IntroductionThe actuation system of an aircraft performs a safety critical function that ensures controllability. Although today’s trend is focused on developing electrical actuation systems, current aircraft and new developments still require hydraulic power to feed the actuators.
AHAS functional breakdownIn this series, the AHAS is defined as the set of actuators that drives the control surfaces and the associated hydraulic systems. An extensive definition should cover the flight control computer (or similar components that perform the same function) in which the actuator control loops are implemented. For simplification purpose, the actuator control loop will be reduced to block diagrams that will still ensure such a functionality. Thus AHAS covers the following main functions:
- Store hydraulic fluid – typically ensured by reservoirs – pressurized or non-pressurized.
- Generate hydraulic power – typically ensured by mechanical pumps, electric pumps, hand pumps, ram air turbine (RAT) and power transfer units (PTU).
- Store hydraulic power – mostly covered by accumulators (also by capacitance effect of the lines).
- Distribute hydraulic power – mostly ensured by rigid lines and hoses as well as dedicated valves to guide the fluid.
- Convert hydraulic power – typically performed by linear or rotary actuators. It can be split into two sub-functions:
- Meter hydraulic power – typically ensured by proportional control valves.
- Transform hydraulic power (in to mechanical power) – usually performed by hydraulic cylinders (linear or rotary).