Hydraulics Flashcards
The following systems receive hydraulic power:
flight controls,
leading edge slats,
trailing edge flaps,
landing gear, wheel brakes,
nose and main gear steering
and thrust reversers.
Hydraulic power is distributed to the flight control system to allow sufficient aircraft control with
only one functioning hydraulic system.
Primary pumps pressurize each system during most operations.
Demand pumps are employed
during high load phases of flight and to provide system redundancy.
Reservoirs requiring refilling display
Reservoir low quantity is displayed as
Reservoir over-full shown as
a white RF.
LO.
OF.
OF and RF are inhibited in flight.
The shutoff switches are for ground maintenance use only and are not required for any flight crew
normal or non-normal procedures.
The demand pumps augment hydraulic power during high load
system demands and also
provide redundancy should the primary pump fail.
The center hydraulic system power is used to
retract and extend the landing gear and provide nose and main gear steering.
The alternate brakes, the reserve brakes,
the leading edge slats, and the trailing edge flaps
all receive hydraulic power exclusively from the center system.
The center hydraulic system has two electrically
driven primary pumps.
Two air-driven demand pumps
(powered by bleed air) augment the center system during
high load system demands and when pressure is low.
Back-up hydraulic power to the center system flight controls is
supplied by the Ram Air Turbine.
The Ram Air Turbine is located
on the underside of the aft fuselage.
To further increase redundancy in the hydraulic system, the Ram Air Turbine, (RAT) can
be deployed automatically or manually.
The RAT extends into the airflow providing hydraulic power to the center system’s primary flight controls only.
The RAT is capable of providing hydraulic and electrical power throughout the flight envelope. In flight it will deploy automatically if:
both engines have failed and C-HYD system pressure is low,
or both AC transfer busses are unpowered,
or all three HYD system pressures are low.
With the primary pump switches ON, each engine driven pump pressurizes
its system when the respective engine is running.
The FAULT light illuminates to show
low pump pressure,
excessive fluid temperature,
or the pump switch is off.
With AUTO selected, the demand pump operates when
system or primary pump pressure is low or when required by control logic.
The demand pump selector’s AUTO position
controls the electrically driven demand pumps.
If the AUTO function fails, or if required by a non-normal procedure —
the demand pump should be selected ON.
The pump will run continuously.
The demand pump also operates when control logic anticipates
With AUTO selected, both the left and right demand pumps operate during
a large system demand.
takeoff and landing phases.
In the AUTO position, the right demand pump operates
continuously when the airplane is on the ground.
In the AUTO position, system logic
commands the center demand pumps to operate
As the landing gear is selected down, system logic commands
in anticipation of gear retraction.
a demand pump to operate.
Once the center system primary flight control hydraulic pressure
is greater than
1,500 psi, the Ram Air Turbine pressure light illuminates green.
Selecting both center system demand pumps ON results in
only the C1 pump operating.
Both center demand pumps
cannot operate together in the ON position
due to system logic.
When excessive fluid temperature is sensed in either
a primary or a demand pump, a
FAULT light appears in the associated pump switch
or below the demand selector.
When low left primary pump pressure is sensed, a
caution level EICAS message is generated.
The hydraulic system has a great deal of pump redundancy.
Therefore, the most likely cause for low system pressure
is loss of fluid quantity.
he center hydraulic system is unique in its ability to
When a low fluid quantity is sensed,
isolate certain components when low fluid quantity is sensed.
both isolations valves close.
The C1 primary pump and the ALTN / RSV BRAKES
are now isolated from the rest of the system.
Let’s look at what happens when the leak is in the main part of the system.
In this case, quantity is indicated as 0.00,
the actual fluid level has stopped at
standpipe level.
System pressure remains low.
Note that the C1 PRIM pump receives fluid below the standpipe level.
If center system pressure drops to a low value,
the left isolation valve will re-open.
Note: that in this case, the C1 PRIM pump can provide pressure to the ALTN / RSV BRAKES and under certain conditions to NOSE GEAR STEERING (after landing with the airspeed below 60 kt).
re-open.
Note: that in this case, the C1 PRIM pump can
provide pressure to the ALTN / RSV BRAKES
and under certain conditions to NOSE GEAR STEERING
(after landing with the airspeed below 60 kt).
The left isolation valve also opens
after landing when airspeed decreases below 60 kt.
The center ELEC C1 pump
can now provide pressure to NOSE GEAR STEERING.
