An emergency landing occurred on Sunday, 27 May 2007 at around 11:40 (Finnish time) off the shoreline of Helsinki. The gearbox clutch of a single-engine Diamond DA40D four-seat aircraft, registration OH-FDA, failed and the aircraft was subsequently forced to make an emergency landing in water. The OH-FDA was owned by the Aviation Club of Helsinki University of Technology.

On 31 May 2007, Accident Investigation Board Finland (AIB) appointed investigation commission C3/2007L for this incident. Chief Investigator Esko Lähteenmäki was named Investigator-in-Charge with Investigator Asko Nokelainen as member of the commission.

The pilot intended to fly two passengers to Tallinn airport and return to Helsinki-Malmi aerodrome in the afternoon. The Full Authority Digital Engine Control (FADEC) unit executed a pre-takeoff engine runup test. The test passed, with the engine providing full power. The takeoff and climb to 1000 ft were uneventful. The pilot was cleared to 4000 ft in Helsinki Terminal Control Area (TMA). When he began to climb he selected full power, at which time engine power fluctuated between 80–100% for 5 to 10 seconds. He then remembered that the pilot of the previous flight on the OH-FDA had told him that he, too, had noticed similar power oscillations. However, the power stabilized at 100% and all engine instrument indications were normal. Nevertheless, the pilot felt that the engine did not provide as much power as it had earlier. When he set the autopilot to a 500 ft/min climb, the engine suddenly overrevved (exceeded maximum RPM). The pilot then reduced power and tried to establish what the matter was. Meanwhile, he reported engine troubles to the air traffic control (ATC) and said that he would turn back to Helsinki-Malmi. At that time he was over the sea next to Harmaja Island, approximately 15 km from Helsinki-Malmi aerodrome.

Even though the pilot checked the emergency checklist regarding engine trouble, he soon realized that the engine would either idle or overspeed. The pilot deemed that the malfunction involved either the fuel supply or the FADEC. He let the engine run at a high rpm so as to make it back to land.

When they were at about 700 ft the pilot realized that they could not make it to the shore. Instead, they would have to make an emergency landing into the sea. The pilot reported the impending emergency landing and his estimated landing spot to the ATC and informed the passengers of the same. He landed the aircraft in the strait between Valkosaari Island and Katajanokka. The aircraft came to a halt approximately 15 metres from Valkosaari shore.

The pilot and the passengers climbed out onto the wings. Momentarily, a boat arrived next to the right wing, into which they boarded. Soon after, a police patrol boat also arrived at the scene, transporting the pilot and the passengers to Helsinki South Harbour where ambulances were already waiting. The Border Guard fastened the aircraft to Valkosaari shore, where it was encircled with oil spill booms.

The aircraft was transported to Katajanokka Quay, where the wings were detached. Then the aircraft was taken to Helsinki-Malmi aerodrome for test and research. The gearbox was disconnected from the engine, at which stage it was noticed that the friction surfaces on the clutch plate were worn.

The FADEC was sent to the German accident investigation authority, under whose supervision the engine manufacturer downloaded vital On-Board Diagnostic (OBD) data from the FADEC.

The cause of the emergency landing was clutch slippage and clutch plate surface wearing out. Clutch slippage was caused by engine oil which entered through a leak in the crankshaft’s lip seal. Fissures were detected in the seal and engine manufacturer assumes they are due to high internal crankcase pressure.

The investigation commission issued three recommendations.

First: The investigation commission recommends that the engine manufacturer design a system indicating changes in the rotational ratio between the crankshaft and propeller, i.e. clutch slippage. Clutch slippage indication should be recorded in the FADEC’s On-Board Diagnostics system.

Second: The investigation commission recommends that the engine manufacturer redesign the clutch breather orifice so that oil leaks internal to the clutch housing could be reliably distinguished from other, external, oil leaks.

Third: The investigation commission recommends that the aircraft manufacturer expedite the time of compliance for mandatory service bulletin to improve the crankcase breathing system.

Download the investigation report pdf-file (659 kB)