EUROCOPTER
FLIGHT MANUAL BO 105 CB-5/CBS-5
7 - 28
MANUFACTURER’S DATA
Rev. 2
7.10 HYDRAULIC SYSTEM (see Fig. 7-15 and 7-16)
The hydraulic system, designed to reduce the main-rotor control forces, is constructed of
modular components and mounted on the left side of the transmission compartment. It
consists of two independent but mainly identical systems, of which System 1 is the prima-
ry operating system while System 2 is the stand-by reserve system. If necessary, opera-
tional control crosses over from the primary system to the stand-by system automatically
and without delay. Both systems offer 100 % specific power support.
Pressure supply is provided by two hydraulic pumps which are permanently driven by the
main transmission and which normally deliver full system pressure. The control of pres-
surization for the three actuators in each system is provided by two selector valves (one
each system) which are linked by an interconnecting plate to ensure that only one system
is active. The selector valves are mechanically coupled with and controlled by a hydraulic
shuttle valve that works on the basis of pressure differentiation. Should the pressure of the
primary system (System 1) fall below approx. 50% of the pressure in the stand-by system,
the shuttle valve will position the selector valves to cause the stand-by system (System 2)
to take over control (cross-over) and System 1 to become inactive.
The pressure of each hydraulic system is monitored by a pressure switch that, if system
pressure falls below approx 70 %, will activate the respective HYDR 1 or HYDR 2 warning
light. The HYDR 1 warning light will also come on (together with the HY BLOCK warning
light) whenever System 2 is activated.
A jammed servo valve in System 1 will cause an electric signal to be released by micro
switches which, by means of a relay, trigger a solenoid valve. This solenoid valve causes
an artificial loss of pressure by blocking pressure in System 1 upstream of the shuttle
valve so that System 2 is activated by the shuttle valve and System 1 is then deactivated
(cross-over). The HY BLOCK warning light comes on whenever the relay controlling the
solenoid valve is activated.
The micro switches, noted above, are designed to recognize a jammed servo valve and
are only integrated in System 1. In System 2, however, a connection provided by springs
will by-pass a jammed servo valve noticed by increased control forces about the affected
axis.
NOTE Should a servo valve jam in System 2, an automatic cross-over to System 1 is
impossible. Therefore, normal operation in System 2 is not allowed as the then
lack of redundancy would cause an inflight system failure to be a safety risk.
A cross-over functional check can be performed using a guarded three-position HYDR
TEST switch located on the center console switch panel. Placing the switch from the
spring-loaded center (off) position to the TEST position activates the relay and, as a re-
sult, the solenoid valve to cause control to cross-over to System 2. Placing the switch to
the RESET position deactivates the self-holding solenoid valve causing control to revert
back to System 1.
All system control and monitoring functions are powered direct from the Main Bus via the
HYDR. CONTROLS circuit breaker on the center console switch panel.
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