What happened
On July 2, 2024, a recreational flight involving 12 passengers was underway on a multi-leg route from Mirosławice to Bornholm, via Bagicz. The aircraft, an An-2 with registration SP-FIY, had completed its first leg without incident. After a scheduled stop in Bagicz, the crew departed for the second leg of the journey at 16:22.
Approximately 17 minutes into the flight, after traveling roughly 28 nautical miles, the pilot observed oil streaks appearing on the left side of the windscreen. Intermittent smoke was also seen emanating from the engine compartment. Although engine parameters, including oil pressure and cylinder head temperatures, remained within normal operating limits, the crew decided to terminate the flight and return to Bagicz to mitigate the risk of a more serious failure. The crew declared an emergency by setting the transponder to 7700. After approximately 12 minutes of flight following the leak's appearance, the aircraft landed safely on runway 25 at Bagicz.
The investigation
The PKBWL examined the aircraft's maintenance history and the engine's condition following the event. An inspection of the ASz-62IR engine conducted by the operator revealed a crack, delamination, and structural loss in the gasket located under the valve cover of cylinder number 1. The investigation also reviewed the maintenance logs, noting that while the aircraft had undergone a 2-month inspection two days prior, the last specific maintenance involving valve clearance adjustments—which requires gasket removal—had not occurred for over nine years.
Findings
- The primary cause of the incident was a cracked valve cover gasket on cylinder number 1.
- The investigation noted that the time elapsed since the last valve-related maintenance (over nine years) likely contributed to the degradation of the gasket's structure due to aging.
- While the engine was being maintained according to the current program, the maintenance schedule for the ASz-62IR engine does not mandate a maximum calendar interval between periodic tasks, which may allow seals to age beyond safe limits.
- The crew's decision to return to the airport was appropriate given the visible oil and smoke.
Safety action
- The PKBWL issued a safety recommendation to the engine manufacturer, WSK PZL-Kalisz, to conduct an analysis of the degradation of life-limited components, such as seals, and to implement a maximum allowable calendar interval between periodic maintenance tasks for the ASz-62IR engine family.