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airspeed, and performance. These instruments are categorized
according to their method of operation. The
categories include pitot-static, magnetic, gyro-scopic,
electrical, or self-contained. Examples of self-contained
instruments and indicators that are useful to the
pilot include the yaw string, inclinometer, and outside
air temperature gauge (OAT).
Gyroscopic instruments, including the attitude indicator,
turn coordinator, and heading indicator, are discussed
in this chapter to give you an understanding of
how they function. Self-launch gliders often have one
or more gyroscopic instruments on the panel. Gliders
without power rarely have gyroscopic instruments
installed.
PITOT-STATIC INSTRUMENTS
There are two major parts of the pitot-static system:
(1) impact pressure lines; and (2) static pressure lines,
which provide the source of ambient air pressure for
the operation of the altimeter, variometer, and the airspeed
indicator.
IMPACT AND STATIC PRESSURE
LINES
The impact air pressure (air striking the glider because
of its forward motion) is taken from a pitot tube, which
is mounted either on the nose or the vertical stabilizer,
and aligned with the relative wind. These locations
minimize disturbance or turbulence caused by the
motion of the glider through the air.
The static pressure (pressure of the still air) is taken
from the static line, which is attached to a vent or vents
mounted flush with the side of the fuselage or tube
mounted on the vertical stabilizer. Gliders using a
flush-type static source with two vents, have one vent
on each side of the fuselage. This compensates for any
possible variation in static pressure due to erratic
changes in glider attitude.
The openings of both the pitot tube and the static
vent(s) should be checked during the preflight inspection
to enure they are free from obstructions. Clogged
or partially clogged openings should be cleaned by a
certificated mechanic. Blowing into these openings is
not recommended because this could damage any of
the three instruments.
AIRSPEED INDICATOR
The airspeed indicator displays the speed of the glider
through the air. Some airspeed indicator dials provide
color-coded arcs that depict permissible airspeed
ranges for different phases of flight. The upper and
lower limits of the arcs correspond to airspeed limitations
specific to the glider in which the instrument is
mounted. [Figure 4-1]
The airspeed indicator is the only instrument that
depends on both pitot pressure and static pressure.
When pitot pressure and static pressure are the same,
zero airspeed is indicated. As pitot pressure becomes
progressively greater than static pressure, indicated air-
Figure 4-1. The airspeed indicator uses both the pitot and static system.
4-2
speed increases. The airspeed indicator contains a
small diaphragm that drives the needle on the face of
the instrument.
TYPES OF AIRSPEED
There are three kinds of airspeed that the pilot should
understand: indicated airspeed, calibrated airspeed, and
true airspeed. [Figure 4-2]
INDICATED AIRSPEED
Indicated airspeed (IAS) is the direct instrument reading
obtained from the airspeed indicator, uncorrected
for variations in atmospheric density, installation error,
or instrument error.
CALIBRATED AIRSPEED
Calibrated airspeed (CAS) is indicated airspeed
corrected for installation and instrument error.
Although manufacturers attempt to keep airspeed
errors to a minimum, it is impossible to eliminate all
errors throughout the airspeed operating range. At certain
airspeeds and with certain flap/spoiler settings, the
installation and instrument error may be significant.
The error is generally greatest at low airspeeds. In the
cruising and higher airspeed ranges, indicated airspeed
and calibrated airspeed are approximately the same.
It is important to refer to the airspeed calibration chart
to correct for possible airspeed errors because airspeed
limitations, such as those found on the color-coded
face of the airspeed indicator, on placards in the cockpit,
or in the Glider Flight Manual or Pilot’s Operating
Handbook (GFM/POH), are usually calibrated airspeeds.
Some manufacturers use indicated rather than
calibrated airspeed to denote the airspeed limitations
mentioned. The airspeed indicator should be calibrated
periodically.
Dirt, dust, ice, or snow collecting at the mouth
of the tube may obstruct air passage and prevent correct
indications, and also vibrations may destroy the sensitivity
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Glider Flying Handbook(32)
