TM 55-1520-240-23-6
7-135
UTILITY HYDRAULIC SYSTEM
(Continued)
7-135
Cargo Ramp and Door System Theory of Operation
NOTE
The following ramp operating and
system information is basic and
applies to any aircraft equipped with
or without 65 . Aircraft equipped with
65 differ in that controls have been
provided in the cockpit to operate
the ramp control valve in cases of
emergency.
When the RAMP switch on the overhead hydraulic
(HYD) panel is set to ON, power is removed from a
normally-open solenoid valve in the pressure control
module. With the solenoid valve open, hydraulic fluid at
3,000 psi, is allowed to flow to the ramp control valve.
1.
When the lever on the control valve is at DN,
pressurized fluid releases a mechanical lock in
each of the ramp actuating cylinders. As the
ramp slowly falls under gravity, fluid is forced out
of the cylinders through the pressure actuated
valve to the control valve. The control valve
routes the fluid to the upper end of the cylinders.
As the ramp falls, the sequence valve is activated
by an extending plunger. The valve routes
pressurized fluid to the motor in the cargo door,
retracting the door into the ramp.
As the door retracts, return fluid from the motor
passes through the sequence valve and into the
pressure-actuated valve. A relief valve in the
pressure-actuated valve boosts the pressure
of this return fluid to 180 psi. This increased
pressure moves a spring-loaded valve spool
to block the fluid returning from the actuating
cylinders, stopping the ramp after it has dropped
about eight inches.
When the door is fully retracted, the motor
stops. This relieves the pressure in the
pressure-actuated valve. The valve spool returns
to its original position and return flow from the
cylinders resumes, allowing the ramp to continue
falling.
2.
When the lever is moved to UP, fluid is routed by
the control valve through the pressure-actuated
valve to the bottom of the ramp cylinders. The
fluid retracts the cylinders to lift the ramp.
As the ramp rises, it strikes the sequence valve
plunger. The valve actuates to allow pressurized
fluid to operate the cargo door motor to extend
the door.
As the door extends, return fluid from the
motor is directed by the sequence valve to the
pressure-actuated valve, where it halts the
ramp in the same manner as during ramp-down
operation.
When the door is fully extended, the motor
stops. This relieves the pressure in the
pressure-actuated valve. The ramp continues its
travel in the same manner as during ramp-down
operation.
3.
When the lever is moved to STOP, pressurized
fluid to the cylinders is cut off and the fluid within
them is locked in. The ramp is locked in position.
4.
A MANUAL OPER knob and a locking pin on
the sequence valve allow the ramp to be raised
or lowered with the door locked in position,
extended or retracted. When the knob is pushed
in and held in place with the locking pin, the valve
cuts off fluid flow to the door motor. The position
of the door then remains fixed, regardless of
ramp movement.
5.
Pressing a manual override button at the bottom
of the pressure-actuated valve allows the ramp
to move continuously instead of stopping while
the cargo door retracts or extends.
Emergency Operation of Cargo Ramp and Door
System Aircraft Equipped With 65
The basic cargo ramp and door hydraulic system
remains the same. Control from the cockpit is provided
by a distinctive HYD panel, 5 second time delay relay,
and ramp control valve peculiar to aircraft equipped with
65 .
NO BREAK - WORK HARDER
RAMP PWR and RAMP EMER switches mounted on the
HYD panel control ramp hydraulic power and operation.
The RAMP PWR switch is a three position switch with
OFF, ON, and EMERG functions. The OFF and ON
functions provide the same control as the RAMP switch
on aircraft without 65 . When the RAMP PWR switch
is set to EMERG, electrical power is directed to the
RAMP EMER switch. The RAMP EMER switch is a
guarded, momentary double throw switch which controls
operation of the ramp control valve.
7-615