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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Hydraulic Systems

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




An ammeter is used to monitor the performance of the aircraft
electrical system. The ammeter shows if the alternator/
generator is producing an adequate supply of electrical power.
It also indicates whether or not the battery is receiving an
electrical charge.

Ammeters are designed with the zero point in the center
of the face and a negative or positive indication on either
side. [Figure 6-35] When the pointer of the ammeter is
on the plus side, it shows the charging rate of the battery.
A minus indication means more current is being drawn
from the battery than is being replaced. A full-scale minus
deflection indicates a malfunction of the alternator/generator.
A full-scale positive deflection indicates a malfunction of
the regulator. In either case, consult the AFM or POH for
appropriate action to be taken.

Ammeter and loadmeter.
Figure 6-35. Ammeter and loadmeter.

Not all aircraft are equipped with an ammeter. Some have
a warning light that, when lighted, indicates a discharge in
the system as a generator/alternator malfunction. Refer to the
AFM or POH for appropriate action to be taken.

Another electrical monitoring indicator is a loadmeter.
This type of gauge has a scale beginning with zero and
shows the load being placed on the alternator/generator.
[Figure 6-35] The loadmeter reflects the total percentage of
the load placed on the generating capacity of the electrical
system by the electrical accessories and battery. When all
electrical components are turned off, it reflects only the
amount of charging current demanded by the battery.

A voltage regulator controls the rate of charge to the battery
by stabilizing the generator or alternator electrical output. The
generator/alternator voltage output should be higher than the
battery voltage. For example, a 12-volt battery would be fed
by a generator/alternator system of approximately 14 volts.
The difference in voltage keeps the battery charged.

Hydraulic Systems

There are multiple applications for hydraulic use in aircraft,
depending on the complexity of the aircraft. For example,
hydraulics is often used on small airplanes to operate wheel
brakes, retractable landing gear, and some constant-speed
propellers. On large airplanes, hydraulics is used for flight
control surfaces, wing flaps, spoilers, and other systems.

A basic hydraulic system consists of a reservoir, pump (either
hand, electric, or engine driven), a filter to keep the fluid clean,
selector valve to control the direction of flow, relief valve to
relieve excess pressure, and an actuator. [Figure 6-36]

Basic hydraulic system.
Figure 6-36. Basic hydraulic system.

The hydraulic fluid is pumped through the system to an
actuator or servo. A servo is a cylinder with a piston inside
that turns fluid power into work and creates the power needed
to move an aircraft system or flight control. Servos can be
either single-acting or double-acting, based on the needs of
the system. This means that the fluid can be applied to one
or both sides of the servo, depending on the servo type. A
single-acting servo provides power in one direction. The
selector valve allows the fluid direction to be controlled.
This is necessary for operations such as the extension and
retraction of landing gear during which the fluid must work
in two different directions. The relief valve provides an outlet
for the system in the event of excessive fluid pressure in the
system. Each system incorporates different components to
meet the individual needs of different aircraft.

A mineral-based hydraulic fluid is the most widely used type
for small aircraft. This type of hydraulic fluid, a kerosene-like
petroleum product, has good lubricating properties, as well
as additives to inhibit foaming and prevent the formation of
corrosion. It is chemically stable, has very little viscosity
change with temperature, and is dyed for identification.
Since several types of hydraulic fluids are commonly used,
an aircraft must be serviced with the type specified by the
manufacturer. Refer to the AFM/POH or the Maintenance