What happened
On 11 September 2007, an Apex Robin DR400/135CDi was conducting a training flight near Stolwijk, Netherlands. The flight, involving an instructor and a student pilot, was performing emergency landing exercises following a flight from Rotterdam Airport to Hoogeveen Airport. While flying at approximately 1,300 feet during a final landing practice, the instructor noticed a significant drop in engine revolutions. The engine power became insufficient to maintain level flight, prompting the instructor to execute emergency procedures. Despite these efforts, the engine failed to regain power, necessitating an emergency landing in a nearby field. The aircraft, registration PH-S/SVT, sustained severe damage to the engine, though the two crew members and one passenger escaped without injury.
The investigation
Investigators analyzed data from the engine's Full Authority Digital Engine Control (FADEC) unit and performed a physical inspection of the engine. The FADEC data revealed a critical spike and subsequent drop in oil pressure shortly before the power loss. Specifically, oil pressure exceeded the manufacturer's limit of 5,200 mbar before plummeting to 2,865 mbar. Following this fluctuation, engine revolutions and manifold pressure failed to return to stabilized levels, indicating a loss of power.
Subsequent teardown inspections of the Thielert TAE 125-012 diesel engine in Germany revealed a hole in the piston head of cylinder number 2 and significant scratches on the cylinder/piston contact surface. The investigation identified that the oil nozzles responsible for cooling the pistons were broken. The failure was traced to a lack of clearance between the oil nozzle pipe and the piston skirt, which caused the nozzle to crack under the stress of engine cycles.
Findings
- The engine failure was caused by thermal overstress of the piston due to insufficient cooling.
- The cooling process was interrupted because the oil nozzle pipe had cracked.
- The crack was caused by mechanical stress resulting from inadequate clearance between the oil nozzle and the piston skirt.
- This failure led to a loss of compression in cylinder number 2 and a subsequent drop in engine power.