What happened
On September 20, 2020, a SZD-48-3 glider, registration PT-POH, was performing a local flight at Formosa Aerodrome in Goiás, Brazil. After approximately two hours of flight, the pilot executed a low pass followed by a loop maneuver. During this sequence, the aircraft's indicated airspeed reached between 308 and 321 km/h, exceeding the manufacturer's established limits for maneuvering (170 km/h) and turbulence (200 km/h).
Following the maneuver, the aircraft began to pitch up unexpectedly and experienced intermittent loss of lift. The pilot reported a malfunction via radio, but the aircraft eventually entered an uncontrollable, abnormal attitude and collided with the ground. The impact resulted in substantial damage to the aircraft's airframe, including the wings, canopy, and tail cone, and the pilot sustained fatal injuries.
The investigation
CENIPA investigators examined the wreckage and analyzed data from onboard flight recorders, including a Naviter Oudie2. The investigation focused on the mechanical integrity of the horizontal stabilizer assembly. Technical analysis at the Aeronautics and Space Institute (IAE) revealed that the housing fixing bolt in the rear lock of the horizontal stabilizer had disengaged.
Investigators found that the bolt's grooves were not properly engaged with the locking spring, which allowed the pin to be in an unlocked position while still providing temporary control. The investigation also noted that a manufacturer's service bulletin (No. BE-029/87), which suggested an additional safety modification to the bolt head to prevent accidental disconnection, had not been implemented on PT-POH. Furthermore, while the maintenance manual did not explicitly cover the removal of this bolt during standard maintenance, its removal was common during aircraft transport, and it was unclear if the bolt had been improperly reinstalled following such an event.
Findings
- The primary cause of the loss of control was the disengagement of the horizontal stabilizer fixing bolt, which led to the loss of elevator control.
- The aircraft was operated at speeds significantly exceeding the manufacturer's recommended limits for maneuvering and turbulence.
- The structural stresses from the overspeed, potentially compounded by turbulent air near the runway threshold, contributed to the total disconnection of the already unsecured bolt.
- The failure to implement the manufacturer's recommended safety modification for the tail bolt attachment increased the risk of accidental disconnection.