4. pitot-static

FLIGHT INSTRUMENTS (Pitot-Static system)

FLIGHT INSTRUMENTS (Pitot-Static system)

All aircraft have instruments to let the pilot
know what is happening to his airplane in
flight, his position (navigation) and the
operation of his engine. Instruments work in
different ways and are operated by different
systems.

Pitot-Static System

The pitot-static system is very simple, but very
important, because it operates some of the
primary flight instruments. The system is
based on a difference in air pressure using ram
air pressure (trapped air pressure) and static
(still air pressure).

The Pitot System
The pitot system consists of a pitot head,
which is usually attached to the wings or
fuselage. Ram air enters the pitot head and,
through tubes, is fed to the airspeed
indicator . The Airspeed Indicator is the
only instrument which uses the pitot system.

The Static System

The static system has static vents or inlets
mounted flush to the fuselage or through
small holed in the pitot head. Still air enters
this system and the pressure of this air is fed
through tubes to the airspeed indicator,
altimeter and vertical speed indicator.

Static Air Pressure

We are using the term static air pressure and
atmospheric pressure in this lesson . Let’s see what this
pressure really is. As you probably know, air has
weight, we can measure the weight of a column of air
above us.
Of course, when we go up into the air, the weight of the
air is less, 50% of the weight of the atmosphere is
below a height of 18,000 feet. This weight difference is
measured in the static pressure system and used by the
instruments.

To understand the effect of decrease of pressure
with height we can take an example with water pressure.
For instance, if you fill a large barrel full of water, and
then punch holes from the bottom to the top. You will
see that the pressure is much greater near the bottom,
and reduces as we go up.

Measuring the “Weight” of the Atmosphere

Barometers

Barometric pressure, (atmospheric pressure) is
measured by a barometer. Barometers measure the
weight of the air at the airport or weather station the
barometer is located. Two types of barometers are
commonly used; the Mercurial barometer and the
Aneroid Barometer.

Mercurial Barometer

Mercury is a liquid metal, which is very
heavy. It is used in mercurial barometers
And is kept in a container, which is open to the
pressure of the air. Another part of the barometer is
an inverted glass tube with all the air removed.
The tube has a graduated scale in inches of
Mercury. When the atmospheric pressure changes,
the mercury in the tube rises or lowers and the
reading is taken from the scale.

Aneroid Barometer

An aneroid barometer is mechanical. The
main part is a diaphragm, which is sealed and all the
air is removed. The diaphragm has a series of lever
connected to a pointer, which moves around a scale.
Similarly as the mercurial barometer as the
atmospheric pressure changes, the diaphragm
expand or contracts and moved the pointer
accordingly.

PITOT-STATIC INSTRUMENTS

The primary flight instruments, which use
the pitot-static system are the:

► Airspeed Indicator
► Vertical Speed Indicator
► Altimeter

The Airspeed Indicator

The airspeed indicator measures the speed of
the airplane through the air through which it is
passing. This is done by measuring the
difference in pressure between the ram air
and static air. The airspeed indicator is the
only instrument to operate using both pitot
and static pressure. The speed of the airplane
through the air is determined by comparing
ram air pressure with static air pressure – the
greater the differential, the greater the speed.

Many different factors affect the airspeed indicator
so corrections have to be made to get true readings.
The airspeed indicator tells the pilot what his speed
would be if he was flying at sea-level at a standard
temperature of 15 degrees C and a barometric
pressure of 29.92 inches of mercury.
The airspeed most commonly used is the one which
is read directly from the airspeed indicator and
referred to as indicated airspeed. Important
airspeeds such as take-off, landing and stall speeds
are always the same indicated airspeed. Other types
of airspeeds used by pilots are calibrated airspeed,
true airspeed and ground speed.

Airspeed Indicator

Airspeed Indicator



The Vertical Speed Indicator (VSI)

A pilot needs to know how fast the aircraft is
climbing or descending. The instrument he
uses is the vertical speed indicator, sometimes
referred to as the Vertical Velocity Indicator
(VVI) .
This instrument measures the difference in
atmospheric air pressure when climbing or
descending.

The way this is done is quite simple. The
instrument case itself traps static air pressure,
but allows a very small amount to leak in or
out of the case.
The other part is a type of diaphragm or small
metal container attached to the static system.
This diaphragm will instantly expand or
contract according to a change in atmospheric
pressure when the airplane goes up or down.
The difference is then measured and shown on
the indicator.



Vertical Velocity Indicator

The Altimeter
One of the most important instruments in

the cockpit is the instrument, which measures the
height above sea level, known as altitude.
There are many ways to measure the altitude of
an aircraft. Probably the simplest is with a
barometric altimeter. It is well known that
pressure of the earth’s atmosphere decreases as
height above the earth increases. If this pressure
difference is measured by some mechanical
means, it can be directly related to the height in
feet, meters, or other linear measurement.

The altimeter is an aneroid barometer,
which, in effect, measures the actual outside
air pressure wherever the aircraft is located
and transforms the information as altitude.

Inside the altimeter is a sealed diaphragm.
When the air pressure outside this diaphragm
changes, the diaphragm will expand (get
bigger) or contract (get smaller). This
change is then, through mechanical means,
shown on the instrument face as aircraft
altitude.

For this instrument to be accurate, we must first set
the (sea level pressure) on the instrument. This
setting is known as the altimeter setting, and is
received from ATC (Air Traffic Control).
Wherever an aircraft flies, it must have a current
altimeter setting (QNH).

At higher altitudes, the standard pressure setting of
29.92 inches of mercury or 1013.2 Millibars is
used so that all aircraft flying can fly at an altitude
relative to each other because they are use the
same standard setting (QFE). These altitudes are
referred to as flight levels.

Altimeter

THE PITOT-STATIC SYSTEM