What happened
On October 24, 2007, at approximately 21:00 UTC, a Neiva EMB-720C, registration PT-RDF, was operating a scheduled air taxi flight from Envira, Amazonas, to Rio Branco, Acre. The aircraft was carrying one pilot and five passengers.
Roughly 50 minutes after takeoff, the pilot noticed the engine oil pressure dropping into the red zone on the pressure gauge. In response to the critical drop in pressure, the pilot shut down the engine and performed an emergency landing at Praia Treze Dias, located on the banks of the Purus River. All six occupants of the aircraft escaped the landing without injury, though the engine sustained minor damage.
The investigation
The investigation focused on the cause of the rapid oil pressure depletion. Upon examining the engine, investigators discovered that the oil drain plug was missing, which had led to the loss of engine oil during flight.
Technical analysis revealed that for this specific engine model, the drain plug is required to be secured to the crankcase using safety wire. However, investigators found no evidence that the plug had been properly safety-wired. It was concluded that engine vibrations likely caused the unsecured plug to loosen, eventually resulting in the oil leak.
Maintenance records indicated that the most recent significant maintenance was a 100-hour inspection completed in December 2006 at an ANAC-approved facility, during which the engine oil had been replaced.