What happened
On 25 September 2015, a Sling 2 aircraft, registration ZU-FZF, was conducting a private scenic flight near Grand Central Airport when the engine began to malfunction. The pilot, accompanied by his son, noticed two red warning lights indicating low engine oil pressure, followed by the engine running roughly. During the flight, the pilot observed dark smoke emanating from the engine compartment and subsequently declared a MAYDAY call to air traffic control.
While attempting to divert to Eagle Creek Airfield, the pilot was unable to reach the runway. The aircraft instead performed a forced landing on the N14 highway near Centurion. The impact with the road surface was severe; the aircraft hit the ground with a high rate of descent and a slight left roll, causing the nose and right main landing gears to collapse. The aircraft bounced across the highway before coming to a stop. Despite the substantial damage to the engine, propeller, and wing structure, the pilot and passenger escaped without injury.
The investigation
An investigation by the SACAA AIID focused on the mechanical failure of the Rotax 912 IS engine. Analysis of the Engine Control Unit (ECU) data logs confirmed that oil pressure began to drop significantly minutes before the engine failed completely. The investigation established that the engine eventually shut down due to a connecting rod failure on the first cylinder, a direct result of metal-to-metal friction caused by a lack of lubrication.
Technical examination of the engine components revealed that the rotary oil seal had failed, allowing oil to vent out of the system. Investigators found that the damage to the seal was consistent with the use of an incorrect technique during recent maintenance. Specifically, the shaft had been inserted through the seal using a method that stretched and weakened the material, a flaw that was not immediately visible to the technician but led to failure under operational pressure.