Incident Overview
On June 3, 2020, a Cessna 177RG (registration SP-CVG) was operating a circuit flight at Mielec (EPML) when the pilot observed increasing oil stains on the windshield. The flight occurred after dusk, rapidly reducing forward visibility from the cockpit. The pilot decided to land immediately, and the landing was completed without further consequences.
Post-Flight Examination
Following the flight, an inspection of the aircraft revealed oil in the air intakes, on the engine cowling, and on the windshield. The operator arranged for an external company at EPML to remove the propeller. After inspecting the aircraft and removing the propeller, mechanics suggested that the oil leak might have originated at the connection between the propeller blades and the hub. However, analysis of photographs taken immediately after the incident did not support this theory, as no oil residue was visible around the blade-hub connection points. The propeller was under warranty and was sent to an overhaul facility for repair. During technical inspection at the overhaul facility, no leaks were detected at the blade-hub interface. Additionally, it was determined that the C207 propeller hub does not contain oil; rather, it is filled with grease. Hubs filled with grease feature a vent fitting (item 43) on the body, whereas those filled with special red-dyed oil have a fill and drain plug (item 42).
Probable Cause
The probable cause of the incident was damage to the O-ring sealing the connection between the propeller hub and the crankshaft during propeller installation, which resulted in an oil leak.