Hydraulic Systems in Aviation

Hydraulic System

• Hydraulic reservoirs are pressurized to ensure positive fluid flow to all hydraulic pumps.
• Foaming in flight will result in low pressure lights blinking and a master caution/hyd warning.
• Ailerons, rudder, and elevators (half of total) are components common to both hydraulic systems A and B.
• With hydraulic system A failure, landing gear retraction and ground spoilers are unrecoverable (no redundancies).
• With hydraulic system B failure, leading edge flaps and slats retraction and autobrakes are unrecoverable (no redundancies).
• Rudder, thrust reversers, and leading edge flaps (extend only) are powered by the standby hydraulic system.
• Engine-driven pumps (EDP) provide approximately 6 times the fluid volume of electric motor-driven pumps (EMDP).
• The hydraulic quantity RF (refill) will appear when the quantity is below 76%, specifically, on the ground, with both engines shut down and after landing with flaps up during taxi.
• During preliminary preflight, a low hydraulic quantity is indicated by the RF (refill) next to A or B quantity displays.
• Appropriate action for seeing RF during preflight is to make a M300 writeup and contact maintenance control via dispatch.
• Normal hydraulic pressure is 3000 psi and maximum is 3500 psi.
• Hydraulic system failure can be caused by an electric motor-driven pump overheating or hydraulic fluid used for cooling and lubrication that overheats.
• The minimum fuel required in each main wing tank for ground operation is 1675 lbs.
• Pushing back without a nose gear steering lockout pin requires both system A hydraulic pumps to be set to OFF.

Hydraulic System Operation

• Power Transfer Unit (PTU) provides additional hydraulic fluid for autoslats and leading edge flaps when system B's engine-driven pump is inoperative.
• PTU does not transfer hydraulic fluid, instead uses system A pressure to power a hydraulic motor-driven pump which pressurizes system B hydraulic fluid.
• The LGUT operates when airborne, No. 1 engine RPM goes below a limit, and when the landing gear lever is positioned up or has not been locked.
• A standby System B hydraulic system is primarily used as a backup if system A or B pressure is lost.
• The standby hydraulic system is activated either manually or automatically when system A and/or B pressure is lost.
• Manual activation is achieved by moving the FLT CONTROL A or B switch to the STBY RUD position or the ALTERNATE FLAPS Master switch to the ARM position.
• Standby system automatically activates when airborne with wheel speed over 60 kts; flaps are extended, and hydraulic system A or B is lost.
• A Master Caution, FLT CONT announcement, and STBY RUD ON lights illuminating at FL350 indicate automatic activation of the standby system.
• The STANDBY HYD LOW QUANTITY light illuminates when the standby reservoir is approximately half empty.
• STANDBY HYD LOW PRESSURE light (amber) indicates low output pressure from the standby pump and it is armed when standby operation is selected or automatic function is activated.
• In the event of FLT CONTROL A switching to STBY RUD position, the B system controls elevators, B system for ailerons, and standby system for rudder.
• If hydraulic system A or B fails, a reverse thrust is provided by the standby hydraulic system.