What happened
On 8 September 2013, a Zenair CH 701SP, registration G-IMME, was conducting a private flight over the Pentland Firth in Scotland. While the pilot was performing gentle manoeuvres at approximately 100 mph, they observed both cabin doors bulging into the slipstream. Shortly after this observation, the left door suffered a fracture and became detached from the aircraft. The incident also resulted in damage to the left tailplane skin. There were no injuries to the pilot or the single passenger on board.
The investigation
The investigation focused on the structural integrity of the aircraft's doors. The aircraft was equipped with a specific type of door known as 'bubble' doors, which feature a single transparent panel with a moulded bulge designed to improve visibility and cockpit space. Unlike the original design which used a tubular frame with screwed-on transparency, this aircraft was fitted with a newer version featuring a composite frame. This specific door configuration had been in use for approximately 9 hours of flight time prior to the incident.
Findings
It was determined that the left door fractured and departed the airframe due to the aerodynamic forces acting upon the bubble-shaped design. The pilot noted that the doors lacked sufficient rigidity to resist the pressure created by their shape while flying at speed. Additionally, the Light Aircraft Association identified these bubble doors as an unapproved modification to the aircraft type.