What happened
On 15 November 2013, a Cessna F172M, registration D-EESE, was conducting an aerial photography mission over Northern Ireland. After completing tasks near Belturbet, the pilot attempted to climb back to a cruising altitude of 2,000 feet. During this climb, the pilot noticed that the engine load gauge was only indicating 74-79% rather than the expected 100%.
As the pilot leveled off at 1,200 feet to investigate, the engine oil pressure began to drop into the amber and then the red sectors. This was accompanied by a simultaneous loss of airspeed and altitude. Following a MAYDAY call, the pilot determined that a water landing was the safest option due to the lack of suitable landing fields. The aircraft was ditched in Lower Lough Erne. During the descent, the occupants observed blue smoke exiting the cabin doors. While the aircraft remained upright after impact, the occupants were unable to open the doors due to external water pressure and had to escape through a window. There were no fatalities and no injuries among the two people on board, though the aircraft was destroyed.
The investigation
The investigation focused on the engine, which had been retrofitted with a Thielert TAE 125-01 diesel engine. Investigators downloaded data from the engine's Full Authority Digital Engine Control (FADEC) system, which revealed that oil pressure had been steadily declining for approximately 15 minutes prior to the ditching.
Upon a detailed teardown of the engine by the manufacturer, investigators identified significant mechanical failures. They discovered that a bolt securing the oil pump to the oil gallery had become loose, and an associated 'o' ring seal was broken. These findings suggested a failure in the oil delivery system.
Findings
- The primary cause of the engine failure was oil starvation of the turbocharger.
- A loose bolt and a broken seal at the oil pump attachment point allowed for a loss of oil pressure.
- This lack of lubrication caused the turbocharger main bearing to degrade, preventing the turbocharger from reaching the necessary rotational speed to maintain engine power.