What happened
On July 26, 2009, at approximately 12:25 UTC, a Zlin Z 526F, registration OK-GIN, was performing glider towing operations at Vysoké Mýto Airport (LKVM). The aircraft was engaged in its seventh tow of the day, pulling a Discus glider (OK-930 and 05), when a critical mechanical failure occurred.
Approximately four minutes after takeoff, while climbing to an altitude of 600 meters AGL, the glider disconnected from the towline. Simultaneously, the pilot experienced intense vibrations from the front of the aircraft, followed by the complete separation of the propeller from the engine and subsequent engine failure. Due to the low altitude and lack of sufficient runway length for a landing attempt back at LKVM, the pilot executed an emergency landing in a field near the village of Javorník, approximately 5 km from the aerodrome. The aircraft landed safely in a poppy field, and the pilot sustained no injuries.
The investigation
The ÚZPLN investigation focused on the structural failure of the engine's crankshaft. The engine, an M-137 A, was dismantled and sent to LOM Praha for metallurgical analysis. The investigation examined the fracture surface of the crankshaft at the propeller flange (Sh0401) and evaluated the maintenance history of both the engine and the V-503A propeller.
Investigators reviewed the maintenance records, noting that while the aircraft and propeller were maintained according to schedule, there was a discrepancy regarding the 15-year engine inspection interval. The investigation also looked for evidence of propeller imbalance, improper installation, or material defects. Metallographic examination revealed that the failure originated from fatigue caused by additional cyclic bending moments.
Findings
- The primary cause of the incident was a fatigue failure of the engine crankshaft.
- The fracture initiated at the threads of the crankshaft and propagated through the helix toward the propeller flange.
- The fatigue was driven by additional cyclic bending stresses acting on the end of the crankshaft.
- While the pilot reported no history of excessive vibration or propeller malfunction, the investigation could not entirely rule out the influence of previous aerobatic flight, as historical engine maintenance records prior to June 2007 were unavailable.
- The investigation found that the failure site was located in a difficult-to-access area partially hidden under the propeller flange, making it nearly impossible to detect via standard maintenance protocols.
- The pilot successfully performed the emergency landing procedures in accordance with the flight manual.
Safety action
- It is recommended that the type certificate holder for M137 series engines, when equipped with V 503A propellers, propose appropriate measures to inspect the condition of the crankshaft at the propeller flange location.