Pitot static system

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Pitot-Static System
Prepared and to be presented by
MD. ATAUL MAMUN
Bangladesh Airlines Training Center Biman

Objectives
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Topic Objectives:
To have idea on earth atmosphere and its impact on
instruments
To learn what is pitot-static system
To learn basic working principles of instruments that use
pitot-static system
To understand the limitations of the system

Introduction
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Within an aircraft the flight crews need to know airspeed,
aircraft altitude, vertical speed etc. for a safe flight.
They get these data from corresponding instruments (airspeed
indicator, altimeter, vertical speed indicator) in the cockpit.
The above mentioned instruments collect data from
environment through pitot probes, static ports etc.
Thus the atmosphere provides much of the basic information
required by a pilot. Before we study pitot-static system
instruments we must first, therefore, understand the properties
of the atmosphere these instruments utilize.

Atmospheric Physics
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The earth’s atmosphere is the surrounding envelope of air (mostly
Nitrogen, 78.09% and Oxygen, 20.95% gas).
The envelope is divided into several layers extending from the
earth’s surface.
The lowest layer is the troposphere, extending to a height of
about 28,000ft (11km) at the equator.
This is the start of the tropopause, which goes on up to about
66,000 ft (20km).
Above this is the stratosphere, extending to the stratopause at
an average height of between 60 and 70 miles.
As all aircraft fly in the troposphere or lower levels of the
stratosphere we will not concern ourselves with other higher layers.

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Atmospheric Pressure:
The atmosphere is held in contact with the earth's surface by
gravity, producing a pressure within the atmosphere.
Gravitational effects decrease with increasing distance from the
earth's center, so that atmospheric pressure decreases steadily
with altitude.
The standard sea-level pressure is 14.7 lb/𝑖𝑛2
and is equal to
29.92 in Hg or 1013.25 mbar.
The rate at which the pressure falls with height is termed as ‘the
lapse rate’. The pressure lapse rate is not linear, but exponential.
Atmospheric Physics cntd.
Standard Pressure Lapse Rate:
1 in Hg per 1000ft. of altitude.

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Atmospheric temperature:
The air in contact with the earth is heated by conduction and
radiation, and as a result its density decreases and the air starts
rising. As it rises its pressure falls, allowing the air to expand,
the expansion in turn causing a fall in temperature.
The air temperature decreases by 1.98°C for every 1,000 feet
increase in altitude from +15°C at MSL to -56.5°C at 36,089
feet (i.e. up to tropopause)
In the stratosphere the temperature at first remains constant at
-56.5°C, then it increases again to a maximum at a height of
about 40 miles

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Figure: ICAO standard atmosphere

What is Pitot-Static System
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A pitot-static system is a system of pressure-sensitive
instruments that is used to determine an aircraft's
airspeed, vertical speed, altitude, and Mach number.
It uses the principle of air pressure gradient i.e. it
measures pressures/pressure differences and uses
these values to determine the speed and altitude.

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Static Pressure:
Static pressure, as the name suggests, is the absolute pressure
(pressure referenced to a vacuum) of the air surrounding the
aircraft.
This is easily obtained whilst the aircraft is stationary on the
ground, but will be affected as the aircraft moves through the
air, giving rise to errors. Modern aircrafts sample static pressure
through pairs of Static Vents.
Basics of Pitot-Static System

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Pitot Pressure:
The pitot pressure is a measure of ram air pressure (the total air
pressure created by aircraft motion)
To understand this, let us consider a probe placed in a flowing
fluid. When the fluid flows at a certain velocity, v over the
probe, it will be brought to rest at the nose
known as the stagnation point.
The stagnation pressure of the fluid,
also known as the total pressure or the
pitot pressure.
Basics of Pitot-Static System cntd.

