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PROTECTIONengineeringuk

Contents

12 ICE FORMATION, ........................................................................ 12-3

12.1 CLASSIFICATION AND DETECTION INTRODUCTION ...................... 12-3

12.2 FACTORSAFFECTINGICEFORMATION ................................. 12-3

12.3 TYPESOFICEFORMATION ..................................................... 12-312.3.1 Hoar Frost ..................................................................... 12-312.3.2 Rime Ice ........................................................................ 12-412.3.3 Glaze Ice ....................................................................... 12-412.3.4 Pack Snow .................................................................... 12-5

12.3.5 Hail ............................................................................... 12-512.4 AREASTOBEPROTECTED...................................................... 12-5

12.4.1 Effects On Aircraft ......................................................... 12-612.4.2 Effects of Icing on The Ground ...................................... 12-7

12.5 ICEDETECTION ........................................................................ 12-7

12.6 METHODSOFICEDETECTION ................................................ 12-712.6.1 Visual (Hot Rod) Ice Detector) ....................................... 12-8Pressure Operated Ice Detector Heads ...................................... 12-9Serrated Rotor Ice Detector Head ............................................... 12-1012.6.4 Vibrating Rod Ice Detector ............................................ 12-11

12.7 ICE FORMATION SPOT LIGHT ......................................................... 12-12

12.8 ANTI-ICING AND DE-ICING SYSTEMS.............................................. 12-1212.9 INTRODUCTION ............................................................................. 12-12

12.9.1 De-icing ......................................................................... 12-1212.9.2 Anti-icing System .......................................................... 12-13

12.10 DE-ICING/ANTI-ICING SYSTEMS-GENERAL................................. 12-13

12.11 FLUID SYSTEMS ........................................................................... 12-1312.11.1 Windscreen Protection .................................................. 12-1312.11.2 Aerofoil Systems ........................................................... 12-1612.11.3 Propeller Systems ......................................................... 12-18

12.12 PNEUMATICSYSTEMS ............................................................. 12-1912.12.1 Air Supplies ................................................................... 12-20

12.12.2 Distribution .................................................................... 12-2012.12.3 Controls and Indication ................................................. 12-2012.12.4 Operation ...................................................................... 12-21

12.13 THERMAL(HOTAIR)SYSTEM .................................................. 12-2212.13.1 Exhaust Gas Heating System ....................................... 12-2312.13.2 Hot Air Bleed System .................................................... 12-25

12.14 ELECTRICALICEPROTECTIONSYSTEN ................................ 12-2712.14.1 Heater Mat .................................................................... 12-2712.14.2 Spray Mat ..................................................................... 12-28Windscreen Anti-icing ................................................................. 12-31

12.15 WINDSCREEN CABIN WINDOW DE-MISTING SYSTEMS..................... 12-33

12.16 RAIN REPELLANT AND RAIN REMOVAL........................................... 12-35

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PROTECTIONengineeringuk12.17 WINDSCREENCLEARINGSYSTEMS ....................................... 12-35

12.18 WINDSCREEN WIPER SYSTEMS ..................................................... 12-3612.18.1 Electrical System........................................................... 12-3612.18.2 Electro-Hydraulic System .............................................. 12-3812.18.3 Windscreen Wiper Servicing ......................................... 12-39

12.19 PNEUMATICRAINREMOVALSYSTEMS.................................. 12-41

12.20 WINDSCREENWASHINGSYSTEM .......................................... 12-41

12.21 RAINREPELLANT ..................................................................... 12-43

12.22 DRAIN MAST HEATING .................................................................. 12-46

12.23 WATER SUPPLY AND DRAIN LINES ................................................. 12-46

12.24 DRAIN MASTS .............................................................................. 12-46

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12 ICE FORMATION,

12.1 CLASSIFICATION AND DETECTION INTRODUCTION

The operation of aircraft in the present day necessitates flying in all weatherconditions and it is essential that the aircraft is protected against the build up ofice which may affect the safety and performance of the aircraft.

Aircraft designed for public transport and some military aircraft must be providedwith certain detection and protection equipment for flights in which there is aprobability of encountering icing (or rain) conditions.

