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Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Pitot-Static Flight Instruments

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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




Each pilot is responsible for consulting the Aircraft Flight
Manual (AFM) or the Pilot's Operating Handbook (POH)
to determine the amount of error that is introduced into the
system when utilizing the alternate static source. In an aircraft
not equipped with an alternate static source, an alternate
method of introducing static pressure into the system should
a blockage occur is to break the glass face of the VSI. This
most likely renders the VSI inoperative. The reason for
choosing the VSI as the instrument to break is that it is the
least important static source instrument for flight.

The altimeter is an instrument that measures the height of
an aircraft above a given pressure level. Pressure levels
are discussed later in detail. Since the altimeter is the only
instrument that is capable of indicating altitude, this is one of
the most vital instruments installed in the aircraft. To use the
altimeter effectively, the pilot must understand the operation
of the instrument, as well as the errors associated with the
altimeter and how each effect the indication.

A stack of sealed aneroid wafers comprise the main
component of the altimeter. An aneroid wafer is a sealed
wafer that is evacuated to an internal pressure of 29.92 inches
of mercury (29.92 "Hg). These wafers are free to expand
and contract with changes to the static pressure. A higher
static pressure presses down on the wafers and causes them
to collapse. A lower static pressure (less than 29.92 "Hg)
allows the wafers to expand. A mechanical linkage connects
the wafer movement to the needles on the indicator face,
which translates compression of the wafers into a decrease
in altitude and translates an expansion of the wafers into an
increase in altitude. [Figure 7-2]

Notice how the static pressure is introduced into the rear of
the sealed altimeter case. The altimeter's outer chamber is
sealed, which allows the static pressure to surround the aneroid
wafers. If the static pressure is higher than the pressure in the
aneroid wafers (29.92 "Hg), then the wafers are compressed
until the pressure inside the wafers is equal to the surrounding
static pressure. Conversely, if the static pressure is less than
the pressure inside of the wafers, the wafers are able to expand
which increases the volume. The expansion and contraction
of the wafers moves the mechanical linkage, which drives the
needles on the face of the altimeter.

Figure 7-2. Altimeter.

Principle of Operation
The pressure altimeter is an aneroid barometer that measures
the pressure of the atmosphere at the level where the altimeter
is located, and presents an altitude indication in feet. The
altimeter uses static pressure as its source of operation.
Air is denser at sea level than aloft—as altitude increases,
atmospheric pressure decreases. This difference in pressure
at various levels causes the altimeter to indicate changes in

The presentation of altitude varies considerably between
different types of altimeters. Some have one pointer while
others have two or more. Only the multipointer type is
discussed in this handbook. The dial of a typical altimeter
is graduated with numerals arranged clockwise from zero
to nine. Movement of the aneroid element is transmitted
through gears to the three hands that indicate altitude. The
shortest hand indicates altitude in tens of thousands of feet,
the intermediate hand in thousands of feet, and the longest
hand in hundreds of feet.