What happened
On February 23, 2011, a Sikorsky SK 61N helicopter, registration PR-MEX, was performing an external cargo transport flight from Urucu Aerodrome to the Jaraqui 1 clearing in Coari, Amazonas. During the final approach for landing, the crew heard a noise similar to a compressor stall. This was immediately followed by a drop in engine parameters and the subsequent shutdown of the number 2 engine.
In response to the emergency, the pilot released the external load and successfully performed a single-engine landing. The aircraft sustained no damage, and both crew members were uninjured.
The investigation
The investigation examined the mechanical state of the engine and the maintenance history of the components. Technical inspections of the engine were conducted at the Heli-One workshop in Canada, with further testing of the Fuel Control Unit (FCU) performed by Columbia Helicopters in the United States.
Investigators identified that the Fuel Control Unit had been contaminated with metal shavings. These shavings originated from the Centrifugal Purifier Filter (FCP), which was found to be unbalanced, causing internal contact and wear. While independent testing suggested the volume of metal shavings alone was insufficient to cause an engine shutdown, the investigation focused on the interaction between multiple mechanical discrepancies.
Findings
- The primary cause of the engine shutdown was the combination of an improperly adjusted Stator Vane Actuator (SVA) and the unbalanced Centrifugal Purifier Filter (FCP).
- The improper SVA adjustment altered the compressor airflow, causing the engine to operate at higher-than-normal temperatures.
- The unbalanced FCP generated metal shavings that contaminated the Fuel Control Unit.
- The simultaneous presence of increased operating temperatures and fuel delivery issues created the conditions for the compressor stall and engine failure.
- The crew was fully qualified, and the aircraft was within weight and balance limits with valid airworthiness certification.
Safety action
Following the discovery of the issue, the operator's engineering and maintenance department implemented a fleet-wide inspection of all Centrifugal Purifier Filters on their S61 engines. This inspection revealed similar imbalances in two other engines. As a result, the operator increased the inspection frequency for these filters from every 600 hours to every 100 hours or 90 days, whichever occurs first.