Synopsis
On 13 February 2017 at 0512Z, a commercial pilot operating a Piper PA-36-300 (registration ZS-LJJ) on a crop spraying flight under Part 137 experienced an engine failure while conducting aerial application in the Settlers area. The pilot performed a forced landing in a field adjacent to the target farm. During the landing, the main undercarriage collapsed and the left wing struck a tree. The aircraft sustained substantial damage, but the pilot was not injured.
Investigation Findings
The Aircraft Maintenance Organisation (AMO) that recovered the aircraft found that the engine could not be rotated by hand. Inspection revealed a crack in the crankcase in the region of cylinders 7 and 8, indicative of an internal catastrophic failure.
A teardown inspection of the engine (serial number L-1235-54A) on 16 April 2018 revealed that the nut on the No.7 connecting rod big-end bearing was missing. The separation of the No.7 big-end bearing involved loosening of the nut installed on the big-end bearing bolts and eventual loss of one nut. Evidence of nut loosening was provided by fretting wear damage on the bolt threads and bolt holes. The No.7 connecting rod had separated from the crankshaft journal, causing substantial damage to the camshaft.
Aircraft and Pilot Information
The aircraft had a valid Certificate of Airworthiness expiring on 20 February 2017 at 4587.4 airframe hours. The last Mandatory Periodic Inspection (MPI) was conducted on 1 December 2016 at 4496.1 airframe hours; the aircraft had flown 91.3 hours since that inspection. The engine was overhauled in January 2015 at 6023.12 engine hours since new and 246.4 hours since last overhaul. The pilot held a valid Commercial Pilot Licence expiring 17 May 2019.
Probable Cause
The investigation concluded that it is likely the engine failure was a result of the separation of the No.7 big-end retainer and nut due to under-torqueing.