What happened
On June 24, 2018, at approximately 18:06 UTC, an AS350 BA helicopter, registration CN-HBA, was conducting a VFR flight from Rabat-Salé to Marrakech. While cruising at 333 feet, the pilot observed the amber "FUEL FILTR" warning light illuminate. The pilot initially reduced engine power, which temporarily extinguished the light, but upon increasing power back to 73.2% torque, the warning reappeared.
Believing an engine failure was imminent, the pilot initiated an emergency autorotation, turning right to find a suitable landing site. During the descent, the main rotor speed (NR) increased to 425 RPM, triggering the high-speed warning horn. The helicopter struck a wheat field near Berrechid, sliding approximately 30 meters. The impact caused a failure of the attachment between the right skid and the forward cross-member. There were no fatalities or injuries to the pilot.
The investigation
The investigation examined the engine performance, maintenance records, and operational documentation. Investigators confirmed that the engine remained functional throughout the event; the fuel filter was partially clogged with impurities, but the bypass system was working correctly, ensuring continuous fuel flow. The engine did not flame out.
Crucially, the board identified a significant discrepancy between the aircraft's official Flight Manual (AFM) and the operator's Quick Reference Handbook (QRH). While the AFM advised landing as soon as possible, the company's QRH used red text to suggest a possibility of engine shutdown, which influenced the pilot's decision-making. Additionally, the investigation noted that the fuel filter had only been in service for 123 hours before clogging, likely due to environmental dust or contaminated fuel.
Findings
- The primary cause was the use of an operational procedure that did not comply with the manufacturer's flight manual.
- Impurities were present within the fuel filter circuit.
- The operator's QRH contained instructions that were inconsistent with the AFM, creating a false perception of imminent engine failure.
- The pilot's training and proficiency checks (LPC/OPC) provided insufficient time to adequately practice complex emergency procedures.
- The company's operations manual (MANEX) contained language that could lead to dangerous interpretations regarding safety-related rule transgressions.