What happened
On July 10, 2010, an Aero Boero AB-115, registration PP-GGR, was conducting a local flight instruction mission at the Ponta Grossa Aerodrome in Paraná, Brazil. The crew consisted of a flight instructor and a student pilot. During the fifth go-around of the training session, the engine experienced a significant drop in power, with RPM falling from 2,40 and airspeed dropping to 60 mph.
The instructor attempted to return to the runway by initiating a left turn. However, during the maneuver, the engine power continued to decline, with RPM dropping further to 1,900 and airspeed falling to 50 mph. Faced with a continuous loss of altitude and power, the instructor executed a forced landing in an unprepared field near the aerodrome. Both occupants emerged uninjured, and the aircraft sustained no damage.
The investigation
CENIPA investigators examined the engine and fuel system to determine the cause of the power loss. Initial tests on the aircraft's magneto and power settings did not reveal immediate abnormalities. To investigate a suspected fuel system failure, the carburetor was replaced and tested, which resulted in satisfactory performance.
Upon subsequent disassembly and inspection of the original carburetor, investigators discovered fragments of porous foam material inside the carburetor bowl and within the mixture valve communication channel. Further analysis revealed that this foam was residue from the aircraft's air filter. The investigation established that the air filter being used was not the type recommended by the manufacturer's maintenance manuals, which specifically advised the use of paper filters.
Findings
- The engine power loss was caused by the partial interruption of fuel flow to the engine.
- As the pilot increased power, the carburetor's venturi effect sucked foam debris from the air filter into the carburetor bowl and through the diffuser.
- The use of an unauthorized foam air filter instead of the manufacturer-recommended paper filter allowed debris to enter the fuel system.
- Inadequate maintenance supervision allowed the installation of non-compliant components.
Safety action
CENIPA issued several safety recommendations following the investigation:
- To ANAC: Conduct a safety inspection of the maintenance facility involved (Agro Aérea Vila Velha Ltda.).
- To the Aeroclube de Ponta Grossa: Improve the supervision of aircraft maintenance processes.
- To the maintenance provider: Ensure all engine and air filter maintenance procedures strictly adhere to the manufacturer's technical manuals.