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Aircraft Instruments
Mechanical Instruments
Primary flight instruments of many aircraft rely on direct measurement of aerodynamic pressure to predict the
altitude, airspeed and climb rate of the aircraft. Other primary instruments rely on the effects of gyroscopic
motion to measure rates of pitch, roll and yaw or aircraft attitude. These instruments use simple mechanical
means to display information to the pilot. Schematic diagrams and explanations of the working of these
instruments are shown below.
Altimeter
The altitude of an aircraft can be obtained by using a
measurement of the static pressure of the surrounding
atmosphere. A single pressure line from the Static Port is all
that is required as input for the instrument. Although the rate
of change of pressure with altitude is well known, based on
an International Standard Atmosphere model, the ambient
sea level pressure at any location of the earth's surface can
vary from day to day due to meteorological conditions. The
instrument will require zeroing before and sometimes during
the flight. This is done by setting the instrument to the
ambient pressure of the airport at the start of the flight and
resetting as required. It is therefore important to remember
that this instrument measures height above sea level and
not height above current ground location. For accuracy of
measurement the instrument usually has at least two
sometimes three indicator needles operating on different
scales. For the instrument shown the larger needle indicates
100's of feet and the smaller 1000's of feet. Ambient sea
level pressure pre-set value is shown in the rectangular box on the right (measured in HectoPascals). Click
on the Altimeter image to view a schematic diagram showing its internal operation.

Airspeed Indicator
The airspeed of an aircraft can be obtained from the
difference between atmosphere static pressure and the
measured total pressure from a pitot tube placed in the
airflow. The two pressures required come from lines to
the Static Port and an second to the Pitot Port. The
difference between these pressures is the dynamic
pressure created by the motion of the vehicle through
the air.
1    2
Dynamic Pressure =  V
2
where V is the velocity of the aircraft and    is the
density of the surrounding air.

As the instrument does not have information on the
actual density of the air at the aircraft's altitude, an
assumed standard sea level density is used (1.225
Kg/m3). The true airspeed may be significantly higher as
the ambient density at the altitude will be lower than the sea level value. A correction will need to be made to
calculate true airspeed.
At high speed, where the surrounding air starts to be compressed by the flight of the vehicle, the assumed
sea level standard density will again be inaccurate and a further correction for compressibility made to the
IAS (indicated airspeed) → EAS (equivalent airspeed) : correction for compressibility
EAS → TAS (true airspeed) : correction for altitude.

TAS= EAS×
   ssl

The instrument only measures speed relative to the surrounding air so actual ground speed will need to be
calculated based on prevailing wind conditions.

The instrument shown gives a reading of indicated airspeed in knots (nautical miles per hour). Click in the
image of this instrument to see a schematic diagram showing its operation.

Vertical Speed (Climb/Descent Rate)

The vertical speed indicator shows the current rate of climb
or descent of the vehicle in feet per minute. It is feed from
a pressure line from the Static port in a similar fashion to
the altimeter. However this instrument does not measure
the absolute pressure but the rate at which the surrounding
static pressure is changing. Its internal metering system is
calibrated to give the rate of change of altitude equivalent
to the measured rate of change of pressure.
Click on the image of this instrument to see a schematic
diagram showing its internal operation.

Static Port

The pressure of the surrounding atmosphere is obtained through a flush mounted static port. This is usually
located on the side of the fuselage in a position which will have a local surface pressure which is closely
matching the stream ambient atmospheric air pressure. The position must be calibrated and not where there
is significant pressure change due to the pressure field of the wings, flow separations from joins or
protrusions on the fuselage, propeller slipstream or jet wake effects.
Pitot Port

A pitot tube is used to measure the kinetic energy of the airflow due to the motion of the aircraft. The tube
protrudes into the airstream and is aligned with the flow. The airstream impacting on the open end of the
tube is brought to rest. The pressure at this opening will thus be the sum of the static pressure of the stream
and its dynamic pressure. This is the total pressure of the flow field.

Pitot-Static Probe
In some cases, especially fro wind tunnel laboratory work, the pitot and static ports are merged into a single
unit, the pitot-static probe. The two tubes one inside the other can supply both the stream static pressure
and the stream stagnation pressure to a measurement system.
Turn and Bank Indicator
In order to determine rate of turn and correct balance for the bank angle required for a turn this instrument
combines two separate measurements.
The first is a simple curved tube filled with liquid, in which sits a ball with reasonable mass. The ball will
position itself in the tube depending on the resultant local
acceleration vector. In steady level flight this will be just due
to the effect of gravity and the ball will sit in the centre. For a
balanced turn the resultant of the gravity vector and the
angular acceleration vector will still keep the ball in the
center. If the ball is off centre then the accelerations are not
aligned correctly with the aircraft fuselage and a sliding or
slipping turn will result.
The rate of turn is measured by a gyroscope inside the
instrument. Click on the Turn and Bank Indicator image to
see a schematic view of the gyroscope and its operation.