What happened
On 15 August 2023, an Aermacchi Bosbok AM-3C, registration ZU-ADM, was conducting a private flight from Wonderboom Aerodrome in Gauteng. The flight, operated under visual meteorological conditions, was intended to be a local circuit returning to the same aerodrome. While cruising at approximately 500 feet above ground level, the aircraft's engine began running roughly before suffering a significant loss of power.
With limited time to react, the pilot identified an empty gravel road in the Rooiberg area and executed a precautionary landing. The aircraft reached the ground safely without any 0 fatalities or injuries to the two occupants on board. The incident took place roughly 5/58 nautical miles from the departure point.
The investigation
SACAA AIID investigators examined the engine following the event. A technical inspection by an approved person revealed that the engine had lost compression in cylinder number 2. The failure was traced to the exhaust valve head, which had separated from the valve stem.
Records indicated that a compression check (both cold and hot blow-by tests) had been performed on the engine cylinders on 24 July 2023, less than a month prior to the incident. At the time of the failure, the engine had approximately 183.7 hours of flight time remaining before its scheduled overhaul. The investigation also confirmed that the aircraft was within its annual inspection period and the pilot held a valid ATPL.
Findings
- The primary cause of the power loss was a fractured exhaust valve stem in the second cylinder.
- The separation of the valve head from the stem resulted in the loss of compression.
- There were no contributing environmental factors, as weather conditions were clear.
- This specific type of component failure had not been previously reported on this particular aircraft type.
Safety action
Following the incident, the maintenance organization sent the engine to a workshop for a complete overhaul as a precautionary measure.