On most light planes, the hydraulic system is limited to powering the brakes and, in some cases, retractible landing gear. Once you get to airliner weights, though, you find that the forces involved often outstrip the ability of the human muscle to overpower. In these cases, engineers rely on hydraulic systems to do the grunt work. This is the case with many systems on the JungleBus, including all basic flight controls, spoilers, landing gear, brakes, nosewheel steering, and thrust reverser deployment. Notably missing are the flaps, which are hydraulically powered on many airliners but are electric on the JungleBus.
Hydraulic systems use a fluid under pressure to do work. This is possible because fluids are mostly incompressible. For a hydraulic system to work, you need a source of pressure (a pump), a closed system that remains leak-free under significant pressure, and enough fluid to pressurize the system to design pressure. Hydraulic systems have a number of failure nodes: pump failure, loss of power to the pump, loss of fluid, and physical damage to the plumbing so it's unable to hold pressure. For these reasons, aircraft engineers build multiple independent hydraulic systems into the airliners they design, usually with multiple sources of power for each system.
The JungleBus has three fully independent hydraulic systems. Systems 1 and 2 are normally pressurized by engine-driven pumps (EDPs), which are connected to their respective engine's accessory gearbox. They each have an electric hydraulic pump installed as backup in case of engine failure. System 3 is powered solely by an electric pump, with a second electric pump as backup. All three systems use Skydrol brand hydraulic fluid and are normally pressurized to 3000 psi. Most aircraft systems that use hydraulic power are powered by at least two of these systems, so the loss of one hydraulic system will not affect most other aircraft systems. Flight critical systems are powered by three systems, so in the highly unlikely event of two fully independent systems failing simultaneously, the crew will still have at least pitch and roll control.
Hydraulic System 1 is normally pressurized by a mechanical pump driven by the left engine's accessory gearbox. The pump takes fluid from the reservoir, which an accumulator keeps at slight positive pressure, and pressurizes the fluid before sending it through a filter and the plumbing to the various System 1 users. The return line routes the fluid through another filter and then either through a heat exchanger or directly to the reservoir, depending on fluid temperature. If the fluid reaches 100º C, a HYD 1 HI TEMP caution message is displayed on the EICAS. At 125º C, the Hydraulic Shutoff Valve (HSOV) automatically closes to isolate EDP 1 from System 1. The HSOV will also close automatically in case of engine failure to decrease drag on the engine and make a windmilling relight easier. The HSOV can also be closed manually via a guarded push button on the hydraulic panel.
In case of engine failure or EDP failure, System 1 can be pressured by Electric Hydraulic Pump 1, powered by alternating current from AC BUS 2. It is controlled by a 3-position switch on the hydraulic panel. With the switch OFF, the pump stays off; selecting ON causes the pump to run continuously regardless of conditions. The normal position is AUTO, which causes the pump to activate automatically in case of engine 1 failure or EDP failure. In AUTO mode, the electric hydraulic pump will also run concurrently with the EDP when the flaps are in any position greater than zero in flight, or on the ground when flaps are greater than zero and thrust levers are in takeoff/goaround position (TOGA) or groundspeed exceeds 50 kts. The idea is to have both engine-driven and electric pumps running during takeoff and landing.
Hydraulic System 1 has the following users:
- Elevator (Left-Hand outboard actuator only)
- Rudder (upper actuator)
- Spoilers (LH & RH, panels 2, 3, and 4)
- Thrust Reverser (Engine 1)
- Brakes (outboard only)
- Emergency/Parking Brake
- Elevators (LH & RH inboard actuators)
- Ailerons (LH & RH inboard actuators)
- Spoilers (LH & RH, panels 1 and 5)
- Thrust Reverser (Engine 2)
- Brakes (inboard only)
- Nosewheel Steering
- Landing Gear
- Emergency/Parking Brake
System 1 and 2 don't have any common points where fluid can migrate, but there is a mechanical connection via the Power Transfer Unit, or PTU. This is basically an extra pump used to pressurize part of System 2; it is motored by hydraulic pressure in System 1. The only purpose of the PTU is to facilitate extension and retraction of the landing gear. To operate, it needs System 1 to be pressurized, and there must be fluid in System 2. It is controlled by a 3-position "OFF-AUTO-ON" knob, which is almost always left in AUTO. In this position, system logic will turn on the PTU if the right engine or EDP 2 fails when the flaps are greater than zero. It's essentially there to quickly raise the landing gear if the right engine fails just after takeoff and Electric Hyd Pump 2 fails or isn't supplying enough pressure to the landing gear. Note that the PTU is useless to raise or extend the landing gear in case of System 2 fluid loss; in this case the crew must extend the gear using a freefall procedure. There is no way to retract the gear with no fluid in System 2.
Hydraulic System 3 is essentially an emergency backup system to ensure critical flight controls remain powered in case of a catastrophic simultaneous failure of Systems 1 and 2. It is pressurized by one electric pump (Electric Hyd Pump 3A) with an additional electric pump (Electric Hyd Pump 3B) for backup. Pump 3A is controlled by an OFF-ON knob on the hydraulic panel, with no automation involved. It gets its power from the AC ESS bus, which remains powered in an electrical emergency. Pump 3B has a 3-position OFF-AUTO-ON knob; in AUTO position it will activate whenever Pump 3A fails. Pump 3B is powered by AC BUS 2. System 3 powers the following hydraulic users:
- Elevator (RH outboard actuators)
- Rudder (lower actuator)
- Ailerons (LH & RH outboard actuators)
The Multi-Function Display (MFD) in the flight deck can bring up a Hydraulic Synoptic Page. It displays hydraulic fluid quantity, temperature, and pressure for all three systems and shows the status of all engine-driven and electric hydraulic pumps, plus the PTU. Finally, the Synoptic Page displays a handy list of all hydraulic users, organized by system. This, in case of hydraulic system failure, the pilots can see at a glance which aircraft systems will be affected.
You can see that the JungleBus could suffer multiple hydraulic failures and the most critical systems will be unaffected. Either engine failure should not affect any of the three systems. Complete System 1 failure would leave the pilots with three of four elevator actuators, all four aileron actuators, one of two rudder actuators, two of six roll spoilers, two of four ground spoilers, one of the two thrust reversers, and two of four brakes. Complete System 2 failure would still leave powered two of four elevator actuators, two of four aileron actuators, both rudder actuators, four of six roll spoilers, two of four ground spoilers, one thrust reverser, and two of four brakes. A combined System 2 + System 3 failure would leave the ailerons unpowered but you'd still have one rudder actuator and four of six roll spoilers for roll control. The only catastrophic combination, of course, is an uncontained engine failure or similarly violent event that results in failure of all three systems a la United 232. Of course, certification standards have improved considerably since then to ensure the physical separation and protection of hydraulic system components. That's good because, despite its smaller size, the JungleBus is just as dependent on hydraulics as the DC-10.