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provide hydraulic power. If the actuator(s) fails the test, the switching valve isolates both circuits to
prevent additional loss of the backup circuit. At the expiration of the 10 second timer, the FCCs no
longer inhibit actuator failure detection logic. With both circuits isolated, this logic Xs LEF actuators
immediately and rudder or aileron actuators only when the actuator is subsequently commanded to
move. It is for this reason that lateral stick and rudder pedals must be cycled during the post-flight
switching valve check.
Switching valve operation is completely hydro-mechanical, separate from electrical inputs or FCS
reset commands. As mechanized, there is no hazard associated with multiple reset attempts to regain
an Xd surface following a hydraulic circuit failure.
Preference is given to the primary circuit at all times. The switching valve transfers to the primary
circuit any time primary circuit pressure recovers above 2,000 psi, regardless of the valve position or the
backup circuit pressure level.
NOTE
If the leak detection test failed due to cavitation in the actuator, the
switching valve resets, runs another test, and completes the switch to
the backup circuit. This self-resetting feature may require several
minutes to complete depending on surface loading, hydraulic system
pressure, and system temperature. In this case, an FCS reset restores
an X'd surface if and when the switching valve successfully transfers to
the backup circuit.
2.6.1.4 Hydraulic Isolation Valves. HYD 2 utility functions that are required only during takeoff,
landing, and ground operations are downstream of isolation valves. The forward isolation valve is
closed when the LDG GEAR handle is UP and all three landing gear are up and locked; isolating the
nose gear, NWS, launch bar, wheel brakes, and anti-skid. The aft isolation valve and the arming valve
are open with WonW and are normally closed inflight; isolating arresting hook retraction, parking
brakes, and emergency brakes. The aft isolation valve is manually opened inflight by hook retraction
or by holding the HYD ISOL switch in ORIDE. The arming valve is manually opened inflight by
emergency gear or emergency probe extension.
2.6.2 Hydraulic Accumulators. Two hydraulic accumulators are provided in the HYD 2B circuit; the
auxiliary power unit (APU) accumulator and the brake accumulator.
The APU accumulator provides hydraulic pressure to start the APU. With a HYD 2 failure, pressure
from the APU accumulator can be used to:
a. Emergency extend the landing gear or refueling probe inflight.
b. Provide emergency nosewheel steering on the ground.
c. Aid the brake accumulator with emergency braking.
A1-F18EA-NFM-000
I-2-41 ORIGINAL
On the ground with engines shutdown, the brake accumulator provides hydraulic pressure to set the
parking brake. With a HYD 2 failure, pressure from the brake accumulator can be used to provide
emergency braking. A fully charged brake accumulator provides a minimum of ten full brake
applications.
The APU and brake accumulator charges are maintained against normal leakage and temperature
fluctuations by a trickle-charge restrictor connected to HYD 2A. Additionally, both accumulators may
be manually recharged inflight using HYD 2B pressure by placing the HYD ISOL switch to ORIDE.
This procedure recharges the brake accumulator if and only if the arming valve is open (emergency gear
or emergency probe extension previously selected). Both accumulators can be charged on the ground
by a hand pump located in the right main landing gear wheelwell.
2.6.2.1 Brake Accumulator Pressure Gauge. The brake accumulator pressure gauge is located on
the lower left main instrument panel and is redlined to indicate pressure below 2,000 psi. The BRK
ACCUM caution is displayed when brake accumulator pressure drops below 2,000 psi. The caution and
redlined pressure indication are warnings that approximately five full brake applications remain before
the brake accumulator is empty. When ac power is not applied, power to the gauge is controlled by the
BRK PRESS switch.
2.6.2.1.1 BRK PRESS Switch. The BRK PRESS switch, located on the forward left console, is spring
loaded to the aft position.
Forward
(unmarked)
Applies maintenance bus power to the brake accumulator pressure gauge when ac
power is not applied.
Aft
(unmarked)
Brake accumulator pressure gauge unpowered when ac power is not applied.
2.6.2.2 HYD ISOL Switch. The HYD ISOL switch, located on the aft left console, is spring loaded to
the NORM position.
ORIDE Opens the aft isolation valve inflight allowing HYD 2B pressure to recharge the
brake and/or APU accumulators. Following emergency landing gear extention, the
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NATOPS Flight Manual 飞行手册 1(49)
