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The Utility hydraulic motor/pump consists of a fixed-displacement,
hydraulically-driven motor that drives a fixed-displacement hydraulic motor
by means of an interconnect shaft. The motor/pump assembly is installed
in the right side of the tail compartment.
A 1.4 GPM restrictor connects the pump pressure port to the main return
line. The restrictor allows a small amount of fluid to circulate within the
pump to assist in damping pressure surges, reduce pump noise and
reduce starting torque.
E. Pressure Snubber and Transmitter:
A pressure snubber is installed upstream of the pressure transmitter to
damp any pressure surges in order to prevent erratic pressure indications.
The pressure transmitter receives 28V DC power from the Essential DC
bus through the UTILITY HYD PRESS circuit breaker. It then provides a
variable output signal, proportional to fluid pressure, to the DAU. The DAU,
in turn, prompts these signals for display on the Engine Instruments
display, ENGINE START system page and HYDRAULICS system page.
F. System Filters:
The Utility hydraulic system main pressure filter is a “non-bypass” filter,
meaning there is no alternate path to bypass the filter element. Fluid will
continue to flow through the element at increasingly reduced rates until
ceasing completely.
The Utility pump bypass filter is a bypass type filter in that a relief valve is
incorporated. If either filter element begins to clog, the pressure differential
between the filter inlet and outlet opens a relief valve, allowing the fluid to
bypass the filter element.
DPIs for each filter provide a visual means of noting filter bypass by
causing a red indicator on the DPI to “pop up” when the pressure
differential between the filter inlet and outlet exceeds a preset value. The
DPI indicator may be reset for monitoring or troubleshooting as required.
G. Hydraulic Pump Control Circuit:
(See Figure 17).
(1) General:
An electrically-operated hydraulic pump control circuit is
incorporated to ensure fail-safe pressure output of the Combined,
Flight and Utility hydraulic system pumps during various scenarios.
It also provides visual indications (in the form of CAS messages)
should the Combined or Flight hydraulic systems overheat or fail.
The circuit is composed of the Utility hydraulic system solenoid
shutoff valve, the Combined and Flight hydraulic system pressure
switches, the Combined and Flight hydraulic system thermal
switches and various relays that control the sequence of circuit
operation. All scenarios described in the following paragraphs
assume proper configuration of the system.
(2) Engine Start Scenario:
As the right engine is started and Flight hydraulic system pressure
increases above 1500 psi, the Flight hydraulic system pressure
switch opens, causing the amber FLT HYD FAIL message to be
OPERATING MANUAL
2A-29-00 PRODUCTION AIRCRAFT SYSTEMS
Page 30
October 11/02
Revision 8
removed from CAS. Since the left engine has not yet been started,
Combined hydraulic system pressure is zero. The amber CMB HYD
FAIL message is displayed on CAS and the Combined hydraulic
system pressure switch is closed. 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.
As the left engine is started (with the right engine running) and
Combined hydraulic system pressure increases above 1500 psi, the
Combined hydraulic system pressure switch opens, causing the
amber CMB HYD FAIL message to be removed from CAS. With the
pressure switch open, power is routed through both the hold relay
and the UTILITY PUMP OFF/ARM switch and delivered to the Utility
hydraulic system solenoid shutoff valve. When the shutoff valve
closes, Utility pump operation stops. At this point, hydraulic pressure
configuration is: Combined = 3000, Flight = 3000 and Utility = 0.
This is the normal ground and flight configuration.
(3) Left Engine Failure On Takeoff Scenario:
Should an engine fail during takeoff, immediate landing gear
retraction is essential. Assuming that the left engine has failed and is
windmilling, the Combined hydraulic system would most likely be
unable to provide landing gear retraction pressure, as the
windmilling engine would only provide pressure fluctuating in the
800 psi range. In this range, the Combined hydraulic system
　
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