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Sep 24, 2025 - SC5091-1 Main Servo / Main Servo Control

The SC5091-1 is a main servo control unit used in helicopters, specifically designed to handle the heavy demands of main rotor operation. In simple terms, it’s part of the “muscle system” that translates a pilot’s cyclic and collective inputs into actual movement of the rotor blades. Without these servos, the pilot’s controls wouldn’t have the hydraulic force needed to adjust blade pitch and keep the aircraft flying the way it’s commanded.

This servo is most commonly associated with the Airbus/Eurocopter EC120 series. In these helicopters, the SC5091-1 sits in the main rotor control system, linking hydraulic power to the mechanical control linkages. It’s a precision component that has to manage high loads and constant motion, and for that reason, it comes with strict maintenance requirements. When servicing, the hydraulic system must be depressurized before disassembly. Many of the internal parts—O-rings, seals, locknuts, washers—are considered non-reusable and must be replaced every time the unit is opened. Later production units (serial numbers above 12) incorporate slight design changes that affect reassembly steps, so technicians have to be aware of these differences. Cleaning must be done carefully, without immersion, and only with approved solvents. During reassembly, torque specs, orientation of check valves, and safety wiring procedures all need to be followed exactly.

Because this is such a critical flight component, it’s not simply a part you “swap and forget.” Overhaul and exchange are the most common ways operators keep these units in service, with specialized shops providing certified rebuilds. The goal is to make sure the servo continues to deliver reliable performance under the constant hydraulic pressure and vibration loads of rotorcraft flight. Right now, the SC5091-1 is primarily tied to the EC120 platform. While other servo part numbers exist for different Airbus and Eurocopter helicopters, the SC5091-1 designation consistently shows up for EC120 main rotor applications.

In short, the SC5091-1 main servo is a highly specialized hydraulic control unit. It takes the pilot’s inputs and turns them into precise, forceful adjustments at the rotor head. It’s one of those hidden but absolutely vital components that keeps a helicopter responsive, safe, and airworthy.