What happened
On July 7, 2012, at 10:10 UTC, a Piper PA 31-350, registration OK-EKV, was conducting aerial photography near Krakow, Poland. During the climb phase of the flight, the crew experienced a sudden, heavy impact originating from the left engine, which was immediately followed by a loss of power and engine shutdown.
The pilot promptly implemented single-engine emergency procedures. Given the aircraft's proximity to Krakow Balice Airport (EPKK), the pilot elected to return to the departure airfield. The aircraft landed safely at EPKK without any damage to the airframe or injuries to the two-member crew.
The investigation
The ÚZPLN investigation focused on a technical examination of the left Lycoming TIO 540J2BD engine. Upon inspection of the engine cowling, investigators discovered significant structural damage to the upper engine casing.
Detailed disassembly of the engine revealed that a segment of the crankshaft counterweight (p/n LW-19210) had become detached. The investigation found that a circular washer (p/n 71907) within the weight's retaining pin hole had failed and broken into several pieces, which were recovered from the oil sump. This failure allowed the retaining pin to shift axially, damaging the retaining ring and allowing the counterweight segment to loosen. The loose component then struck the engine casing, the connecting rod of the sixth cylinder, and the camshaft, causing the camshaft to deform and disengage from the gears.
Findings
- The primary cause of the engine failure was the detachment of a crankshaft counterweight segment.
- The failure originated from the structural failure of a circular washer, likely due to material fatigue.
- The loss of the washer allowed the retaining pin to move, leading to the subsequent destruction of the engine casing and internal rotating components.
- Standard maintenance procedures do not include a specific inspection of this internal component, meaning the defect could not have been detected without a complete engine teardown.
- The aircraft was maintained in accordance with all applicable regulations, and the pilot was fully qualified and competent.