Accident Sequence
On 6 August 2005 at 1125 hours UTC, a Cessna 150M (registration G-BOVS) was conducting a training flight near Rumney, north east of Cardiff, South Glamorgan, Wales. The aircraft was returning to Cardiff Airport at 1,400 ft on the published Cardiff Docks arrival when the engine began to vibrate. The vibration initially slight, rapidly worsened. After checking carburettor heat and magnetos, the pilot transmitted a PAN call to Cardiff Radar and turned the aircraft away from the bay area towards fields to the north-east of Cardiff, anticipating a precautionary landing.
The vibration became severe, accompanied by a loud mechanical banging sound and decreasing airspeed. The pilot reduced engine rpm and sought the nearest suitable field, transmitting a MAYDAY call. The aircraft was landed in a grass field at minimum airspeed with 40° flap selected. The grass was wet and, despite wheel brake application, the aircraft overran the field boundary, passing through brambles and a wire fence. The left wing struck a fence post, turning the aircraft through 90° before it came to rest on a minor road with the nose landing gear collapsed. The instructor and student vacated the aircraft normally and without injury.
Damage and Injuries
The aircraft sustained damage to the nosewheel, engine cowling, and left wing, resulting in an insurance total loss. No injuries were reported among the two persons on board.
Engineering Examination
Subsequent examination of the aircraft found that the No 2 engine cylinder had a large circumferential crack around the base of the fins. The AAIB did not perform a metallurgical examination, but from discussions with the maintenance organisation, the failure was almost certainly due to metal fatigue originating in corrosion pitting on the outer surface of the cylinder at the base of the fins. This is a known problem with parts that have a long calendar history and/or very low utilisation.
Interrogation of the CAA’s Mandatory Occurrence Report database revealed 18 cases of cracking and/or separation of cylinders over the previous ten years, involving several different makes and models of engine but all sharing similar construction and layout. The date of original manufacture of the subject cylinder is not known, but it was recorded as being fitted to this engine in August 1996 and had run 1,570 hours since that time. The standard of the cylinder suggested it was of a considerably earlier manufacturing date.