COMPONENTS
fully in three-sixteenths of an inch of piston
movement.
The complete backup flight control system is
The shutoff valve may open momentarily during
mounted on a protective armor plating that measures
backup system operation to discharge excess fluid
only 8 by 16 inches and is located close to the rudder
volume. This action may be a result of unequal
and stabilizer power packages. Flight and combined
stabilizer in-and-out stroke volume or thermal
hydraulic system pressure line switches control the
expansion of the fluid. The shutoff valve also opens
operation of this system.
when the flow rate exceeds the flow capacity of the
The two switches in the pressure lines to the
backup pump. The latter condition could occur when
backup flight system are wired normally closed at zero
the flight system is operating normally and high rate
pressure. The backup pump outlet pressure switch is
inputs are applied to the actuators.
wired to normally open at zero pressure. The switches
actuate at 900 to 1,100 psi on rising pressure and 700 to
OPERATION
900 psi on decreasing pressure. Closing of the
combined or flight system pressure switches energizes
Pressure line isolation is accomplished by the use
the backup system motor pump. Closing the outlet
of check valves. To prevent backup system leakage to a
pressure switch lights the backup hydraulic system
failed combined system, a soft-seat check valve is
indicator light on the annunciator panel in the cockpit.
installed upstream of the standard metal-seat check
valve. These valves are found in the combined system
When pressure in the flight and/or combined
pressure line.
hydraulic system decreases to 700-900 psi, the system
is automatically activated. The system isolates a
A three-position backup system hydraulic test
portion of the combined system in the tail of the aircraft
switch is located in the cockpit. The central
by check valves in the pressure lines and a shutoff valve
spring-loaded OFF position provides automatic
in the return line. When the shutoff valve closes, it
function in flight. The momentary hold positions,
stores a full charge of fluid in the backup system
COMBINED and FLIGHT, are for a ground test of the
reservoir.
system when the aircraft is on external electrical
power. Selection of either position will energize the
The reservoir mounts on top of the motor-pump
motor pump when aircraft pressure is less than 700 to
assembly. It has a capacity of 0.84 quarts. The return
900 psi.
system shutoff valve is an integral part of the reservoir
end flange inside the reservoir pressurizing spring. The
A cartridge-type filter element housed within the
soft-seated, poppet-type shutoff valve is held open
reservoir head and a pressure line filter protects the
when the reservoir is at the full position. When
system from contamination. Since the backup motor
pressure drops and the reservoir piston moves about
pump is energized when either or both primary systems
three-sixteenths of an inch away from the full position,
fail, the following three operational conditions can
the spring-loaded valve closes and prevents flow from
exist:
the reservoir. The shutoff valve also acts independently
1. With the backup and flight systems operating,
as a relief valve to relieve reservoir pressure above 95
normal flight control is available. The backup system
psi.
performs as an isolated system with the return shutoff
Return fluid flow from the rudder and stabilizer
valve closed. The variable displacement backup motor
actuators fills the backup system reservoir. When the
pump has a maximum rated output of 3 gpm at 1,000-psi
reservoir approaches the full position, it mechanically
output pressure to zero gpm at cutoff pressure
opens a shutoff valve, allowing return flow to go to the
(3,000-3,200 psi). The pump cannot match the high rate
combined system reservoir. In normal flight, the 40-psi
capacity of the flight system. Backup motor pump
return system pressure is enough to maintain the
pressure will drop to zero when demand exceeds 3 gpm.
backup reservoir piston at the full position. The shutoff
Zero pressure causes the cockpit indicator light to go
valve fully opens against its spring pressure.
out. When pressure increases to 900-1,000 psi, the light
will come on again, indicating backup system
If return system pressure drops below the reservoir
operation.
pressurizing spring pressure of 15 psi, the reservoir
piston moves and displaces fluid through the shutoff
2. With the backup system and combined system
valve. As the piston moves, the shutoff valve closes
operating, normal flight control is available. The
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