What happened
On January 11, 2020, a PIPER-PA34-220T, registration SP-CSL, was performing a flight from Warsaw Babice (EPBC) to Leszno (LZIB) carrying a pilot and three passengers. Approximately 40 minutes into the flight at FL100, passengers in the rear seats observed a dark liquid trailing from the trailing edge of the wing behind the left engine. Suspecting an oil leak, the pilot decided to return immediately to the departure airport.
During the return flight, the pilot reduced power to the left engine and noted a small but steady decline in oil pressure. While on final approach, following a further reduction in power, the oil pressure dropped significantly, triggering the low oil pressure warning light for the left engine. The aircraft landed safely without further incident; however, during taxiing, the left engine began to run unevenly, prompting the pilot to shut it down and continue taxiing on the right engine only.
The investigation
Following the incident, an inspection of the engine revealed metal shavings on the oil filter. Subsequent teardown of the Continental O–360–KB engine at a repair facility uncovered scoring on the cylinder walls of all cylinders. Specifically, in cylinder number 5, investigators found a burnt exhaust valve stem and clear evidence of overheating on the piston rings. Testing of the piston rings showed a loss of elasticity and improper clearance at the ring grooves.
Additionally, the investigation focused on the engine's low-pressure fuel injection system. Upon removing the brass injectors, it was discovered that they lacked legible markings. It is believed these markings were lost during a cleaning process during previous maintenance.
Findings
- The primary cause of the event was engine overheating during operation.
- The loss of elasticity in the piston rings allowed oil to enter the combustion chamber, leading to increased oil consumption and the gradual drop in oil pressure.
- Increased crankcase pressure, caused by exhaust gases leaking into the crankcase, resulted in the external oil leak observed by the passengers.
- A contributing factor was the potential installation of fuel injectors into the incorrect cylinders, made possible by the lack of identifiable markings on the injectors. This mismatch likely contributed to uneven engine performance and the overheating of the valve and rings in cylinder number 5.