What happened
On September 29, 2013, an Embraer EMB-711ST (registration PT-VHR) departed from Coronel Alexandre Raposo Aerodrome (SNOZ) bound for Carutapera (SNCP) in Maranhão, Brazil. The aircraft was carrying a pilot and three passengers.
Approximately 35 minutes into the flight, the pilot observed smoke emanating from the right side of the engine. Shortly before attempting an emergency landing in an open field near Serrano do Maranhão, the engine ceased functioning. The pilot successfully executed the landing with the landing gear retracted and flaps extended. All four occupants escaped the aircraft without injury, though the aircraft sustained minor damage.
The investigation
CENIPA investigators examined the engine's lubrication system during disassembly. They discovered significant sludge and metal shavings at the bottom of the oil sump. Upon inspecting the oil pressure regulating valve, investigators found a green rubber particle lodged in the valve seat, which held the valve open and caused a drop in oil pressure.
Further analysis revealed that the green rubber originated from the intake valve guide seals. The investigation established that the seals on cylinders #1, #3, #5, and #6 had been improperly installed. Specifically, the maintenance process failed to adhere to the manufacturer's Critical Service Bulletin (CSB95-4), which provides specific instructions for inspecting and installing intake valve guide seals (P/N 646678) based on the height of the valve guides.
Findings
- The installation of seals P/N 646678 on the valve guides of cylinders #1, #3, #5, and #6 was performed in violation of the manufacturer's Critical Service Bulletin CSB95-4.
- Because the valve guides were at an incorrect height (0.875 inches instead of the required 0.695 to 0.715 inches), the rocker arms caused damage to the improperly installed seals.
- The damaged seals released synthetic green rubber particles into the lubrication system, which eventually blocked the oil pressure regulating valve.
- The damaged seals also allowed oil to leak into the combustion chambers, resulting in the visible smoke and the eventual engine failure.
- Improper aircraft maintenance was the contributing factor to the occurrence.