What happened
On April 7, 2015, a Robinson R44 Raven II, registration CS-HGF, was conducting an instrument approach training flight from Cascais Airfield (LPCS) to the Montijo military air base (LPMT). The flight was part of a structured training program involving an instructor pilot and a student pilot.
During the final phase of the third approach to LPMT, at an altitude of approximately 300 feet and near minimums, the crew experienced abnormal vibrations and noises. This was accompanied by several warning indications on the instrument panel, including the "FUEL FILTER", "AUX FUEL PUMP", and an intermittent "CLUTCH" light. In response to the abnormal aircraft behavior, the instructor pilot took control, declared an emergency, and executed a landing at the airfield. The crew members were uninjured, though the aircraft sustained minor damage to the landing skids and suffered a catromic engine crankshaft failure.
The investigation
The GPIAAF conducted a safety investigation to examine the mechanical failure. The investigation included a detailed assessment of the Lycoming IO-540 engine at a manufacturer-authorized workshop. Investigators analyzed the fractured components, specifically focusing on the crankshaft (PN: 13E47622) and the bearing geometry to identify the failure mode.
Findings
Technical analysis of the engine revealed that the Lycoming crankshaft had suffered a full fracture. The failure originated at the short cheek near bearing 4. Detailed material analysis identified the presence of "beach marks" on the fracture surface, which are characteristic of fatigue propagation caused by repeated cyclic mechanical loads.
Investigators found that the bearing 4 cap contained cracks due to a geometric notch. This flaw facilitated the fatigue process, which eventually led to the sudden, catastrophic fracture of the crankshaft. While the engine was operating under a legal Time Between Overhaul (TBO) extension, the investigation concluded that this extension was ineffective in detecting the developing fatigue. The investigation also noted that the operator's maintenance program could have been more robust in monitoring engine health.
Safety action
As a result of the findings, the GPIAAF issued safety recommendation No. PT.SIA 2021/03 to the operator, Helibravo. The recommendation directs the operator to work with its airworthiness management organization to review the eligibility criteria for engine TBO extensions in commercial operations. Specifically, the operator should implement more rigorous technical validation methods, such as periodic spectrometric oil analysis and the use of engine parameter monitoring technology (e.g., tracking vibrations, MAP, or EGT) to ensure the timely detection of component degradation.