Standby Hydraulic System

The Standby Hydraulic System is provided as a backup if System A and / or B pressure is lost. The standby system can be activated manually or automatically and uses a single electric motor-driven pump to power:

  • Thrust Reversers
  • Rudder
  • Leading Edge Flaps and Slats (extend only)
  • Standby Yaw Damper.

Manual Operation

Positioning either FLT CONTROL Switch to STBY RUD:

  • Activates the Standby Electric Motor-Driven Pump
  • Shuts off the related hydraulic system pressure to ailerons, elevators and rudder by closing the Flight Control Shutoff Valve
  • Opens the Standby Rudder Shutoff Valve
  • Deactivates the related Flight Control LOW PRESSURE Light when the Standby Rudder Shutoff Valve opens
  • Allows the standby system to power the rudder and thrust reversers.
  • (airplanes with SEL 11) llluminates the STBY RUD ON, Master Caution, and Flight Controls (FLT CONT) Lights.

Positioning the ALTERNATE FLAPS Master Switch to ARM, (refer to Flight Controls for a more complete explanation):

  • Activates the Standby Electric Motor-Driven Pump
  • Closes the Trailing Edge Flap Bypass Valve
  • Arms the ALTERNATE FLAPS Position Switch
  • Allows the Standby System to power the leading edge flaps and slats and thrust reversers.

Automatic Operation

Automatic operation is initiated when the following conditions exist:

  • Loss of System A or B, and
  • Flaps extended, and
  • Airborne, or wheel speed greater than 60 kts, and
  • FLT CONTROL Switch A or B Hydraulic System ON.
  • (airplanes with SEL 11) The main PCU Force Fight Monitor (FFM) trips. Automatic operation:
  • Activates the Standby Electric Motor-driven Pump
  • Opens the Standby Rudder Shutoff Valve
  • Allows the Standby System to power the rudder and thrust reversers.
  • (airplanes with SEL 11) Illuminates the STBY RUD ON, Master Caution, and Flight Controls (FLT CONT) Lights.

Standby Hydraulic System Leak

If a leak occurs in the Standby System, the Standby Reservoir quantity decreases to zero. The LOW QUANTITY Light illuminates when the Standby Reservoir is approximately half empty. System B continues to operate normally, however, the System B Reservoir fluid level indication decreases and stabilizes at approximately 72% full.

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