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At the stagnation point, kinetic energy of the fluid is
converted into pressure energy.
Kinetic energy=pressure energy
1
2
m 𝑣2
= 𝑃𝑉
»
1
2
ρ 𝑣2
= 𝑃
» 𝑣α 𝑃
So, by measuring dynamic pressure we can determine
the fluid velocity.
(P=dynamic pressure=difference
between pitot and static pressure)

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Figure: Measuring airspeed by using pitot and static pressures

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Air Speed Indicator

Airspeed indicator measures the difference between the pitot
and static pressures in terms of the 1/2ρV2 formula i.e. it
measures a differential pressure which varies with the square of
the airspeed.
Air Speed Indicator cntd.
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Pointer movement means capsule deflection. At low speeds
small pointer deflection means large speed variations and vice
versa.
So direct magnification of deflection would give a non-linear
scale reading which is inconvenient to read.
To make the dial linear an arrangement needed so that the
pointer movement is increased for small deflections and
decreased for large deflections i.e. a variable magnification
which is called, in this case, the square-law-compensation

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Figure: Square-law-compensation by rocking lever/sector-arm mechanism
Square-law-compensation

Vertical Speed Indicator
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VSI is a very sensitive differential pressure gauge, designed to
indicate the rate of altitude change from the change of static pressure
alone.
This indicator consists basically of three main components,
I. a capsule,
II. an indicating element and
III. a metering unit with an orifice/
calibrated leak
The orifice is opened to the interior of the
case to apply static pressure to the
exterior of the capsule. It has a time-lag
response characteristic.

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Level flight: zero differential
pressure across capsule
aircraft descending: metering unit maintains
case pressure lower than capsule pressure
aircraft ascending: metering unit maintains
case pressure higher than capsule pressure
Vertical Speed Indicator cntd.

Altimeter
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Figure: Aneroid Barometric altimeter
An altimeter operates on the aneroid barometer principle, i.e.
it responds to changes in atmospheric pressure.
The Altimeter has a sealed evacuated capsule inside a sealed
case.

Altimeter cntd.
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The air pressure on the outside of the capsule tends to squash it,
this being opposed by the leaf spring and the spring action of
the corrugated metal itself. As barometric pressure increases or
decreases, the capsule will be compressed or expanded
respectively.
By the use of an amplifying lever and chain linkage the expansion
and contraction of the capsule is transmitted to a pointer that
moves over a scale, calibrated to show barometric pressure, with
the leaf and tensioning springs maintaining tension in the linkage.
The aneroid comes from the Greek aneros, 'not wet‘.

Altimeter errors due to changes in atmospheric pressure
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The basis for the calibration of altimeters is the standard atmosphere.
If the atmospheric pressure at MSL is not standard, the altimeter will
be in error.
If an aircraft were on the ground on an airfield at sea level with
standard pressure (1013.25 mb, 29.92 in Hg) the altimeter would
indicate zero feet.
If the atmospheric pressure now falls to say, 1012.2 mb (29.89 in Hg)
the altimeter would indicate +30 feet.
If atmospheric pressure had risen to 1014.2 mb (29.95 in Hg) it would
have indicated -30 feet. Similar errors would occur in flight.
There is a Baro correction knob to set the pressure of the day in
millibars, (or inches of Hg), so that the altimeter displays the correct
height.

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Altimeter errors due to changes in atmospheric
temperature cntd.
The standard atmosphere assumes certain temperature values at
all altitudes and consequently non-standard values can also cause
errors in altimeter readings.
Figure: Effect of atmospheric temperature on an altimeter

Altimeter Dial
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Altimeters used to have three
pointers rotating at different rates,
one revolution of a pointer indicating
one thousand, ten thousand and one
hundred thousand feet of altitude
respectively.

‘Q’ Code for altimeter setting
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It is essential for maintaining adequate separation between aircraft
and for terrain clearance during take-off and landing. Meteorological
data is transmitted from ATC, forming part of the ICAO “Q" code of
communication. The three code groups used in connection with altimeter
setting procedures are QNH, QFE and QNE.
QFE Setting the pressure prevailing at an airfield to make the
altimeter read zero on landing and take-off.
QNE Setting the standard sea-level pressure of 1,013.25 mbar
(29.92 in Hg) to make the altimeter read the airfield elevation.
QNH Setting the pressure scale to make the altimeter read airfield
height above sea-level on landing and take-off

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Mechanical or conventional altimeters suffer from friction in their
bearings and mechanical linkages; this leads to the indication lagging
actual altitude by as much as 10% (called ‘hysteresis’). As the aircraft
climbs hysteresis error increases. These limitations can be overcome by
replacing the mechanical linkage between the capsules and pointer
with an electrical servo mechanism.
In servo altimeter a two-phase drag-cup type motor is coupled by a
gear train to the pointer and counter assembly, and also to a
differential gear which drives a cam. The reference phase of the motor
is supplied with a constant ac voltage from the main source, and the
control phase is connected to the amplifier output channel.
Servo Altimeter

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Servo Altimeter cntd.

