What happened
On 12 September 2015, a Cessna 182 F, registration ZS-DOU, was conducting a commercial skydiving operation near Mossel Bay Airport (FAMO). After dropping skydivers at flight level 105, the pilot began a descent to return to the aerodrome. During this return leg, the engine began running roughly. The pilot attempted to address the issue by applying carburettor heat, which briefly restored normal engine performance, but the engine soon began running poorly again before cutting out completely at approximately 3,000 feet.
Unable to reach the runway at FAMO, the pilot executed a forced landing in an open field roughly 2 NM south-west of Runway 10. While the aircraft touched down safely, the nose wheel struck an irrigation ditch during the landing roll. This impact caused the aircraft to flip onto its roof, resulting in substantial structural damage to the airframe, propeller, and wings. The pilot escaped the wreckage without any injuries.
The investigation
SACAA AIID investigators examined the engine's maintenance history and physical condition. A teardown and inspection of the Continental engine revealed no mechanical defects or evidence of shock loading. The investigation focused on the meteorological conditions and the pilot's management of the carburettor heat system.
Using METAR data from George Airport, investigators calculated the dew point depression to determine the likelihood of ice formation. The analysis confirmed that the atmospheric conditions were highly conducive to carburettor icing, with probabilities indicating serious icing potential during descent. The investigation also reviewed the aircraft's pilot operating handbook (POH) procedures regarding the use of carburettor heat.
Findings
- The primary cause of the engine stoppage was carburettor icing.
- The pilot did not monitor the temperature and dew point to identify icing-prone conditions prior to the flight.
- The pilot's application of the carburettor heat control did not follow the prescribed POH procedure, which requires using full throttle to stabilize the engine after applying heat.
- The introduction of hot air via the carburettor heat control inadvertently enriched the fuel mixture, contributing to the engine's eventual failure as the ice accumulation and rich mixture became unsustainable.