What happened
On May 1, 2013, at approximately 19:40 LMT, a pilot initiated a visual flight from a grass airstrip in Łęgowo, Poland, in a Zenair CH-601HD Zodiac, registration OK-LUA47. The aircraft was carrying one passenger, a seven-year-old child.
After reaching an altitude of approximately 100–150 meters AGL, the right canopy lock suddenly opened, causing the canopy to deform. The pilot attempted to hold the canopy in place with his right hand while controlling the aircraft with his left. After flying roughly 3 km, the left canopy lock also disengaged, causing the canopy to flap violently. This-induced aerodynamic forces caused the aircraft to pitch nose-down and significantly restricted the pilot's ability to control the aircraft.
Unable to maintain the centerline of the runway, the pilot opted for an emergency landing in a nearby field. During the maneuver, the pilot encountered medium-voltage power lines at an altitude of approximately 10 meters. In an attempt to fly beneath the wires, the pilot performed a hard three-point landing across field furrows. The impact caused the nose gear to break away and the left main gear leg to fail, leading to a nose-over. The engine was torn from the airframe and the propeller was destroyed. Both occupants sustained minor injuries.
The investigation
The PKBWL investigation examined the mechanical condition of the canopy locking mechanism and the structural integrity of the canopy. The investigators found that the locks were properly adjusted, but noted significant asymmetry in the pins and the receiving slots. This asymmetry was attributed to the lack of structural stiffness in the canopy.
Additionally, the investigation looked into the landing gear failure. It was determined that the left main gear fork and wheel broke at a welded joint that had been weakened by corrosion, which had been caused by long-term use and insufficient anti-corrosion protection. The investigation also noted that the aircraft's flight manual contained no procedures for handling a canopy opening in flight.
Findings
- The primary cause of the accident was mechanical wear of the canopy locks and a reduction in the structural stiffness of the canopy due to operational use.
- A contributing factor was the design characteristic of the canopy, which prioritized lightweight construction at the expense of structural rigidity, making it susceptible to loss of stiffness over time.
- The use of unpaved, grass airstrips contributed to increased vibrations and structural stress on the canopy during taxiing and takeoff.