What happened
On November 14, 2015, a Cessna F172H was conducting a flight route originating from Opole-Kamień Śląski (EPKN), traveling via Łososina Dolna (EPNL) and Nowy Targ (EPNT), before returning to EPKN. Prior to departure, the pilot performed a standard pre-flight inspection and replenished the engine oil to a level midway between the minimum and maximum marks.
While flying approximately 15NM from Nowy Targ (EPNT) toward Łosos and following the flight plan, the pilot noticed a systematic and slow decrease in oil pressure. In response to this developing situation, the pilot notified Kraków FIS and alerted other aircraft in the vicinity of the situation before deciding to execute a precautionary landing at EPNT. The aircraft landed safely without any injuries to the occupants.
The investigation
Following the landing, the oil pressure gauge remained at the lower limit of the green operating range. An external inspection of the aircraft and the engine compartment revealed no visible external leaks. However, the oil level on the dipstick had dropped to the minimum level. After consulting with the aircraft owner, the aircraft was secured at EPNT, and the incident was reported to the continuing airworthiness management organization, which recommended a special inspection.
Preliminary inspections identified oil droplets on the rear mooring hook and on the lower engine breather. While inspections of the oil cleanliness and filters showed no metal particles, and cylinder compression tests were within normal limits, a critical finding was discovered during the inspection of the crankcase breathers. While the lower breather was found to be clear, the upper breather, located at the front of the crankcase, was found to be blocked.
Findings
- The blockage of the upper crankcase breather was the primary cause of the incident.
- The investigation determined that the blockage likely occurred in two stages: an initial partial obstruction caused by an insect during the summer months, followed by a complete blockage caused by the accumulation of oil deposits during subsequent operations.
- This blockage led to an increase in internal pressure, which forced oil to leak through the lower breather, resulting in the observed drop in oil pressure.