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When the aircraft altitude changes the capsule respond that and the
displacement of the capsules in transmitted to the I-bar, changing its
angular position w.r.t. the E-bar. The ‘E and I Bar’ converts capsule
movement into an electrical signal; amplitude being proportional to
the amount and phase the direction of that movement. This signal is
amplified and fed to a servomotor to drive the pointer and height
counters in the correct direction. It also, via the worm gear, cam and
cam follower, drives the E bar back to a null position. Indication is
similar to the mechanical altimeter.
The ‘set ground pressure’ knob puts a bias on the E bar, which is then
driven to a null by the servo as before, with the bias appearing as a
change of indicated altitude.
Servo Altimeter cntd.

Typical pitot probes, static ports and their locations
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A pitot probe consists of a pipe
facing into the airflow, with
electrical heating to prevent
icing and a water drain at its
lowest point.
The static vents are cross
connected, by pipework,
in pairs to balance out any
pressure difference caused
by sideslip of the aircraft.

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If failure of the primary pitot-static pressure source occurs, for
example, complete icing up of a probe due to a failed heater
circuit, then it is obvious that errors will be introduced in the
indications of the instruments
As a safeguard against failure, therefore, a standby system may
be installed in aircraft employing pitot-static probes whereby
static atmospheric pressure and/or pitot pressure from alternate
sources can be selected and connected into the primary system.
The required pressure is selected by means of selector valves
connected between the appropriate pressure sources and the
flight instruments, and located in the cockpit within easy reach of
the flight crew.
Alternate Pressure Sources

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Alternate Pressure Sources
Figure: Alternate pitot pressure and static pressure system

Pitot-Static Heating
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To prevent icing, the pitot tubes and static ports have an
arrangement of heating (usually electrical heating.) The
heating elements require 28V DC or 115V AC.
The heating circuit has a control switch as well as an indication
light to know whether or not the circuit is functioning correctly.
In the circuit shown, K1 and K2 are current sensing relays. If
the Pilot has failed to switch the heating on; or a heater
element has gone open circuit; no current will flow through the
relay coil. The relay will de-energise, connecting 28 VDC
from the left essential bus to the Master Caution Logic; thus
illuminating the appropriate Pitot Heat Caution lamp.
(The loss of A330 flight AF 447 in mid-Atlantic, June 2009 was due to icing of the pitot probes. )

Pitot-Static Heating Circuit
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In order for a pitot-static system to operate effectively under all flight
conditions, provision must also be made for the elimination of water
that may enter the system as a result of condensation, rain, snow, etc.,
thus reducing the probability of `slugs' of water blocking the lines.
Such provision takes the form of drain holes in probes, drain traps and
drain valves in the system pipelines.
Drain holes are drilled in probe pitot tubes and casings, and are of
such a diameter that they do not introduce errors in instrument
indications.
Drains

Pipelines
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Pitot and static pressures are transmitted through seamless and
corrosion-resistant metal pipelines. The diameter of pipelines is related
to the distance from the pressure source to the instruments to eliminate
pressure drop and time -lag factors.
It is very important to ensure that there are no leaks in the pipework,
as this would give rise to inaccurate readings. Even though they don't
have to handle high pressures, the instruments are very sensitive to
small changes in pressure so that even very small leaks can cause
errors in the instruments.
The tubing and hoses that are used are not very strong and should be
inspected carefully for damage. The fittings and connections should be
installed with care and torqued to specified values as stated in the
AMM.

Pressure (Position) Error
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Measuring airspeed and altitude by pitot and/or static port has two challenges:
1. to design a probe which will not cause any disturbance to the airflow over it
2. To find a suitable location on the aircraft where the probe will not be affected
by the disturbance due to aircraft movement itself.
A pressure error is introduced in the instrument due to this problem.
Pressure or position error (PE) is defined as the difference between the local static
pressure and the free-stream static pressure.
Altimeter and airspeed indicators suffer from PE most.
By using pressuring error correction transducers, we can minimize the pressure or
position error.

Pitot static system

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