In addition to the requirements outlined above, certain basic standards have to bemet by all aircraft whether or not they are required to be protected by therequirements. These basic requirements are intended to provide a reasonableprotection if the aircraft is flown intentionally for short periods in icing conditions.The requirements cover such considerations as the stability and control balancecharacteristics, jamming of controls and the ability of the engine to continue tofunction.

12.2 FACTORS AFFECTING ICE FORMATION

Ice formation on aircraft in flight is the same as that on the ground; it can be

classified under four main headings, i.e. Hoar Frost, Rime, Glaze Ice and PackSnow. Dependent on the circumstances, variations of these forms of icing canoccur and two different types of icing may appear simultaneously on parts of theaircraft.

Ice in the atmosphere is caused by coldness acting on moisture in the air. Wateroccurs in the atmosphere in three forms, i.e. invisible vapour, liquid water and ice.The smallest drops of liquid water constitute clouds and fog, the largest dropsoccur only in rain and in between these are the drops making drizzle. Icingconsists of crystals, their size and density being dependent on the temperatureand the type of water in the atmosphere from which they form. Snowflakes areproduced when a number of these crystals stick together or, in very cold regions,

by small individual crystals.

12.3 TYPES OF ICE FORMATION

12.3.1 HOAR FROST

Hoar frost occurs on a surface which is at a temperature below the frost point ofthe adjacent air and of course, below freezing point. It is formed in clear air whenwater vapour condenses on the cold airframe surface and is converted directly toice and builds up into a white semi-crystalline coating; normally hoar frost isfeathery.

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PROTECTIONengineeringukWhen hoar frost occurs on aircraft on the ground, the weight of the deposit is

unlikely to be serious, but the deposit, if not removed from the airframe, mayinterfere with the airflow and attainment of flying speed during take-off, thewindscreen may be obscured and the free working control surfaces may beaffected. Hoar frost on aircraft in flight usually commences with a thin layer ofglaze ice on the leading edge, followed by the formation of frost which graduallyspreads over the whole surface.

Again the effects are not usually serious, though some change in the landingcharacteristics of the aircraft can be expected.

12.3.2 RIME ICE

This ice formation, which is less dense than glaze ice, is an opaque, roughdeposit. At ground level it forms in freezing fog and consists of a deposit of iceon the windward side of exposed objects. Rime is light and porous and resultsfrom the small water drops freezing as individual particles, with little or nospreading, a large amount of air is trapped between the particles.

Aircraft in flight may experience rime icing when flying through a cloud of smallwater drops with the air temperature and the temperature of the airframe belowfreezing point. The icing builds up on the leading edge, but does not extend farback along the chord. Ice of this type usually has no great weight, but the dangerof rime is that it will interfere with the airflow over the wings.

If the super-cooled droplets are small enough and the temperature is low, eachdroplet freezes instantly on impact as an individual particle and being a non-adhesive dry powder in the slipstream the accumulation on the aircraft is notserious. This is called "opaque rime".

12.3.3 GLAZE ICE

Glaze ice is the glassy deposit that forms over the village pond in the depth ofwinter. On aircraft in flight, glaze ice forms when the aircraft encounters largewater drops in clouds or in freezing rain (or super-cooled rain) with the airtemperature and the temperature of the airframe below freezing point. It consists

of a transparent or opaque coating of ice with a glassy surface and results fromthe liquid water flowing over the airframe before freezing. Glaze ice may bemixed with sleet or snow. IT WILL FORM IN GREATEST THICKNESS ON THELEADING EDGES OF AEROFOILS AND IN REDUCED THICKNESS AS FARAFT AS ONE HALF OF THE CHORD. Ice formed in this way is dense, toughand sticks closely to the surface of the aircraft, it cannot easily be shaken off andif it breaks off at all, it comes away in lumps of an appreciable and sometimesdangerous size.

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PROTECTIONengineeringukThe main danger of glaze ice is still aerodynamic but also the weight of the ice

produces unequal loading and propeller blade vibrations. Glaze ice is the MOSTSEVERE and most dangerous form of ice formation on aircraft because of itshigh RATE OF CATCH. Super-cooled rain is rare in the British Isles but is morecommon on the Continent and East coast of North America.

