What happened
On 16 August 2003, a Zlin Z5/0LX, registration G-MATE, was conducting a private aerobatic practice session near Finningley, South Yorkshire. After successfully testing the aircraft's inverted oil system, the pilot began a flight sequence from an altitude of 3,800 feet. The first nine manoeuvres of the routine were completed without issue.
During the tenth manoeuvre, a vertical hesitation roll, the engine began emitting a loud screeching noise. As the pilot returned to level flight and adjusted the throttle, a loud bang occurred, followed by smoke and oil escaping from the engine cowling. The engine suffered a severe loss of RPM and significant vibration.
Unable to maintain altitude and with no oil pressure remaining, the pilot declared a 'MAYDAY' to RAF Waddington. Realising that the nearby airfield at Finningley was unreachable, the pilot selected a ploughed field for an emergency landing. Upon touchdown, the aircraft's landing gear caught a rut in the soft ground, causing the aircraft to flip onto its back. The pilot sustained minor injuries and was able to exit the cockpit through the broken canopy.
The investigation
The AAIB examined the engine and the aircraft's history. The investigation revealed that the No 4 cylinder had completely fractured, with the piston and parts of the connecting rod jammed within the barrel. The crankcase had also ruptured. Laboratory analysis of the crankshaft showed that the No 4 journal had been subjected to extreme temperatures exceeding 600°C, indicating a rapid breakdown of lubrication.
Investigators also noted that the aircraft had been involved in a collision with a stationary Pitts Special in July 2003. While the engine was not running during that collision, the investigation highlighted that a shock load inspection should have been performed following such an event, as per manufacturer guidelines.
Findings
- The engine failure was caused by a sudden interruption of oil supply to the crankshaft bearings.
- The No 4 bearing suffered catastrophic failure due to extreme overheating, which subsequently led to the destruction of the connecting rod and cylinder.
- While the use of RTV sealant during a previous crankcase replacement was noted, there was no evidence that excess sealant had blocked the oil galleries.
- The engine had previously undergone a crankcase replacement earlier in 2003.