What happened
On September 25, 2023, a Cessna FR172H, registration SP-DVW, departed from Piotrków Trybunalski (EPPT) to perform a mission involving the aerial dispersal of fox vaccines. Approximately 37 minutes into the flight, the pilot observed zero oil pressure. Due to uncertainty regarding the reading—noting no visible oil leaks in the cockpit and no rise in oil temperature—the pilot decided to perform a precautionary landing at the nearest airfield.
While attempting to locate the airfield in Łowicz, the pilot was unable to find the landing site from the air. After notifying Warsaw FIS and requesting coordination for a landing at Tomaszów Mazowiecki, the pilot accepted a suggestion to divert to a closer airfield in Nieborów. Approximately 42 minutes after takeoff, the engine began to lose power and RPM fluctuated, followed by a total engine failure at an altitude of 2,500 ft AMSL. The pilot executed an emergency landing in a ploughed field near a railway line, landing into the wind and along the furrows. The impact caused the nose gear strut to break, resulting in a nose-over of the aircraft. The crew sustained no injuries and evacuated the aircraft independently.
The investigation
The investigation focused on the engine's condition and the source of the oil loss. Post-accident inspection revealed an oil leak originating from a damaged quick-release drain valve on the oil pan. Upon removal of the oil pan, the upper portion of a broken drain valve was discovered.
Technical analysis of the broken valve identified a smooth fracture surface characteristic of fatigue, alongside a brittle fracture area. A machining mark (notch) was found at the initiation point of the crack. The engine, a Continental IO-360 D, had operated for 1,472 hours since its last major overhaul in 2004. The investigation also noted that these specific types of valves were discontinued in 2016 in favor of more functional designs.
Findings
- The engine failure was caused by the fatigue failure of the oil drain valve.
- The fracture was initiated at a machining notch, likely resulting from long-term engine operation.
- The loss of oil through the broken valve led to severe internal engine damage and subsequent seizure.
- The quick-release drain valve is not subject to mandatory periodic technical inspections, though manufacturers recommend checking for leaks and replacing the valve if necessary.