CHAPTER 12
BASIC HYDRAULIC/PNEUMATIC AND EMERGENCY
POWER SYSTEMS
c system may be used. If the application requires only a
INTRODUCTION
medium amount of pressure and a more accurate
control, a combination of hydraulics and pneumatics
The Navy uses hydraulic and pneumatic power
may be used. If the application requires a great amount
systems extensively in naval aircraft. These systems
of pressure and/or extremely accurate control, a
have a number of favorable characteristics; they
hydraulic system should be used.
eliminate the need for complicated systems of gears,
cams, and levers. Also, they transmit motion without
the slack or delay inherent in the use of solid machine
TYPES OF POWER SYSTEMS
parts. The fluids used are not subject to breakage as are
mechanical parts, and the mechanisms are not
LEARNING OBJECTIVE: Identify the two
subjected to great wear.
types of power systems used on naval aircraft.
The different parts of a fluid power system can be
Hydraulic and pneumatic systems in aircraft
conveniently located at widely separated points, since
contain power systems and several subsystems, the
the forces generated are rapidly transmitted over
number depending upon the design of the aircraft. The
considerable distances with small loss. These forces
power systems are sometimes called the heart of the
can be conveyed up and down or around corners with
system, and the subsystems are known as the muscle.
small loss in efficiency and without complicated
The power systems include all the components
mechanisms. Very large forces can be controlled by
normally installed in the system, from the reservoir to,
much smaller ones, and can be transmitted through
but not including, the selector valve. In pressurized
comparatively small lines and orifices.
reservoir systems, this also includes all components
used to control and direct the pressurizing agent to the
If the system is well adapted to the work it is
reservoir. The utility hydraulic system includes
required to perform, and if it is not misused, it can
systems used for landing gear, arresting gear,
provide smooth, flexible, uniform action without
nosewheel steering, and many other systems. In
vibration, and it is unaffected by variation of load. In
accordance with military specifications, which set up
case of an overload, an automatic release of pressure
the requirements for aircraft hydraulic systems, all
can be guaranteed, so that the system is protected
hydraulically operated systems considered essential to
against breakdown or strain. Fluid power systems can
flight safety or landing must have provisions for
provide widely variable motions in both rotary and
emergency actuation. The hydraulic/pneumatic and
straight-line transmission of power. The need for
emergency power systems are discussed here.
control by hand can be minimized. In addition, fluid
power systems are economical to operate.
HYDRAULIC/PNEUMATIC POWER
The question may arise as to why hydraulics is
SYSTEMS
used in one application, pneumatics in another, or a
combination of hydraulics and pneumatics, also
System design must prevent the failure of a single
k n o w n a s h y d ro p n e u m a t i c s , i n s t i l l a n o t h e r
part, such as a pump, pressure line, or filter, from
application. Many factors are considered by the user
disabling the aircraft. Special consideration is given to
and/or the manufacturer when determining which type
the hydraulic flight control system. System design
of system to use in a specified application. There are
specifications require two separate systems for
no hard and fast rules to follow; however, past
operating the primary flight controls. All aircraft that
experience has provided some sound ideas that are
use hydraulically operated flight controls have at least
usually considered when such decisions are made. If
two hydraulic power systems. The systems supply
the application requires speed, a medium amount of
pressure to the utility or normal system in addition to
pressure, and only a fair amount of control, a pneumati
the flight controls. The flight control portion is given
12-1
