Course ObjectiveDescribe the historical development of

aircraft instruments.Identify the first instruments used on

aircrafts and their purpose.Identify why standards and requirements are

needed for aircraft instrumentsDescribe what an airworthiness requirement

is and who sets it.

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Course Objective (cont….)Describe the requirements regarding

location, visibility and grouping of aircraft instruments.

Identify the different types of instrument grouping categories and the indicators in each category.

Identify the different types of instrument displays used on aircraft.

List the different types of panels used on the aircraft.

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Text BookAircraft Instruments, 2nd Edition

By EHJ Pallett

Chapter 1: Requirements and StandardsChapter 2: Instrument elements and mechanismsChapter 3: Instrument displays, panels and layoutsChapter 5: Primary Flight InstrumentsChapter 6: Heading Indicating Instruments

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A. Historical BackgroundThe first successful airplanes flown by the

pioneers were “stick and string”, with temperamental engines.

The airplane was maneuvered by the pilot (without cockpit)lying,sitting or crouching in the open

Instruments designed specifically for use in an airplane were non-existent.

5aircraft instruments

Cont’dIt is a little difficult to specify in what

sequence instruments were introduced into airplanes.Magnetic compass – to fly from point A to B.Fuel gauge – a glass sight gauge to

monitor amount of petrol in the tank

Clock – to calculate speed from a time/distance method, as an aid to

navigation.

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Boeing 767

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Boeing 747 Cockpit

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Boeing 777-200

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UNIT -I

INSTRUMENT DISPLAYS, PANELS AND

LAYOUTS

Man-Machine Loop System In flight an airplane and its operating crew form a man-machine system

loop.Depending on the size and type of aircraft, this loop could be fairly simple

or very complex.The function of the crew within the loop is that of CONTROLLER and

the extent of the control function is governed by the simplicity or otherwise the machine as an integrated whole.

Depending upon the control function the pilots’ duty can be that of :a controller – if the pilot is flying the aircraft manually, ora monitor – if autopilot is flying the aircraft.

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InstFeedback

Pilot aircraft

Man-Machine Closed-loop systemController/monitor controlled

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Cont…Instruments play a vital role as they are the means of

communicating data between systems and the controller/pilot.As a result the content and form of the data displayed is very

important.The most common forms of data display applied to aircraft instruments

are:

1. Quantitative – In which the variable quantity being measured is presented in terms of a numerical value and by the relative position of a pointer or index .

2. Qualitative – In which the information is presented in symbolic or pictorial form.

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1. Quantitative Displays There are three principal methods:

i. The circular scale or clock type.ii. Straight scale iii. Digital, or counter

I. Circular Scale May be considered as the classical method of

displaying information in quantitative form . It has clock type presentation or display.

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Circular scaleThe main components in a circular scale quantitative display

are:Scale baseScale marksScale spacingScale lengthPointer or index

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Scale BaseAlso known as Graduation circle.It refers to the line which may be actual or

implied ,running from end to end of scale and from which the scale marks and line of travel of the pointer are defined.

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Scale marksAlso called graduation marks.Are the marks which constitute the scale of the instrument.The number of marks should be chosen carefully:

Too few marks dividing the scale, vital information may be lost and reading errors may occur.

Too many marks, time will be wasted since speed of reading decreases as the number of markings increases.

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Scale marks - rulesThe scales will be divided so that the marks represent units of

1, 2 or 5 or decimal multiples thereof.The size of the marks to be numbered are generally the largest

while those in between are shorter and usually all of the same length.

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Scale spacingThis is governed by physical laws related to the quantity to be

measured. Hence, there cannot be complete uniformity between all

quantitative displays.In general, have two groups:

Linear – scales with evenly spaced marks, orNon - linear – non-evenly spaced marks

Square law type (airspeed), orLogarithmic type (rate of altitude changes)

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Linear and non-linear scales

linear

logarithmic

Square law

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ALTITUDE METERE

NumberingThe sequence of numbering always increases in a clockwise

direction, except for instruments having a centre zero.

