What happened
On May 9, 2017, at 17:40 LMT, an instructor and a student pilot departed from the EPDE aerodrome in a Diamond DA20-C1 for a scheduled training flight. After passing the Kilo EPDE waypoint, the crew set the engine to cruise RPM. Approximately three minutes into the flight, the crew experienced uneven engine operation. To rectify the issue, the crew activated the fuel pump, which restored normal engine performance.
Three minutes later, the crew deactivated the fuel pump. Within one minute of doing so, the engine irregularity recurred, prompting the crew to reactivate the pump again. As the aircraft approached the Whisky EPDE waypoint, the engine experienced a sudden surge/jerk despite the fuel pump being active, resulting in a loss of approximately 5 knots of airspeed. The crew decided to return to the departure aerodrome, initially applying maximum power before returning to cruise RPM for the landing. The aircraft landed safely without further incident.
The investigation
The operator's maintenance organization conducted a thorough inspection of the Continental IO-240 engine, which included partial disassembly. The investigation covered the following technical checks:
- Inspection of the ignition system, including cleaning and bench testing of the spark plugs.
- Inspection of the fuel system, involving the removal, cleaning, and blowing out of fuel injectors with compressed air.
- Cylinder leak testing following a hot engine run, which confirmed the cylinders were within standard limits.
- Inspection of the valve train, including checking valve springs, rocker arms, and hydraulic lifter clearances.
- Replacement of the exhaust manifold gasket.
Following these procedures, an engine test run confirmed the engine was in proper technical condition, and the aircraft was returned to service.
Findings
The investigation determined that the cause of the engine irregularity was excessive carbon buildup on the exhaust valve guides and stems.