What happened
On August 14, 2019, a Cessna 172N, registration LZ-SPB, was conducting a training flight departing from Lesnovo Airfield. The flight, operated by Avio Detachment Varna Ltd, was intended to take place in the training area above the Iskar Dam. During the climb to approximately 3,600 feet, the trainee pilot noticed smoke or fumes emanating from the left side of the cockpit dashboard.
In response to the smoke, the instructor decided to terminate the flight and return to the airfield. As the aircraft began a right-hand turn to initiate the return, engine vibrations became apparent. During the final approach to runway 10, the engine ceased operation approximately three to four seconds before touchdown. The crew managed to perform a successful landing with no injuries to the two occupants. Upon inspection on the ground, the crew discovered that oil had spilled across the lower surface of the airframe.
The investigation
The AAIU investigation focused on the mechanical state of the engine and the integrity of the oil system. Post-flight inspections at a maintenance hangar revealed that the engine oil filter had been torn away from its adapter plate. The investigation established that the filter was only being held in place by the locking wire. The inspection also found significant damage to the mounting assembly of the adapter plate at the crankcase and destruction to the valve lifter housing of the fourth cylinder. The engine had lost nearly all its oil, with only 200 ml remaining in the crankcase.
Findings
- The primary cause of the incident was the tearing of the engine oil filter from the adapter plate.
- This failure was driven by plastic deformations in the mounting sleeve.
- These deformations were the result of improper maintenance or repair procedures.
- The aircraft's airworthiness documentation contained inconsistencies regarding the serial number between the registration and noise certificates.