What happened
On September 28, 2007, at approximately 16:30, a Cessna 172N aircraft, registration OH-CAU, performed an emergency landing on the Porvoo motorway near the Sipoonlahti exit in Sipoo, Finland. The aircraft, operated by Malmi Aviation Club, was on a local flight from Helsinki-Malmi aerodrome when the engine power suddenly dropped from 75% to between 25% and 30%.
The pilot initially attempted to turn toward the village of Box and notified Malmi air traffic control of the power loss. After determining that the aircraft could not reach Malmi aerodrome, the pilot decided to land on the motorway. Due to heavy road traffic, the landing could not be completed before the Hangelby Bridge, forcing the pilot to land in the right-hand lane of the motorway.
During the landing sequence, the aircraft's right wing struck a light pole, and the left wing subsequently hit a traffic sign. The left main undercarriage climbed the roadside barrier, causing the nose gear to break off. The aircraft's nose dropped, and the propeller blades were severed upon impact with the crash barrier. There were no injuries to the pilot or the two passengers, though the aircraft sustained significant damage. No damage was caused to road traffic.
The investigation
The investigation focused on the sudden loss of engine power in the aircraft's TAE 125-01 diesel engine. Investigators examined the fuel system and discovered metal shavings within it, which were traced back to a damaged high-pressure fuel pump.
Fuel analysis conducted in both Finland and Germany confirmed that the JET A1 fuel used met all standard requirements. However, the investigation compared the lubricity of the fuel using the HFRR method. The aircraft's fuel showed an HFRR value of 0.835 mm, whereas the fuel used for the high-pressure pump's original certification by EASA had a lower, more lubricative value of 0.780 mm.
Findings
The investigation established a causal chain of events leading to the engine failure. A metal chip, having broken off from the bearing surface of the high-pressure pump's piston actuator, caused the pressure regulation valve to become stuck in a nearly open position. This resulted in the common rail maintaining only enough pressure to sustain engine idling, causing the significant power loss.
The degradation of the pump's bearing surface was likely initiated by the presence of water in the fuel. It is believed that water entered the fuel tank through a poorly secured fuel filler cap. While no water was detected in fuel samples taken immediately before or after the flight, the investigation concluded that this contamination was the root cause of the mechanical failure.