Pitot-Static Instrument Family
pitot-static instrument family is
made up of all the aircraft instruments that obtain their information
from either a pitot or static source of air. Pitot tube air is sometimes
called ram air because it is air forced through a tube. Static air is
air measured from a relatively non-moving or ‘static’ source
located on or in the aircraft. The three pitot-static instruments are
the airspeed indicator, the altimeter, and the vertical speed indicator.
indicator is used to determine the speed of air flowing over
the aircraft. The airspeed indicator uses a pitot tube that is positioned
to give a good reference of the actual airspeed flowing over the wings.
This pitot tube utilizes a small ram air tube that runs the air from the
pitot to the airspeed indicator. A static air source, an air opening located
in a relatively undisturbed place on the side of the fuselage or behind
the pitot tube opening, is then sent to the airspeed indicator and the
two are compared to determine airspeed. This is usually accomplished by
a small barometer, which measures air pressure differential inside the
instrument. The barometer looks like a miniature accordion and expands
and collapses due to the pressure differentials moving a needle that is
seen on the airspeed indicator.
The airspeed indicator
is critical in certain flight regimes or phases. During takeoff and landing,
it is crucial that the aircraft be flown at the correct airspeed for the
flap setting and pitch attitude which you are flying in. If a pilot allows
the airspeed to get too slow, he risks the onset of a stall. If he allows
it to get too fast, he may not climb or on the landing he may float for
a considerable distance.
The airspeed indicator
commonly used on light piston single-engine aircraft is color coded and
expresses its speed in knots or miles per hour. A white
arc indicates the arc in which it is safe to use flaps. The
green arc is the normal operating
range of the aircraft. The yellow arc
is the caution range for the airplane. It is fine to fly the airplane
in the yellow range, unless the aircraft is doing maneuvers such as steep
banks or dramatic pitch changes, or unless turbulence is encountered.
The red line is the highest speed
at which the aircraft should be flown. If you fly above this airspeed,
it does not necessarily mean the wings will come off, but the aircraft
was not designed to be flown this fast, and structural damage could occur
in certain conditions.
measures the vertical height of the airplane above a reference point,
usually sea level. The actual height of an object above sea level is called
true altitude. To measure this altitude correctly, the altimeter is equipped
with a barometer, which measures air
pressure. Standard air pressure at sea level is measured at 29.92"
of mercury (hg). The pressure decreases one inch of mercury for every
thousand feet of increase in altitude.
As you will learn
in the weather chapter, air pressure also varies according to changing
weather and atmospheric conditions. The altimeter in your aircraft is
equipped with a window to allow you to change your altimeter setting to
reflect current atmospheric conditions and thus reflect true altitude.
As you fly on cross-country flights, you will occasionally need to reset
your altimeter to the closest available airports along your route.
When you are flying
above 18,000 feet in higher performance aircraft, however, you will use
pressure altitude rather than true altitude for your altimeter. You will
do this by setting the altimeter window to 29.92"—the
standard altimeter setting. This is done for a couple of reasons. At heights
above 18,000' airplanes travel with such speed that it is difficult to
reset the altimeter every few minutes to local altimeter conditions. A
setting of 29.92" allows aircraft to fly on the same pressure setting
for increased safety, even in changing atmospheric conditions. As the
atmospheric pressure changes, the altimeter will show the change, and
the pilot will make the adjustments necessary to maintain consistent altitudes.
for using a pressure altitude is that by 18,000' the atmosphere is different
than that found at the reporting local airport due to temperature and
the static system of the aircraft to obtain air pressure information.
This pressure then moves an internal barometer to indicate changes in
can also be affected by changes in temperature. Air molecules become more
dense as the temperature becomes lower. A temperature change can adversely
affect the altimeter’s measurements, but they are much less a factor
than changes in elevation. A common memory aid used to consider the effects
of high and low pressure changes due to weather and accompanying temperature
changes is, “When flying from high to low or hot to cold, look out
Speed Indicator or VSI is used to display the rate of climb
or descent in feet per minute. The instrument is a secondary instrument
because it depends on a barometer for information and tends to lag behind
the actual pitch movements of the aircraft. The VSI uses static system
information to observe trend changes in pressure, which are then displayed
on the instrument. The VSI is often used as a backup instrument on private
pilot maneuvers such as steep turns.