12.3.4 PACK SNOW

Normally, snow falling on an aircraft in flight does not settle, but if the temperatureof the airframe is below freezing point, glaze ice may form from the moisture inthe snow. The icing of the aircraft in such conditions, however, is primarily due towater drops, though snow may subsequently be embedded in the ice so formed.

12.3.5 HAIL

Hail is formed when water droplets, falling as rain, pass through icing levels andfreeze.

Air currents in some storm clouds (Cumulo-nimbus) may carry the hail verticallythrough the cloud a number of times, increasing the size of the hailstone at eachpass until it is heavy enough to break out of the base of the cloud and fall towardsearth.

Aircraft encountering this type of ice formation may suffer severe damage in theform of dented skin, cracked windscreens, blocked intakes and serious damage

to gas turbine engines.

12.4 AREAS TO BE PROTECTED

The following areas are critical areas on the aircraft where ice forms andwhere protection is essential.

a. all aerofoil leading edges

b. engine air intakes (including carburettor intakes)

c. windscreens

d. propellers

e. pitot static pressure heads

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IcingAreas to be ProtectedFigure 1

12.4.1 EFFECTS ON AIRCRAFT

The build up of ice on the aircraft is known as 'ice accretion' and, from theforegoing, it is evident that if ice continues to be deposited on the aircraft one, ormore, of the following effects may occur.

a. Decrease in Lift

This may occur due to changes in wing section resulting in loss of streamlinedflow around the leading edge and top surfaces.

b. Increase in Drag

Drag will increase due to the rough surface, especially if the formation is rime.

This condition results in greatly increased surface friction.c. Increased Weight and Wing Loading

The weight of the ice may prevent the aircraft from maintaining height.

d. Decrease in Thrust

With turbo-prop and piston engines, the efficiency of the propeller will decreasedue to alteration of the blade profile and increased blade thickness. Vibrationmay also occur due to uneven distribution of ice along the blades.

Gas Turbine engines may also be affected by ice on the engine intake, causingdisturbance of the airflow to the compressor. Furthermore, ice breaking away

from the intake, may be ingested by the engine causing severe damage to thecompressor blades and other regions within the engine.

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PROTECTIONengineeringuke. Inaccuracy of Pitot Static Instruments

Ice on the pitot static pressure head causes blockage in the sensing lines andproduces false readings on the instruments.

f. Loss of Inherent Stability

This may occur due to displacement of the centre of gravity caused by the weightof the ice.

g. Radio antennae

Reduced efficiency

h. Loss of Control

Loss of control may occur due to ice preventing movement of control surfaces.(This is not usually a problem in flight but may occur on the ground).

12.4.2 EFFECTS OF ICING ON THE GROUND

The effects of ice accretion on the ground are similar to those occurring in flightbut the following additional effects may be caused.

a. Restriction of the controls may occur if ice is not removed from hinges andgaps in the controls.

b. The take off run may be increased because of the increase in weight and

drag.c. The rate of climb may be reduced because the weight and drag are

increased.

12.5 ICE DETECTION

The ANO Schedule 4 states that:

In the case of an aircraft of MTWA exceeding 5700 kg (12500 lb), means ofobserving the existence and build up of ice on the aircraft must be provided.

The equipment will be carried on flights when the weather reports or forecasts

available at the aerodrome at the time of departure indicate that conditionsfavouring ice formation are likely to be met.

12.6 METHODS OF ICE DETECTION

Ice detection systems use one of the following methods of detecting andassessing the formation of ice.

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PROTECTIONengineeringuk12.6.1 VISUAL (HOT ROD) ICE DETECTOR)

This consists of an aluminium alloy oblong base (called the plinth) on which is-mounted a steel tube detector mast of aerofoil section, angled back toapproximately 300 from the vertical, mounted on the side of the fuselage, so thatit can be seen from the flight compartment windows. The mast houses a heatingelement, and in the plinth there is a built-in floodlight.

Hot Rod Ice DetectorFigure 2

The heating element is normally off and when icing conditions are met iceaccretes on the leading edge of the detector mat. This can then be observed bythe flight crew. During night operations the built-in floodlight may be switched onto illuminate the mast. By manual selection of a switch to the heating element theformed ice is dispersed for further observance.

