What happened
On the morning of 10 May 2024, a private flight traveling from Kroonstad Aerodrome to Bram Fisher Airport in the Free State province was interrupted by mechanical failure. The aircraft, a De Havilland DH-82A Tiger Moth II with registration ZS-BGL, was operating under visual meteorological conditions when the pilot noticed the engine running roughly during the cruise phase.
After approximately 1.8 hours of flight, the engine lost power entirely. The pilot attempted to rectify the issue by adjusting the fuel mixture and applying carburettor heat, but these efforts were unsuccessful. To avoid a more serious accident, the pilot executed a forced landing on the N1 Highway, approximately 12 nautical miles southwest of Kroontstad Aerodrome. There were no injuries to the pilot or the passenger, and the aircraft sustained no damage during the landing.
The investigation
Following the incident, the aircraft was moved to Rand Airport for a detailed engine teardown. Investigators examined the Gipsy Major 1H engine to identify the source of the power loss. While the engine showed rotational continuity and compression was present in cylinders 1, 2, and 4, the investigation revealed that cylinder No. 3 failed to produce compression.
Upon removing the No. 3 cylinder, inspectors found that the intake valve could slide freely within its guide. Crucially, the examination of the exhaust valve seat area revealed significant carbon build-up. This accumulation prevented the valve from sealing correctly, which compromised the airtightness of the combustion chamber.
Findings
- The primary cause of the engine power loss was poor compression in the No. 3 cylinder.
- This lack of compression was directly caused by the exhaust valve failing to seal properly due to carbon deposits in the valve seat.
- The build-up of carbon was likely a result of the gradual accumulation of lead oxybromide, a byproduct of tetra-ethyl lead found in Avgas 100LL fuel.
- Operating the engine at low power settings for cooling purposes may have contributed to the accumulation of these deposits.