X-Plane has three hydraulic systems per aircraft that can power a variety of actuators. Most notably, each flight control surface can be powered by any combination of hydraulic actuators.

Hydraulic power sources

Each of the three hydraulic systems can be pressurized from

  • an engine-driven pump
  • a rotor-driven pump (so helicopters with free-wheeling transmissions can retain hydraulic power in auto-rotation)
  • an electrically operated pump
  • a ram-air driven turbine (RAT)
  • a bleed-air driven pump

By default, engine- and rotor-driven pumps operate continuously, but they can be unloaded with the datarefs
sim/cockpit2/hydraulics/actuators/engine_pump int[8] y boolean and
sim/cockpit2/hydraulics/actuators/prop_pump int[8] y boolean respectively.
Note that these datarefs do nothing if the pump hasn’t been assigned to provide pressure to any system in Plane Maker.

The electric pumps are switched with the datarefs
sim/cockpit2/hydraulics/actuators/electric_hydraulic_pump_on int y boolean
sim/cockpit2/hydraulics/actuators/electric_hydraulic_pump2_on int y boolean
Again, switching them on does nothing unless the pump has been assigned a hydraulic system in Plane Maker.
If assigned in Plane Maker, the pumps will consume the assigned amps from their electrical busses, and can be failed with their failure datarefs
sim/operation/failures/rel_hydpmp_ele and /rel_hydpmp_el2

The RAT (ram-air turbine) automatically activates if it has been assigned in Plane Maker, and the airplane is off the ground (weight-off-wheels) and all other hydraulic sources have failed and some time has passed for pressure to bleed from the system. Alternatively, the RAT can also be controlled manually with the dataref
sim/cockpit2/hydraulics/actuators/ram_air_turbine_on int y boolean

Hydraulic PTU

The power transfer unit can be installed with Plane Maker to allow power transfer between system 1 (left) and system 2 (right). The PTU can either be uni-directional, allowing power transfer only in the specified direction, or bi-directional, in which case it will keep the weaker system powered by the stronger system, as long as the weaker system has enough hydraulic fluid left. Note that the PTU only transfers power, but no fluid is transferred between systems.

When installed in the plane, the PTU has three possible states during flight:
sim/cockpit2/hydraulics/actuators/PTU int y enum

  1. PTU is off, and no power transfer takes place.
  2. PTU is armed and watching for pressure differential
  3. PTU is running currently, trying to equalize pressure. When pressure differential is minimized, it will fall back to state 1

In auto mode, which is the default, the PTU is watching for a pressure differential and will turn on and off as needed to keep it minimal. This causes the characteristic on-off-cycle that on some airplanes can be heard as a “barking dog” sound. This is a purely mechanical operation, no electric power or sensors are needed for this to work.

Hydraulic actuators (power consumers)

Each of these can be assigned on or more hydraulic actuators, so they can be powered from one or multiple systems:

  • nosewheel steering
  • flaps
  • slats
  • landing gear (see the article on hydraulic landing gear actuators for more detail)
  • wheel brakes (through an accumulator that will store pressure for a few brake applications)
  • thrust reversers

Each of these will not be able to operate without hydraulic pressure from at least one selected source, or operate at reduced deflection, speed, or efficacy at low pressures.

Notice the primary flight controls are missing from this list – they have moved to the the flight control configuration in Plane Maker, Control Geometry, Hydraulic tab.

Hydraulic flight control actuators

The following primary and secondary flight controls can be assigned to hydraulic systems on a per-wing basis:

  • aileron group 1
  • aileron group 2
  • elevator 1
  • elevator 2
  • rudder 1
  • rudder 2
  • spoiler group 1
  • spoiler group 2
  • speed brake group 1
  • speed brake group 2
  • yawbrake

Each of these actuators can be assigned any combination of hydraulic systems. Each of these assignments can be made separately for any of these wings:

  • wing 1-4 left and right
  • left and right horizontal stabilizer
  • vertical stabilizers 1 and 2

Note that Plane Maker will only display the options for wings the plane actually has, and only when an element of the wing is selected to have that type of control. This keeps the table simple for most airplanes.

That makes it possible to create complex assignments of individual spoiler tabs that will stop operating when pressure is lost from the specified system.