Overview
The 747-400 has four independent hydraulic systems (1, 2, 3, 4) operating at 3,000 psi. Each engine drives one Engine-Driven Pump (EDP); systems 1 and 4 also have Air-Driven Pumps (ADPs) powered by bleed air, and systems 2 and 3 add Electric Demand Pumps. A Ram Air Turbine (RAT) deploys automatically on loss of all AC power to provide backup pressure to system 1.
Architecture
Components
| Hydraulic Systems | 4 independent (Sys 1, 2, 3, 4) |
| System Pressure | 3,000 psi nominal |
| Engine-Driven Pumps (EDPs) | 4 (one per engine) |
| Air-Driven Pumps (ADPs) | 2 (Sys 1 and Sys 4) — pneumatic |
| Electric Demand Pumps | 2 (Sys 2 and Sys 3) — AC |
| Auxiliary Pump | Sys 4 has an aux electric pump for parking brake |
| RAT | Ram Air Turbine — deploys on all-engine flameout or loss of AC; powers Sys 1 |
| Reservoirs | 4 (pressurized by bleed air) |
| Fluid | Skydrol — phosphate ester (BMS 3-11) |
System Usage
| System | Primary Users | |---|---| | Sys 1 | Lower rudder, inboard ailerons, inboard elevator, body gear steering, lower outboard elev, normal brakes, autopilot L | | Sys 2 | Outboard ailerons, outboard elevators, flaps (normal), autopilot R | | Sys 3 | Inboard ailerons, inboard elevator, lower rudder, nose gear steering, normal brakes, autopilot C | | Sys 4 | Upper rudder, outboard ailerons, outboard elevators, slats, alternate brakes |
Normal Operation
- All four EDPs ON during normal flight (each engine driving its pump)
- ADPs/Demand pumps in AUTO — supplement EDPs at low engine speed or on engine loss
- System pressure monitored on EICAS HYD page
Failures & Abnormals
Loss of one hydraulic system results in degraded redundancy on shared surfaces but full flight control retained. Plan for alternate landing distance per QRH if Sys 1 (normal brakes) lost.
RAT deploys automatically and provides pressure to Sys 1 only. Many surfaces lose redundancy. Land at nearest suitable airport per QRH.
Limits
- System pressure: 3,000 psi nominal, EICAS alert below ~2,400 psi
- Reservoir quantity: monitor on EICAS — low quantity triggers EICAS message
- Hydraulic fluid temp: max ≈ 215 °F (varies by system)