Incident Overview
On 14 November 2015, a Cessna F172H aircraft was on a flight from Opole-Kamień Śląski (EPKN) to Nowy Sącz-Łososina Dolna (EPNL) to Nowy Targ (EPNT) and back. About 15 nautical miles from Nowy Targ airport, the pilot noticed a slow decrease in oil pressure. The pilot decided to perform a precautionary landing at Nowy Targ (EPNT). Before landing, the pilot reported the situation to Krakow Flight Information Service (FIS Kraków) and informed other aircraft in the area via radio. The aircraft landed safely.
Post-Landing Inspection
After landing, the oil pressure gauge indicated the lower limit of the green arc. External inspection of the aircraft and engine compartment revealed no visible oil leaks. The oil level on the dipstick showed minimum. After contacting the aircraft owner, it was decided to secure the aircraft at Nowy Targ. The event was reported to the continuing airworthiness management organization, which recommended a special inspection. Preliminary inspection found oil droplets on the tail tie-down hook and on the lower engine breather (attached to the bottom rear of the crankcase). An oil and filter cleanliness check showed no metal particles, and cylinder compression measurements were within normal limits.
Investigation Findings
Further examination focused on the oil leaks. The lower breather was checked and found to be clear. However, the upper breather (attached to the top front of the crankcase) was found to be blocked. The blockage likely occurred in two stages: partial blockage by an insect during the summer, followed by complete blockage from oil deposits during subsequent operation. After cleaning the breather and changing the oil, an engine test run showed no anomalies. A subsequent test flight confirmed the oil level remained constant. The aircraft then flew back to Opole-Kamień Śląski without issue.
Cause
The cause of the incident was a blocked crankcase breather, which caused oil leakage due to increased pressure and forced a precautionary landing.