AIRCRAFT INSTRUMENTS
When the first aircraft came into existence, the
flight instrument, which is discussed in the inertial
main goal was to launch the aircraft and keep it
navigation systems chapter.
airborne as long as possible. At first, it was not
possible to keep the aircraft in the air for longer
than a few minutes. However, as engines and
aircraft structures were improved, the aircraft was
FLIGHT INSTRUMENT SYSTEMS
able to remain aloft for a longer time. Along
with these improvements came the need for
Learning Objective: Recognize operating
instruments. The first aircraft instruments were
principles and features of various aircraft
fuel and oil pressure instruments. These instru-
flight instrument systems, including the
ments warned the pilot of engine trouble so the
pitot-static, airspeed indicator, angle-of-
aircraft could be landed before the engine failed.
attack, gyroscope, and miscellaneous flight
Later, when the aircraft could fly over con-
instrument systems.
siderable distances, weather became a problem.
This led to the development of instruments that
To maintain instruments properly, you, as an
helped pilots fly through snowstorms, thunder-
AE, must know the basic principles of the flight
storms, and other bad weather conditions.
instrument systems. AEs frequently work with
The instruments used in aircraft years ago are
equipment and systems that use the principles of
reasonably simple compared with those in current
density and pressure.
aircraft. The jet aircraft has brought many
You must consider density and pressure when
complex problems to instrument engineering.
discussing altimetry and airspeed. Although very
Instrumentation is basically the science of
light, air has weight and is affected by gravity.
measurement. Measurements that are common on
By its weight, air exerts pressure on everything
all aircraft are posit ion, direction, speed, altitude,
it touches. Since air is a gas, it exerts pressure in
engine condition, fuel on board, and fuel
all directions. The weight of the air pressing down
consumption. In addition, jet aircraft instruments
from above determines the air pressure at any
include Mach number, angle of attack, and tail
given altitude.
pipe temperature indicators.
The weight of the atmosphere presses the
There are two ways of grouping aircraft
molecules closer together, making them more
instruments--by their operating principles and by
numerous per unit of volume. This action takes
the job they perform. Instrument operating
place at the bottom of the atmosphere, or where
principles include gyroscopic, pressure or
it rests upon the earth's surface. Therefore, the
temperature sensing, magnetism, electrical energy,
air at the bottom of the atmosphere is more dense
or a combination of any of these. This chapter
than at higher altitudes. Air pressure at sea level
deals with instruments and indicating systems in
on an average day will support a column of
relation to the jobs they perform--flight, engine,
mercury 29.92 inches high (fig. 6-1).
and equipment instruments. The flight instru-
ments discussed are those instruments that provide
Atmospheric pressure is a force per unit
aircraft performance information to the pilot.
area, and force is equal to mass multiplied by
These instrument systems include the airspeed,
acceleration. Therefore, a pressure change occurs
altimeter, vertical speed, attitude, turn and bank,
if either the mass of the atmosphere changes or
and angle-of-attack. Along with the heading
the molecules within the atmosphere accelerate.
indicator, these instruments provide primary flight
Although altitude exerts the dominant control,
reference to the pilot. The heading indicator is a
temperature and moisture alter pressure at any
