What happened
On September 30, 2020, a Beechcraft A36, registration PT-LST, was conducting a private flight from Birigui, São Paulo, to Cáceres, Mato Grosso. While cruising at flight level 085, the engine began to exhibit vibrations, followed by fluctuations in power and RPM. Shortly thereafter, the oil pressure dropped to zero, and the engine lost all traction.
As the aircraft descended through flight level 065, a significant amount of oil leaked onto the windshield, and smoke entered the cabin, severely restricting visibility. The pilot performed an emergency forced landing in an open field at Fazenda Vale do Rio Verde, near Camapuã, Mato Grosso do Sul. The aircraft struck the ground heavily, resulting in substantial damage to the airframe. The pilot sustained serious injuries.
The investigation
CENIPA investigators examined the Continental IO550-B engine and the aircraft's maintenance history. While fuel samples from the left wing appeared normal, the right wing sample showed unusual coloration and distillation properties; however, investigators determined this was likely due to a recent fuel tank replacement and not a cause of the failure.
Upon disassembly of the engine, investigators found significant metal debris (swarf) in the primary oil filter and the engine crankcase. The investigation revealed that the piston pin plugs in cylinder number 4 had worn down due to abnormal friction against the cylinder liner. This wear generated metal particles that accumulated on the piston skirt, increasing friction and heat, eventually leading to the piston seizing. Additionally, a misaligned oil passage in the intermediate bearing caused reduced oil flow to the cylinder number 4 connecting rod bearing, contributing to the thermal failure.
Crucially, the investigation found that the engine's Time Between Overhaul (TBO) had been repeatedly extended. While the manufacturer recommended an overhaul every 12 years or 1,700 hours, the engine had been operating for 18 years without a major overhaul, with extensions granted annually since 2013.
Findings
- Engine component failure: Abnormal wear of the piston pin plugs in cylinder number 4 caused metal accumulation and increased friction, leading to the piston seizing.
- Reduced lubrication: Misalignment in the intermediate bearing oil passage likely reduced oil flow to the cylinder number 4 connecting rod bearing, exacerbating high temperatures.
- Maintenance practices: The engine's TBO had been extended by over 58% beyond the manufacturer's recommended limit, which hindered the detection of gradual component deterioration.
- Incomplete records: Aircraft maintenance logs for the airframe, engine, and propeller were not up to date and contained erasures.