The Air Accident Investigation Unit (AAIU) said the most likely cause of the incident was the deterioration over time of a key engine component in the Robinson R44 Raven II aircraft, which in turn triggered substantial engine damage during the training flight from Cork Airport on August 18, 2014.
However, investigators said they were unable to determine why the engine’s number three small -end bushing had deteriorated to such an extent.
There were no injuries in the incident and the AAIU said there are no safety recommendations arising out of its investigation.
The commander and a student pilot departed on a training flight on the day and were over Cork Harbour when the pilot reported to air traffic controllers that there was a vibration in the helicopter.
He made a mayday call and started an autorotation — using airflow rather than engine power to land — and selected a large field for the emergency landing.
However, as he came in to land at just 500ft above the ground, he was forced to make two adjustments to his flight path to avoid cables, a hedge and some trees, before landing heavily but upright, and sliding to a stop in the field.
Both crew walked away uninjured.
On receipt of the pilot’s written report, the AAIU said it became clear the incident was more serious than was first thought, and an investigation was launched.
It found the engine’s number three con rod had fractured in flight, causing significant secondary damage to the number three and four cylinder barrels, and to the crankcase.
The report said it appeared as if the number three small end bushing had deteriorated over time until an excessive amount of clearance formed between it and the engine’s number three piston pin.
This excessive clearance triggered a sequence of events during which the con rod was overloaded and failed. Subsequently, flailing of the failed con rod parts caused extensive secondary damage to other parts of the engine. The AAIU said the reason for the deterioration of the number three small end bushing could not be determined.
The report found that the same aircraft and the 2003-built engine had an event involving a loss of oil pressure in September 2009. The engine was overhauled after that incident and the work recorded in the aircraft’s maintenance records, which were found to be in order.
The AAIU also said there were no reported problems with the engine at the time of last year’s incident.