What happened
On April 8, 2015, a Shorts SC7 Skyvan, registration OE-FDN, was conducting a training flight from Landsberg to Klatovy when the crew experienced a sudden, loud impact from the right side of the aircraft. The impact was immediately followed by a drop in RPM, torque, and oil pressure in the right engine, accompanied by visible smoke trailing from the rear of the engine.
As the flight progressed, the crew noticed a strong smell of fuel in the cabin and a continuous drop in the fuel level of the right wing tank. The instructor took control of the aircraft, implemented single-engine emergency procedures, and successfully landed the aircraft at Klatovy Airport. Upon taxiing to the parking stand, it was discovered that fuel was leaking from the right fuselage tank into the cabin and onto the ground, contaminating an area of approximately 80 to 100 square meters. Local firefighters were called to manage the spill.
The investigation
The ÚZPLN investigation focused on the mechanical failure of the Honeywell TPE331-2-201A engine. Technical analysis revealed that fragments from the second and third-stage turbine wheels had breached the combustion chamber and the engine casing. These high-energy fragments also pierced the aircraft's fuselage and the rubber seal of the fuel tank, creating the leak.
Investigators examined the engine's maintenance history and found that the 3rd-stage turbine wheel had been installed at 603 cycles. However, errors in recording the equivalent cycles (TEC) during a 2004 engine modification meant that the actual accumulated cycles were not accurately tracked in the CAMO records. Furthermore, the investigation looked into the Hot Section Inspection (HSI) performed previously, noting that the maintenance organization did not remove the rivets during the fluorescent penetrant inspection (FPI), which was contrary to the required maintenance manual instructions.
Findings
- The primary cause of the incident was the complete loss of power in the right engine due to the failure of the second and third-stage turbine wheels.
- The failure of the 3rd-stage turbine wheel was caused by a fatigue crack originating from a rivet hole.
- Inaccurate engine cycle tracking meant the component exceeded its manufacturer-specified service life.
- The failure to remove rivets during the FPI maintenance check likely prevented the detection of existing cracks.