CHAPTER 5: FLIGHT INSTRUMENTS
AND SYSTEMS
The airspeed indicator, the altimeter, the variometer, the yaw string, and the
compass are your primary tools for tracking the glider’s position and movement
through the air. Because of their crucial role, it is important that you understand
how these instruments work and how you should interpret the information they
provide.
Several secondary instruments may also prove useful in particular situations.
When flying over unfamiliar terrain, a Very high frequency Omnidirectional
Range (VOR) receiver or Global Positioning System (GPS) unit may come in
handy to provide navigation assistance. For motor-gliders that are permitted to
fly in clouds, gyroscopic instruments such as an attitude indicator, a turn
coordinator, or a turn and slip indicator are required.
Additionally, many modern gliders are equipped with other flight systems, such
as radios, emergency locator transmitters, and oxygen systems.
In this chapter, you will learn about the various flight instruments and systems
that you may encounter in a glider.
5.1 The Atmosphere
Most of the primary flight instruments use properties of the atmosphere to
provide information about the glider’s speed, altitude, and vertical movement. In
order to understand how these instruments work, we need to first understand
the atmosphere.
Properties of the Atmosphere
Air has weight. The higher you are in the atmosphere, the less air there is on top
of you pressing down. Therefore, the pressure in the atmosphere decreases as
you increase your altitude.
Air is also compressible, meaning that it will change in density and volume as
the pressure changes. If you raise a parcel of air, it will expand because of the
decreasing pressure, and its density will decrease.
The temperature of any compressible gas will increase as the pressure is
increased, and decrease as the pressure is decreased. These processes are called
adiabatic heating and cooling. You have probably noticed this effect if you have
ever used a spray can. The can gets colder as you use it. This is because the
pressure in the can decreases as you let the contents out, and the decrease in
pressure causes the decrease in temperature. The opposite effect can be observed
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Section 5.1
when using a bicycle pump. As you work the pump, you will notice it warming
up because the air inside is being compressed.
The same effects apply to the atmosphere. As the pressure decreases with
altitude, the temperature decreases.
The Standard Atmosphere
The actual atmosphere can be quite chaotic, with the temperature and pressure
varying considerably with location and altitude. To provide a common reference
with which to compare aircraft performance, the Standard Atmosphere was
defined. At sea level, the standard temperature is 59° F (15° C), and the standard
pressure is 29.92 in. Hg. (inches of mercury).
Since temperature and pressure change with altitude, standard lapse rates were
also defined. (A lapse rate is the change in a value with altitude.)
Figure 5.1 – Standard pressure
As Figure 5.1 shows, the standard pressure decreases nearly linearly from sea
level up to about 10,000 feet. The pressure lapse rate in this range is roughly -1.0
in. Hg. per 1,000 feet increase in altitude.
Section 5.1
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Figure 5.2 – Standard temperature
The standard temperature lapse rate is linear from sea level up to 35,000 feet. The
temperature lapse rate is -3.6° F per 1,000 feet increase in altitude. Above 35,000
feet, the standard temperature is constant at –70° F.
Keep in mind that the real atmosphere is never standard. There are always
variations in temperature and pressure due to weather.
Measuring Pressure
Pressure is the force that air exerts over an area. Still air has an associated
ambient pressure that results from the weight of the air above it. Moving air,
when brought to a stop, has additional pressure that results from its kinetic
energy. The ambient pressure is called the static pressure. The combination of the
ambient pressure and the “kinetic” pressure is called the total or pitot pressure.
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Section 5.1
Figure 5.3 – Measuring static and pitot pressure.
Static pressure is determined by measuring the pressure of the air without
disturbing the airflow. Typically, a pair of small static ports, which are usually
less than 1/8th inch in diameter, are located on either side of the fuselage. Using
two ports helps to average out any errors that might be caused by the glider’s not
flying perfectly straight through the air.
Pitot pressure is normally measured using a pitot tube. A pitot tube points into
the flow, so that the airflow is brought to a stop at the mouth of the tube. The
pitot tube is usually located on the vertical stabilizer or in the nose of the glider.
Pitot pressure is never less than static pressure.
5.2 Primary Instruments
Altimeter
The altimeter measures the static pressure. Since the static pressure changes with
altitude, the altimeter can be calibrated to display the height of the glider above a
given level. Usually, this level is chosen to be mean sea level (MSL).
How it Works
A schematic of an altimeter is shown in Figure 5.4. The altimeter consists of an
aneroid barometer, or bellows, which is sealed so that no air can flow into or out
of it. This bellows is located inside a sealed case that is connected to the static
pressure ports on the glider. When altitude increases, the static pressure
decreases, and the bellows expands, driving the indicating pointers through a
series of gears.
Section 5.2
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Figure 5.4 – Altimeter schematic
Of course, the altimeter will not function correctly if the static ports are blocked
or clogged.
Reading the Altimeter
Most altimeters have three pointers. The large pointer indicates hundreds of feet,
the middle pointer indicates thousands of feet, and the small pointer indicates
tens of thousands of feet.
Figure 5.5 – Reading the altimeter
Adjusting the Altimeter
Since weather systems will cause the pressure to change, the altimeter is
equipped with an adjustment knob that can be used to set it for current atmospheric conditions. This knob can be seen in Figure 5.6 on the lower left corner of
the altimeter. Notice also the Kollsman window on the right side of the altimeter
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Section 5.2
where the number 29.9 is indicated. This is the sea level barometric pressure in
inches of mercury.
Figure 5.6 – The altimeter
Federal Aviation Regulations require that below 18,000 feet, you set the altimeter
to the current reported setting from a station located within 100 nautical miles of
the aircraft. If no station report is available, you can set the altimeter to the
elevation of the departure airport.
If you have been flying for a long time or over a long distance, the atmospheric
pressure will probably have changed, so you should obtain a new pressure
setting from a ground station if you need to know your precise altitude.
Above 18,000 feet, you should set the altimeter to 29.92 in. Hg. (Note: you should
only fly above this altitude while in a “wave window, or with Air Traffic Control
clearance.) If all pilots in a given area have their altimeter set the same,
temperature and pressure variations will affect all the altimeters the same, so that
separation between aircraft can be maintained.
Altimeter Errors
Clearly, if the atmospheric pressure changes, the altimeter reading will be
affected. If you think of the atmosphere as a layer of constant density but varying
thickness, it is easy to see how different pressures affect the altimeter. Where the
layer is thick, the pressure is high, and where the layer is thin, the pressure is
low. (While technically incorrect, this assumption greatly simplifies the
discussion, while not detracting from its applicability.)
Remember, the altimeter measures pressure, which is essentially the weight of
the air above you. It doesn’t so much measure how high you are above sea level
as how far you are below the “top” of the atmosphere.
Section 5.2
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