The 747 is fitted with four independant hydraulic systems. Each system is powered by an “Engine Driven Pump” (EDP) and a bleed air driven or electrically driven “Demand Pump”. An additional AUX pump is installed in system #4 for ground operations (some airlines have an AUX pump also in system #1). These four hydraulic systems power the primary flight controls, the stabilizer trim, the spoilers, flaps, autopilot servos, elevator feel, nose wheel steering and body gear steering, the wheel brakes and the landing gear actuators (on PW engines also the thrust reversers). In order to provide a positive supply of hydraulic fluid to each pump, the fluid reservoirs are pressurized by regulated bleed air from the pneumatic system.

PS1 provides an authentic real-time simulation of all the individual pumps, the pressurization/depressurization time intervals, the controls, valves, fluid reservoirs, the system logic in general and the associated EICAS interface.

Also modeled in PS1 are the following hydraulic malfunctions:
- Controller failure (Demand Pump control fault, or indications inop.).
- HYDIM deactivation (Hydraulic Interface Module).
- Demand Pump failure.
- Engine Driven Pump failure.
- Low reservoir air pressure.
- Leak in hydraulic fluid reservoir.
- System overheat.

Using specific airline configurations in the simulator you can determine:
- whether systems 2 and 3 are bleed air driven or electrically driven.
- whether Demand Pumps 1 and 4 automatically operate when the flaps are out of up in flight.

Limitations in version 1.3: The simulator has not the optional AUX pump in system #1 (installed in some airlines). Also, some newer EICAS versions can now display messages on the EICAS Hyd Synoptic to inform as to which systems are disabled by which hydraulic channel. This option is not available in the simulator.

Sample screen shots: Hydraulics simulation

The lower EICAS is shown in the lower right-hand corner of this screen shot (the “hydraulics synoptic” is currently selected). A clipping of the overhead panel can be displayed at the same time -- the hydraulics panel in this example. Flaps 20 are selected. The Demand Pumps in hydraulic system 1 and 4 are automatically activated to support the trailing edge flap servos (a Demand Pump also automatically activates if the associated Engine Driven Pump has low pressure).

(This screen shot shows a BA 744). The number 2 engine is shutdown and the Demand Pump in system #2 is turned off. Why is hydraulic system #2 still pressurized? At 250 knots IAS the RPM of the “windmilling” engine is high enough to power the EDP.

(This screen shot shows a NW 744). The IAS (145 kts) has dropped into an area where windmilling power is insufficient to power the EDP. Hydraulic system #2 starts depressurizing (Demand Pump is off). Stabilizer trim speed is now only half of the normal drive speed. Also, a triple channel autoland is now unavailable and the weather minima for the landing have to be checked accordingly. (The red gear configuration warning is just activated because flaps 30 are set while the gear is not down -- the siren is silenced by the Override Switch).

(This screen shot shows a LH 744). The hydraulic systems have been left depressurized for a long period of time. The reservoir bleed air pressure has dropped below 22 psi. Accordingly, the RSVR PRESS messages are displayed next to the reservoir symbols on the lower EICAS screen. These messages will be removed as soon as the pneumatic system is powered.

Hydraulic fluid is leaking out in system #1. Both, the Demand Pump and the EDP in system #1 can no longer maintain the normal pressure of 3000 psi. Hydraulic system #3 has failed, too: The Demand Pump and/or the EDP in system #3 are overheating (case drain temperature over 105°C). To cool down the pumps the crew must manually turn off the pumps in system #3.

The consequences of the two inoperative hydraulic systems: Only one autopilot remains available. A triple or dual channel autoland can not be performed. During the following manual flare the throttles has to be manually retarded as well.
(Note: In the simulator the Master Warning/Caution Light is located in the upper left corner of the PFD in order to not obstruct the outside view and to maintain a compact layout on the PC monitor).

Normal operations: Ready for engine start. Demand Pumps 1, 2 and 3 are set to AUTO while for system #4 the “AUX” pump has been turned on (the normal wheel brakes are powered by system #4).

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© 1995-2001 Hardy Heinlin. All rights reserved.