Event Overview
On 25 September 2023, at 12:21 local time, a pilot holding a CPL(A) license departed from Piotrków Trybunalski Airport (EPPT) aboard a Cessna FR172 (registration SP-DVW) for a mission to distribute vaccine baits for foxes. Approximately 37 minutes into the flight, the pilot observed zero oil pressure indications. Uncertain of the reading due to no visible oil leaks from the cabin and stable oil temperature, he decided to perform a precautionary landing at the nearest airfield to assess the situation.
After 41 minutes of flight, the pilot attempted to land at the Łowicz landing site but could not locate it from the air. He began to climb and notified FIS Warsaw of the engine problem, requesting coordination for a landing at Tomaszów Mazowiecki Airport, 31 nautical miles away. FIS Warsaw suggested the closer Nieborów landing site, 5 NM distant, which the pilot accepted, changing course eastward.
Engine Failure and Forced Landing
About 42 minutes after departure, the engine began to lose power and rpm fluctuated. One minute later, the engine ceased operation while the aircraft was at 2,500 ft AMSL. The pilot selected an approximately 800-meter-long field for landing. The forced landing was executed on a smooth, freshly plowed field near a railway line, into the wind and along furrows. During touchdown, the nose wheel fork broke, causing the aircraft to nose over.
Post-Accident Examination and Cause
The crew exited the aircraft without injuries and rescue services arrived about seven minutes later. Examination of the aircraft after the event revealed an oil leak from the engine via a damaged oil quick drain valve installed on the oil sump. After removing the oil sump, the upper part of a fractured drain valve was found inside.
Verification of the engine showed extensive damage to its components and parts due to oil loss and consequent engine seizure. The airframe also sustained damage to structural elements and skin.
A detailed inspection of the damaged valve and its operational history determined that the fracture surface of the valve's fork exhibited a smooth region indicative of fatigue cracking, initiated at a material indentation (notch) likely resulting from prolonged engine operation. The Continental IO-360 D engine had accumulated 1,472 hours since its last major overhaul on 12 March 2004, with 28 hours remaining to its next overhaul. The smooth fracture surface developed after the initial fork separation, caused by contact of mating surfaces (known as "hammering"), and final valve failure occurred when the second fork fractured in a ductile manner.
According to the valve manufacturer's catalog, valves of this type were produced until 2016; newer, more functional designs are now used. Quick drain valves are not subject to periodic inspection during engine maintenance; the manufacturer's installation manual only specifies checking for leaks and replacing seals or the valve if necessary.
Cause
The accident was caused by fatigue fracture of the oil quick drain valve due to prolonged engine operation.
Aircraft and Registration
- Aircraft: Cessna FR172
- Registration: SP-DVW