What happened
On June 3, 2020, while performing a circuit pattern at Mielec (EPML), the pilot of a Cessna 177RG, registration SP-CVG, observed increasing oil spots appearing on the cockpit windshield. The flight was being conducted after dusk, and the accumulating oil caused visibility from the cockpit to deteriorate rapidly. In response to the loss of visibility, the pilot decided to perform an immediate landing. The landing was completed successfully without any further consequences.
Following the flight, a post-flight inspection revealed the presence of oil on the engine cowling, the air intakes, and the windshield. The aircraft operator arranged for the propeller to be removed by an external maintenance company located at the airport. Initial assessments by mechanics suggested the leak might have originated from the junction between the propeller blades and the hub; however, photographic analysis of the scene did not support this, as no oil traces were found in that specific area.
The investigation
The investigation focused on identifying the source of the oil leak. The propeller, which was still under warranty, was sent to a repair shop for detailed inspection. During the technical examination of the propeller, no leaks were found at the connection between the blades and the hub. Furthermore, the investigation clarified the internal composition of the C207 type hub, noting that such hubs are filled with grease rather than oil. The investigation noted that grease-filled hubs feature a vent, whereas oil-filled hubs (using red-colored oil) are equipped with specific filling and draining plugs.
Findings
- The damage to the O-ring sealing the connection between the propeller hub and the crankshaft during propeller installation was the probable cause of the oil leak.