the lower wing fabric was also significantly degraded; manual pressure was sufficient to detach the lower fabric from its ribs.
Technical analysis showed that the aircraft's maintenance history was mostly up to date, though the 10-year major overhaul limit had been slightly exceeded. The investigation also noted that the leading edge design placed the edges of the upper and lower fabric behind the actual leading edge, which allowed air pressure to enter the wing interior once the fabric began to lift.
Findings
- The primary cause of the incident was the loss of adhesive bond between the wing's upper fabric and the underlying structure.
- A lack of overlap between the upper and lower fabric layers allowed dynamic air pressure to penetrate the wing once the fabric separated.
- There was an absence of specific guidance or inspection methods to help operators detect the degradation of the fabric-to-structure bonding.
- The aircraft's maintenance interval for a major overhaul had exceeded the 10-year limit by approximately 5%.