What happened
On May 9, 2021, a pilot was conducting a recreational flight in a Gyrocopter TERCEL, registration SP-XERO, departing from the Pomeranian Aeroclub’s aerodrome in Toru $ ext{ń}$ (EPTO). While flying over a dense forest area at approximately 2,400 ft, the pilot noticed increasing vibrations in the fuselage. In an attempt to stabilize the aircraft, the pilot reduced engine speed and descended to 1,400 ft while heading toward the aerodrome.
Shortly after increasing engine speed to maintain level flight, a sudden shock and intense vibration occurred throughout the aircraft. The pilot immediately shut down the engine and deactivated the electrical power and fuel supply. Due to the loss of engine power, the pilot performed a controlled dive to maintain airspeed and selected a landing site near a railway line. During the descent, the main rotor blades struck treetops and a power line. The aircraft subsequently struck the ground, hitting a granite post which caused the landing gear to collapse and the aircraft to overturn. The pilot escaped the wreckage without injury.
The investigation
The PKBWL investigation focused on the cause of the intense vibrations and the subsequent structural failure. The commission examined the condition of the Kašpar Ka-2/3-LT propeller and conducted extensive metallurgical testing through the Warsaw University of Technology. The analysis included visual inspections, ultrasonic flaw detection, X-rays, and micro-hardness measurements of the propeller components.
Findings
- The primary cause of the accident was the inflight separation of one of the three blades from the rear pushing propeller.
- Metallurgical analysis revealed that the blade failure was due to a fatigue crack in the mounting area near the thread.
- The crack was likely initiated by the grub screw, which caused a local increase in stress by crushing a portion of the thread.
- The propeller material, a structural steel, was found to be sensitive to brittle fracture and the notch effect.
- The design of the thrust bearing locknut was identified as inadequate, as the grub screws are positioned in a highly stressed area of the assembly.
Safety action
- The PKBWL issued a safety recommendation to the propeller manufacturer, Kašpar a synové - strojírna, to implement changes in both material and design to increase the durability of the Kašpar Ka-2/3-LT propeller blades.