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pressure switch would cycle open and closed, resulting in
intermittent operation of the Utility pump. To prevent intermittent
Utility pump operation, the Combined hydraulic system pressure
switch is also used to energize a hold relay as system pressure
drops below 800 psi. With the landing gear control handle in the UP
position and any landing gear door not closed, the hold relay
remains energized, overriding any pressure switch cycling. Since
the relay is held, no electrical power can be routed to close the
Utility hydraulic system solenoid shutoff valve, thus the Utility pump
operates, supplying 3000 psi of hydraulic pressure to the landing
gear system. At this point, hydraulic pressure configuration is:
Combined = 0, Flight = 3000 and Utility = 3000.
With all landing gear doors closed and Combined hydraulic system
pressure greater than 1500 psi, the hold relay is de-energized,
allowing the Utility hydraulic system solenoid shutoff valve to be
energized closed. When the shutoff valve closes, Utility pump
operation stops and pressure drops to zero. Should Combined
hydraulic system pressure remain below 800 psi, however, the
Utility hydraulic system solenoid shutoff valve remains de-energized
(open) and Utility hydraulic system pressure remains at 3000 psi.
(4) Combined Hydraulic System Fluctuation Scenario:
If the landing gear doors are closed or the landing gear control
handle is in the DOWN position, and Combined hydraulic system
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-29-00
Page 31
October 11/02
Revision 8
pressure fluctuates in a range of greater than 1500 psi to below 800
psi, the Utility hydraulic system will also cycle on and off. Utility
hydraulic system will be 3000 psi when Combined hydraulic system
pressure is below 800 psi and 0 psi when Combined hydraulic
system pressure is above 1500 psi.
(5) Combined Hydraulic System Inflight Failure Scenario:
If the Combined hydraulic system fails during flight, the pressure
switch closes as Combined system pressure falls below 800 psi.
This causes the amber CMB HYD FAIL message to be displayed on
CAS.
With the pressure switch closed, the hold relay is de-energized, thus
no electrical power can be routed to close the Utility hydraulic
system solenoid shutoff valve. Since the shutoff valve is open, the
Utility pump operates, supplying 3000 psi of hydraulic pressure to its
using systems and components. At this point, hydraulic pressure
configuration is: Combined = 0, Flight = 3000 and Utility = 3000.
(6) Flight Hydraulic System Inflight Failure Scenario:
If the Flight hydraulic system fails during flight, the pressure switch
closes as Flight system pressure falls below 800 psi. This causes
the amber FLT HYD FAIL message to be displayed on CAS.
Since the Utility hydraulic system solenoid shutoff valve receives its
power from the Combined hydraulic system pressure switch, the
valve remains closed and the Utility hydraulic pump does not
operate. All components that normally receive hydraulic pressure
from the Flight hydraulic system remain operative through the use of
Combined hydraulic system pressure, with the exception of the right
thrust reverser and the Utility hydraulic system motor.
At this point, hydraulic pressure configuration is: Combined = 3000,
Flight = 0 and Utility = 0.
(7) Flight Hydraulic System Overheat Scenario:
If the Flight hydraulic system reservoir fluid temperature reaches
220 ±5°F (104 ±3°C), the thermal switch on the reservoir closes
causing a blue FLT HYD HOT advisory message to be displayed on
CAS. (The same logic applies to the Combined hydraulic system.) In
the case of the Flight hydraulic system, however, in addition to
causing the FLT HYD HOT message to be displayed, power is also
routed to energize and hold the Utility hydraulic system solenoid
shutoff valve closed. This prevents Utility hydraulic pump operation.
This feature may be overridden by the flight crew, however, provided
Flight hydraulic system is above 2000 psi, by selection of the
UTILITY PUMP ON/OFF switch to ON.
If Flight hydraulic system reservoir fluid temperature drops to 175
±10°F (79 ±6°C), the thermal switch opens, clearing the CAS
message.
3. Controls and Indications:
(See Figure 18.)
OPERATING MANUAL
2A-29-00 PRODUCTION AIRCRAFT SYSTEMS
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October 11/02
Revision 8
A. Circuit Breakers:
The Utility hydraulic system is protected by the following circuit breakers
(CBs):
Circuit Breaker Name: CB Panel: Location: Power Source:
UTILITY HYD PRESS CPO B-3 ESS 28 VDC Bus
　
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