What happened
On 6 August 2005, a Cessna 150M, registration G-BOVS, was conducting a training flight when the engine began to vibrate while the aircraft was at 1,400 ft. The vibration intensified rapidly and was accompanied by a loud mechanical banging noise. As the airspeed decreased, the pilot attempted to troubleshoot the issue by checking the magnetos and carburettor heat.
Following the escalation of the engine malfunction, the pilot declared a MAYDAY call to Cardiff Radar and diverted the aircraft away from the Cardiff Docks arrival area toward fields to the north-east. The pilot performed a precautionary landing in a wet grass field, touching down at minimum airspeed with 40 degrees of flaps selected. Due to the wet surface, the aircraft overran the field boundary, passing through a wire fence and brambles. The left wing struck a fence post, causing the aircraft to rotate 90 degrees before coming to a stop on a minor road. The nose landing gear collapsed during the excursion. The two crew members, an instructor and a student, exited the aircraft without injury.
The investigation
Investigators examined the engine and found a large circumferential crack around the base of the fins on the No 2 cylinder. While a metallurgical examination of the specific failure was not conducted, discussions with maintenance organisations indicated that this type of cylinder separation has occurred in various engine models over the last decade. The failure appeared to originate from corrosion pitting on the outer surface of the cylinder base. The cylinder in question had been installed in August 199 6 and had accumulated 1,570 hours of use since that time.
Findings
- The primary cause of the engine failure was a circumferential crack in the No 2 engine cylinder.
- The crack originated from corrosion pitting at the base of the cooling fins.
- The failure was likely caused by metal fatigue.
- The cylinder involved was an older component with a long period of service or low utilisation.
- The CAA's Mandatory Occurrence Report database had previously recorded 18 similar instances of cracking or failure in parts of this specific construction.