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

2m𝑣2 = 𝑃𝑉

»1

2ρ𝑣2 = 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

Engineering

Transcript of Pitot static system