What happened
On September 19, 2004, at approximately 17:36, a private Lake LA-4-200 Mod. "Buccaneer" amphibious aircraft, registration OH-AKF, crashed into Lake Äkäslompolojärvi in Kolari, Finland. The aircraft had departed from the shores of Lake Jerisjärvi, intending to fly near Rovaniemi and land on the Ounasjoki river. Due to poor weather conditions, the crew was flying at a low altitude of between 50 and 100 meters, following the Äkäsjoki waterway.
As the aircraft approached the northern shore of Lake Äkäslompolojärvi at near-cruise speed, the upper rear attachment ear of the left horizontal stabilizer failed completely due to fatigue. This caused the stabilizer to twist downward, which subsequently broke the forward attachment ear under the pressure of the airflow. The failure also caused the elevator trim torque tube to snap and the structure connecting the elevators to tear. This resulted in a sudden, severe downward pitching force that the pilot could not counteract at such a low altitude. The aircraft struck the water on its right wingtip, causing it to roll left and impact the surface with the left wing and forward fuselage. The aircraft sank to a depth of approximately 10 meters, and both occupants perished due to drowning.
The investigation
The investigation examined the structural integrity of the stabilizer attachments, the flight conditions, and the crew's credentials. Investigators found that the aircraft's airworthiness and registration were valid, and the aircraft carried sufficient fuel for the planned VFR flight. The investigation also noted that the person in the pilot's seat was flying with an expired medical certificate, while the aircraft owner, sitting in the adjacent seat, held a valid license. The investigation established that the stabilizer attachment ear had been subjected to forces exceeding its fatigue strength over a prolonged period. No evidence of chafing was found, suggesting the final failure of the remaining attachment point happened rapidly under aerodynamic load.
Findings
- The primary cause of the accident was a fatigue fracture in the upper ear of the left horizontal stabilizer's rear attachment fitting.
- The progression of this fatigue fracture led to the complete breakage of the attachment point during flight.
- The resulting downward movement of the stabilizer caused a near-total loss of pitch control.
- The low flight altitude and the use of flaps prevented the pilot from recovering from the sudden nose-down pitch.
- The aircraft's stabilizer attachments were not inspected at the departure point because the aircraft's tail was submerged in water.
Safety action
Because incipient fatigue fractures in these types of attachment points are nearly impossible to detect using current maintenance and inspection protocols, the investigation recommends that the competent aviation authority implement measures to ensure the reliability of horizontal stabilizer attachments.