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PROTECTIONengineeringuk12.6.2 PRESSURE OPERATED ICE DETECTOR HEADS

Pressure Operated Ice Detector

Figure 3

These consist of a short stainless steel or chromium plated brass tube, which isclosed at its outer end and mounted so that it projects vertically from a portion ofthe aircraft known to be susceptible to icing. Four small holes are drilled in theleading edge of this tube and in the trailing edge are two holes of less total areathan those of the leading edge. A heater element is fitted to allow the detectorhead to be cleared of ice. In some units of this type a further restriction to the airflow is provided by means of a baffle mounted through the centre of the tube.

Each system comprises an ice detector head, a detector relay and a warninglamp. When in normal flight, pressure is built up inside the tube by the air-

stream, this pressure is then communicated by tubing, to the capsule of anelectro-pneumatic relay tending to expand it and separate a pair of electricalcontacts. When icing conditions are met, ice will form on the leading edge andclose off the holes. As the holes in the trailing edge will not be covered by ice theair-stream will now tend to exhaust the system, collapsing the relay capsule andso closing the relay contacts. Generally these contacts operate in conjunctionwith a thermal device, to illuminate a warning indicator in the flight compartmentand to switch on the heater in the detector head; the latter clears the head of iceand is then switched off allowing continued detection of icing conditions. A heaterenergised by the detector relays, automatically clears the ice from the head, but acam holds the lamp on for a further 4 minutes and the heater for a further 30seconds.

IINNLLEETTHHOOLLEESS

OUTLET HOLESMAST

ELECTRICALCONNECTORS

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PROTECTIONengineeringukShould icing conditions persist and the detector heads again ice up, the cam is

automatically re-set and the time cycle repeated.The pilot will switch on the de-icing system when the warning lights indicate icing conditions. In some systemsthe warning phase is connected to automatically switch on the de-icing system.This cycling will continue until such time that the icing conditions no longer exist.

12.6.3 SERRATED ROTOR ICE DETECTOR HEAD

Serrated Rotor Ice DetectorFigure 4

This consists of a serrated rotor, incorporating an integral drive shaft coupled to asmall ac motor via a reduction gearbox, being rotated adjacent to a fixed knife-edge cutter. The motor casing is connected via a spring-tensioned toggle bar to amicro-switch assembly. The motor and gearbox assembly is mounted on a staticspigot attached to the motor housing and, together with the micro-switchassembly, is enclosed by a cylindrical housing. The detector is mounted throughthe fuselage side so that the inner housing is subjected to the ambient conditions

with the outer being sealed from the aircraft cabin pressure.The serrated rotor onthe detector head is continuously driven by the electrical motor so that itsperiphery rotates within 0.050 mm (0.002 in) of the leading edge of the knife-edgecutter. The torque therefore required to drive the rotor under non-icing conditionswill be slight, since bearing friction only has to be overcome. Under icingconditions, however, ice will accrete on the rotor until the gap between the rotorand knife-edge is filled, whereupon a cutting action by the knife edge will producea substantial increase in the required torque causing the toggle bar to moveagainst its spring mounting and so operate the microswitch, to initiate a warningsignal. Once icing conditions cease, the knife edge cutter will no longer shaveice, torque loading will reduce and allow the motor to return to its normal position

and the micro-switch will open-circuit the ice warning indicator.

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PROTECTIONengineeringuk12.6.4 VIBRATING ROD ICE DETECTOR

This ice detector senses the presence of icing conditions and provides anindication in the flight compartment that such conditions exist. The systemconsists of' a solid state ice detector and advisory warning light. The ice detectoris attached to the fuselage with its probe protruding through the skin. The icedetector probe (exposed to the airstream) is an ice-sensing element thatultrasonically vibrates in an axial mode of its own resonant frequency ofapproximately 40 kHz.

Vibrating Rod Ice detectorFigure 5

When ice forms on the sensing element, the probe frequency decreases. The icedetector circuit detects the change in probe frequency by comparing it with areference oscillator. At a predetermined frequency change (proportional to icebuild-up), the ice detector circuit is activated. Once activated, the ice warninglight in the flight compartment is illuminated and a timer circuit is triggered. The

operation of the time circuit switches a probe heater on for a set period of time toremove the ice warning indicator and returns the system to a detector mode,providing that icing conditions no longer exist. If, however, a further ice warningsignal is received during the timer period, the timer will be re-triggered, thewarning light will remain on and the heater will again be selected on. This cyclewill be repeated for as long as the icing conditions prevail.

