operates a hydraulic pump. The turbine and pump
assembly is generally installed on the inner surface of a
door installed in the fuselage. The door is hinged,
allowing the assembly to be extended into the
slipstream by pulling a manual release in the cockpit.
Figure 12-41 shows a typical ram air unit. This type of
emergency system is intended for use only when
normal hydraulic pumps are completely inoperative.
Because of differences in system designs, aircraft
emergency system operating pressures will differ from
one aircraft to another. The ram air turbine system
shown in figure 12-41 provides a means for emergency
hydraulic and electrical power when the normal
aircraft hydraulic system has failed. The
turbine-driven hydraulic pump supplies fluid under
pressure to the primary flight controls as well as to an
The turbine system shown in figure 12-42 consists
Figure 12-40.--Schematic diagram of typical emergency
of a dropout governor-controlled turbine, a hydraulic
power system (electric-motor driven).
pump connected in parallel to the normal hydraulic
system, a ram air turbine actuator, and a turbine-retract
accumulator, a relief valve, a pressure switch, a
control valve. You can pull the release handle, located
snubber, and a control switch in the cockpit.
in the cockpit within easy reach of the pilot, to operate
The main difference in a system of this type and a
the system. A mechanical latch releases the turbine
normal (engine-driven) system is that instead of
assembly into the airstream. The spring-loaded
operating continuously, the pump operates only when
turbine actuator initiates extension of the turbine
pressure is needed in the system. For example, if the
assembly, and the airstream force completes the
normal power system is inoperative, the pilot turns on
the emergency system switch in the cockpit. Turning
turbine, the turbine blades remain at a constant setting
this switch on energizes a pressure switch that is
until near maximum rpm. At this point, the governor
connected into the emergency hydraulic system
senses the shaft rpm and begins to vary the blade angle
pressure line. The pressure switch is actuated
to prevent excessive turbine speed. At this speed, the
automatically by hydraulic pressure.
pump is delivering its maximum amount of fluid. As
the turbine slows down, usually due to a decrease in
For example, when emergency system pressure
airspeed, the fluid delivery from the pump will also
drops below a predetermined point, the pressure switch
decrease. This type of system allows the aircraft to be
turns the pump motor on. When the pressure builds up
controlled in flight by supplying the necessary
to the designed operating psi, the pressure switch turns
hydraulic and electrical power.
the pump motor off.
The turbine is maintained in the fully extended
The system is protected from excessive pressures
position by a hydraulic lock in the turbine actuator.
by a relief valve, which is set to open at a pressure
When the RAM AIR TURBINE RETRACT button
slightly above system operating pressure. Emergency
switch is depressed, electrical power is supplied to the
power systems of this type are generally equipped with
solenoid-operated turbine retract control valve.
an accumulator for storing a reservoir supply of fluid
Hydraulic pressure from the hydraulic power system is
under pressure. This prevents the pump motor from
ported to the retract side of the turbine actuator (fig.
having to cut in repeatedly to maintain operating
12-42) through a restrictor, which controls the retract
pressure in the system.
speed. As the turbine door reaches the closed position,
the spring-loaded hook-type lock is cammed up until it
Ram Air Turbine-Driven System
drops over the roller, locking the door closed. When
In this type of emergency hydraulic system, ram
the button switch is released, electrical power is
removed from the control valve and the retract side of
air is used to turn the blades of a turbine that, in turn,