What happened
On March 20, 1996, an Embraer T-27 Tucano, registration PT-ZTW, was performing a local meteorological observation flight departing from São José dos Campos (SBSJ). While returning to the airfield, cruising at flight level 200 and approximately 25 NM from the SJC VOR, the engine power suddenly reduced to idle, and the power lever became inoperable.
At the time of the failure, the aircraft was operating in Instrument Meteorological Conditions (IMC). As the pilot descended and eventually reached visual conditions, it became clear that the aircraft lacked the altitude necessary to reach runway 1s at SBSJ. To avoid potentially crashing into a populated area, the pilot opted for a forced landing in a flat, marshy region known as "Banhado." During the final approach, the aircraft struck the canopy of a tree, causing damage to the right wing, propeller, and right horizontal stabilizer, before coming to a stop on the ground. The pilot emerged from the wreckage uninjured.
The investigation
CENIPA investigators focused on the mechanical failure of the engine components. Microscopic analysis of the P3 tube revealed fatigue fractures and evidence of corrosion. The investigation established that the brazing process used to join the tube parts utilized a silver alloy, which was subsequently cleaned with a chemical compound containing chlorine. This chlorine penetrated the pores of the brazed junction, initiating a corrosion process.
Further examination via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of corrosion products and chlorine at the crack initiation site. The investigation determined that the combination of corrosion pitting and engine vibrations led to fatigue crack propagation, eventually causing the tube to fracture. This fracture resulted in a loss of reference pressure for the fuel control unit, which minimized fuel flow and prevented the engine from maintaining normal propeller RPM.
Findings
- Design Deficiency: The design of the P3 tube, which requires brazing and subsequent cleaning with chlorine-based compounds, created a susceptibility to corrosion.
- Manufacturing Deficiency: The specific materials used in the brazing alloy and the cleaning process contributed to the initial stages of the fracture.
- Adverse Weather: The pilot's descent through IMC for a significant portion of the flight contributed to the inability to reach the runway after the engine failure.
- Engine Failure: The fracture of the P3 tube caused a critical loss of fuel pressure regulation, leading to the power loss and inoperable throttle.
- Operational Note: The aircraft had departed without being refueled, though sufficient fuel remained for the flight; however, the pilot failed to perform standard checks, only noticing the low-level light during the return leg.