As in the case of marks, numbering is always in steps of 1, 2 or 5 or decimal multiples thereof.

The numbers may be marked on the dial either inside or outside the scale base.

Scale lengthThe distance between the centers of the marks indicating the

minimum and the maximum values of the chosen range of the measurement and measured along the scale base .

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Numbering… Cont’dGoverning factors in the choice of scale length for a particular

range are:the size of the instrument

the accuracy with which it is needed to be read.the conditions under which it is to be observed.

Theoretically, the length of a scale designed for observing at a distance of 30 in and capable of being read to 1% of the total indicated quantity, should be about 2 inches regardless of the shape.

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Scale length - standardizationBut to retain legibility of aircraft instruments at different

conditions, the following standard have been adopted:Instruments displaying information which is to be read

accurately and at frequent intervals have scales about 7 inch in length fitting into standard 31/4 in cases, and

Instruments requiring only occasional observation, or from which only approximate readings are required, have shorter scales and fit into smaller cases.

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High Range Long-Scale DisplayFor the measurement of some quantities such as turbine

engine rev/min, airspeed and altitude, high measuring ranges are involved with the result that very long scales are required.

To overcome these problems, we useConcentric scalesFixed and rotating scalesCommon scale, triple pointerSplit pointer

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High Range Long-Scale Display

Common scale, triple pointers

Concentric scales

Split pointer

Fixed and rotating scales

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Concentric scalesTo accommodate a lengthy

scale, we split it into two concentric scales – the inner is made a continuation of the outer.

The presentation is using two interconnected pointers of different sizes to avoid mis-reading

Found in turbine engine rev/min indicators.

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Fixed and rotating scalesA single pointer rotates

against a circular scale and drives a second scale instead of a pointer.

The rotating scale is visible through the aperture in the main dial.

Commonly used on airspeed indicators.

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Common scale, triple pointersThree concentric pointers of

different sizes register against a common scale.

Common in altimeters where the large pointer indicating hundreds, the intermediate pointer thousands and the small pointer tens of thousands.

Disadvantage – interpretation of a reading is difficult.

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Split pointerUses an inner and outer scale

and two different sized pointers – appearing as one.

The two pointers rotate together and after completing a revolution, the tip of the longer pointer will be covered and only the shorter pointer continues its movement to register against the inner scale.

Applied to airspeed measurement29

Angle of observationThe angle at which an instrument is to be observed affects the

choice of the correct scale length, and case size

A standard is laid down that no part of an instrument should be obscured by the instrument case when observed at angles up to 300 from the normal.

A method adopted is the fitting of instrument mechanisms inside square cases.

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Parallax errorObserving an instrument at an angle results in errors due to

parallax.The magnitude of error due to parallax is dependant on:

The angle at which the data is observed.The clearance distance between the pointer and the dial

plate.It could be solved by using ‘platform scale’ where the scale

marks are set out on a circular platform which is secured to the main dial plate so that it is raised to the same level as the tip of the pointer.

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Platform scale

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Scale range and operating rangeInstrument scale lengths and ranges usually exceed the

operating range of the system with which the instrument is associated thus leaving part of the scale unused.

This is done to improve the accuracy with which readings may be observed.

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Reading accuracy

Equal scale length and operating range Scale range exceeding operating range34

Straight ScalesIn addition to the circular scale presentation, a quantitative

display may also be of the straight scale (vertical or horizontal) type.

In the field of aircraft instruments, there are very few applications of the straight scale and pointer display, B/C they are not suitable for monitoring of the majority of quantities to

be measured, but the moving tape or thermometer type is utilized.

Advantages:To economize the panel spaceImproved observational accuracy.Can be installed horizontally or vertically.

Sequence of numbering is from bottom to top or left to right.35

Comparison of scales

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Digital DisplayIt is also called veeder counter.Data are presented in the form of letters and numbers, known

as alpha-numeric display.It is most common to use counter in combination with the

circular type of display.It is used in the application to the altimeters and, there are two types of counters,

static counter – baro settingdynamic counter – altitude

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Digital Displays

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Dual indicator displayDesigned principally as a means of conserving panel space.There are two basic forms:

Two separate indicators and scales are embodied in one case.

