Figure 6-1.-Mercurial barometer.
given altitude. Figure 6-2 shows the standard
pressure and temperature at given altitudes.
Conditions are very seldom standard for
temperature or pressure; therefore, you must
correct the formula to find density altitude or true
airspeed. Let's consider an airfield under the
Figure 6-2.-The standard atmosphere.
influence of a low-pressure climatic condition,
where the temperature is very hot. Together, these
the altitude. The VSI indicates how fast the
two conditions may reduce the density of the air
aircraft is climbing or descending. They all operate
to such an extent that it affects aircraft engine
performance. This makes takeoff capability
on air that comes in from outside the aircraft
during flight.
marginal, especially for a helicopter. The density
The pitot tube mounts on the outside of the
of air also directly affects aircraft movement
aircraft (fig. 6-3) at a point where the air is least
through the air, and thus the true airspeed of the
likely to be turbulent. The tube points in a
aircraft. The denser the air, the more difficult it
forward direction parallel to the aircraft's line of
is for the aircraft to move through it.
flight. One general type of airspeed tube mounts
PITOT-STATIC SYSTEM
on a streamlined mast extending below the nose
of the fuselage. Another type mounts on a boom
The aircraft pitot-static system (fig. 6-3)
includes instruments that operate on the principle
extending forward of the leading edge of the wing,
of the barometer. The system consists of a pitot
Although there is a slight difference in their
tube, static air vents, and three indicators, which
construction, they operate identically.
Pitot stands for impact pressure, the pressure
connect with pipelines that carry air. The three
of the outside air against the aircraft flying
indicators are airspeed, altimeter, and the vertical
through it. The tube that goes from the pitot tube
speed indicator (VSI).
The airspeed indicator shows the speed of the
to the airspeed indicator applies the outside air
pressure to the airspeed indicator. The airspeed
aircraft through the air and the altimeter shows
