What happened
On 8 January 2012, a Cessna 150M, registration ZS-MAV, departed from Grand Central Aerodrome (FAGC) for a flight intended to serve as a Private Pilot License renewal test. The flight was being conducted by a flight instructor accompanied by a student pilot. While flying at approximately 7,000 feet over the Emeridge Dam area, the engine began to run roughly. Despite the instructor's attempts to rectify the issue using carburetor heat and adjusting the mixture, the engine lost power and eventually failed.
As the aircraft's descent rate reached roughly 100 feet per minute, the crew determined they could not return to Grand Central Aerodrome. They opted to perform a forced landing in a grass field within the Modderfontein Conservancy. During the landing roll, the aircraft struck an earth embankment, which caused damage to the nose wheel and the right wing. There were no injuries to the two occupants.
The investigation
Following the incident, the engine was recovered and examined by an approved Aircraft Maintenance Organisation. The investigation focused on the mechanical state of the Continental O-200-A engine. Technicians found that the engine could not be rotated, prompting a full engine strip.
Upon inspection of the number 2 cylinder, investigators discovered that the exhaust valve had seized in the open position. This mechanical failure caused the piston to strike the valve, resulting in a bent valve stem and damage to the piston. Significant carbon buildup was also noted on the piston crown. The investigation confirmed that the weather conditions were clear and visibility was excellent, meaning meteorological factors did not contribute to the accident.
Findings
- The primary cause of the engine failure was the number 2 cylinder exhaust valve seizing in the open position during flight.
- The subsequent contact between the piston and the valve led to the catastrophic engine failure.
- The forced landing was considered unsuccessful in terms of preventing aircraft damage, as the impact with the embankment caused structural damage to the wing and landing gear.