Two indicators in a case but with the pointers registering against a common scale.

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Dual indicator display

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Colored DisplayProvides a means of indicating specific operational

ranges of the systems with which they are associated, so as to make more rapid assessment of conditions

prevailing with the scanning of instruments.Color may be applied to scales;

in the form of sectors and arcs which embrace the number of scale marks appropriate to the required part of the range, and

in the form radial lines coinciding with appropriate individual scale marks.

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Colored displays …cont’dIt is usual to find that colored sectors are applied to those

parts of a range in which it is sufficient to know that a certain condition has been reached rather than knowing actual quantitative values.

Depending on the condition to be monitored, the colors may beRedYellowGreen

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Range markingsArcs and radial lines are usually called range markings.Definitions of range markings are

RED radial line – maximum and minimum limitsYELLOW arc – take off and precautionary rangesGREEN arc – normal operating rangeRED arc – range in which operation is prohibited.WHITE arc (airspeed only) – indicates the airspeed range

over which the aircraft landing flaps may be extended in the take-off, approach and landing configurations of the aircraft.

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Range markings

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2. Qualitative Displays.These are of special type in which the information is presented

in ,a symbolic or pictorial form, to show the condition of a system whether the value of an output is

increasing or decreasing, by the movement of a component and so on.Example – applications,

Engine synchronizing displaysTo show the movement of flight controls

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Qualitative displays

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Engine synchronizing Position of flight

control systems

A qualitative display example

Director DisplaysAre those which are associated principally with flight attitudes

and, navigational data and presenting it in a manner which indicates the

movements ofGyro Horizon ILS Indicator ,

Are integrated instrument systems of present day aircrafts. There are three elements making up the integrated display

A pointer registering a bank angle against a bank-angle scale;An element symbolizing the aircraftAn element symbolizing the natural horizon.

Both the bank pointer and horizon symbol are stabilized by gyroscope.

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Director displays

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Cont….

ILS + RMI = HORIZONTAL SITUATION INDICATOR (HSI)

Head-up Displays:To present vital flight data at the same level as the pilot’s

line-of-sight when viewing external references, i.e., when he/she maintains a “head up” position.

The principle of the method is to display data on the face of a special Cathode Ray Tube and, to project them optically as a composite symbolic image on to a transparent reflector plate, or directly on the windscreen.

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Head-up display system

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Visual Approach MonitorIt is a head-up display designed to aid pilots when

approaching airfields not equipped with ILS or other approach aids.

The display unit is mounted on a sliding tray located at a glare shield panel in front of the pilot, when required, the tray is pulled out to automatically raise the lens through which the display is projected.

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Cont.The display provides the following cues, based on

selection on a control module:The vertical approach anglehorizontal attitudespeed errors

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Visual approach monitor

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Light Emitting DisplaysThere are two types of light emitting displays used in

aircraft instruments.These are –

Liquid Crystal Display (LCD), and Light Emitting Diode (LED)

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Liquid Crystal Display (LCD)Are found in many state-of-the-art aircraft instruments.The display can be configured to form letter and number

patterns, or it can form a full picture.Commonly, liquid crystals are gray, but they can be full

color.Liquid crystals are fluid materials that contain molecules

arranged in crystal forms.The molecules are typically twisted and therefore “bend”

the light that passes through the crystal.

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LCD – constructionConsists of two glass plates, coated on their inner surface

with a thin transparent conductor such as indium oxide.The conductor on the front plate is etched into a standard

display format of seven bars or segments, each segment forming an electrode.

Each bar is electrically separate and is selected by a logic/driver circuit which causes the bars to illuminate in patterns forming the digit to be displayed.

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Cont.A mirror image of the digits with its associated electrical

contacts is also etched into the oxide layer of the back glass plate,

but this is not segmented since it constitutes a common return for all segments.

The space between the plates is filled with a liquid crystal material, referred to as a nematic (Greek – nemator means thread) material.

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LCD

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L.C.D.

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