What happened
On April 20, 2014, a Comp Air 8 aircraft, registration OH-XDZ, crashed into a forest near Jämijärvi, Finland, during a skydiving event. The aircraft was performing its eighth flight of the day, transporting ten skydivers to an altitude of 4,23 and 4,230 meters. During the flight, the skydivers requested a new jump line because the current one was too long. In response, the pilot increased engine power and initiated a left turn.
During the maneuver, the aircraft began to descend and accelerate, a change in speed that the pilot did not immediately realize. As the pilot reduced engine power to idle to manage the speed, a sudden decrease in airflow over the stabilizer caused a rapid nose-down movement. This change in angle of attack generated a downward force that caused the right wing strut to buckle upward. The right wing subsequently pivoted on its attachment and moved downward against the jump door, rendering the door unusable for evacuation. The aircraft immediately lost controllability and entered an external spiral dive around its vertical axis. While the pilot and two skydivers managed to escape through the pilot's door, the remaining eight skydivers were unable to exit and eight fatalities occurred upon impact with the ground.
The investigation
OTKES examined the structural integrity of the aircraft, the pilot's experience, and the aircraft's loading conditions. The investigation focused on the right wing strut, which showed evidence of a fatigue crack on the inner surface of the strut profile. This crack had developed over a long period and was not detectable during visual inspections.
Investigators also looked into unauthorized structural modifications. It was discovered that winglets had been added to both wings, including extensions in the wing plane. These modifications were not included in the original construction permit, and their impact on the aircraft's strength and flight characteristics had not been investigated. Furthermore, the investigation found that the aircraft's center of gravity was likely behind the limits permitted by the flight manual during the jump line approach.
Findings
- The immediate cause of the accident was the failure of the right wing strut due to forces exceeding its strength, triggered by a negative load factor during the nose-down moment.
- A fatigue crack was present in the wing strut, which may have been exacerbated by engine exhaust heat, vibrations, and the stresses of frequent take-offs and landings at high weights.
- The aircraft's center of gravity was likely outside of safe limits during the maneuver.
- The pilot's limited experience with high-power turboprop aircraft and insufficient training regarding weight and balance/loading effects contributed to the situation.
- Unauthorized structural modifications (winglets) increased the aerodynamic forces acting on the wing and struts without prior investigation of their impact.
- The high frequency of take-offs and landings, often at near-maximum weight, increased the structural load on the aircraft.