DETECTORPROBE

VIBRATING

ROD

SENSORUNIT

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12.7 ICE FORMATION SPOT LIGHT

Many aircraft have two ice formation spot lights mounted one each side of thefuselage, in such a position as to light up the leading edges of the mainplanes,when required, to allow visual examination for ice formation.

Note: In some aircraft this may be the only method of ice detection.

Spotlight Ice detectors

Figure 6

12.8 ANTI-ICING AND DE-ICING SYSTEMS

12.9 INTRODUCTION

There are various methods of ice protection which can be fitted to an aircraft butthey can be considered under one of two main categories, de-icing and anti-icing.

12.9.1 DE-ICING

In this method of ice protection, ice is allowed to form on the surfaces and is then

removed by operating the particular system in the specified sequence.

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PROTECTIONengineeringuk12.9.2 ANTI-ICING SYSTEM

Ice is prevented from forming by ensuring that the ice protection system isoperating whenever icing conditions are encountered or forecast.

12.10 DE-ICING/ANTI-ICING SYSTEMS - GENERAL

There are four primary systems used for ice protection. These are:

1. Fluid

2. Pneumatic

3. Thermal

4. Electrical

12.11 FLUID SYSTEMS

These may be used either as an anti-icing or de-icing system. When used as ananti-icing system it works on the principle that the freezing point of water can belowered if a fluid of low freezing point is applied to the areas to be protectedbefore icing occurs. When used as a de-icing system the fluid is applied to theinterface of the aircraft surface and the ice. The adhesion of the ice is brokenand the ice is carried away by the airflow. The system is normally used onwindscreens and aerofoils and has also been used successfully on propellers. Itis not used on engine air intakes - which are usually anti-iced.

12.11.1 WINDSCREEN PROTECTION

The method employed in this system is to spray the windscreen panel with anALCOHOL based fluid. The principal components of the system are:

Fluid storage tank

Hand operated or electrically driven pump

Supply pipelines

Spray tubes

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The next diagram shows a hand pump installation on the HS 125 aircraft where itis used as an auxiliary system.

Windscreen Auxiliary De-icing SystemFigure 8

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PROTECTIONengineeringuk12.11.2 AEROFOIL SYSTEMS

The fluids used for aerofoil ice protection are all GLYCOL based and haveproperties of low freezing point, non-corrosive, low toxicity and low volatility.They have a detrimental effect on some windscreen sealing compounds andcause crazing of perspex panels.

The components in the system are the tank, pump, filter, pipelines, distributors,controls and indicators normally consisting of a switch, pump power failurewarning light and tank contents indicator.

When icing conditions are encountered, the system may be switched onautomatically by the ice detector or manually by the pilot.

Fluid is supplied to the pump by gravity feed from the tank and is then directedunder pressure to the distributors on the aerofoil leading edges. After an initial'flood' period, during which the pump runs continuously to prime the pipelines andwet the leading edge, the system is then controlled by a cyclic timer which turnsthe pump ON and OFF for predetermined periods.

The leading edge distributors appears in one of two forms, i.e. strip and panel.

Strip Distributor

The distributor consists of a 'U' channel divided into two channels, called theprimary and secondary channels, by a central web. The outer part of the channelis closed by a porous metal spreader through which the de-icing fluid seeps towet the outer surface. The primary and secondary feed channels areinterconnected by flow control tubes to ensure an even spread of fluid over theouter surface.

The strips are let into the leading edge so that the porous element is flush withthe surface of the leading edge curvature. This type of distributor is rarely usedand would only be found on very old aircraft.

Panel Distributors

This type of distributor consists of a micro porous stainless steel outer panel, a

micro-porous plastic sheet and metering tube. The fluid passes through themetering tube that calibrates the flow rate into a cavity between the plastic sheetand a back-plate. This cavity remains filled when the system is operating and thefluid seeps through the porous stainless steel outer panel. The airflow thendirects the fluid over the aerofoil.

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PROTECTIONengineeringukThe outer panel is usually made of stainless steel mesh although a newtechnique of laser drilling of stainless steel sheet is appearing on some newaircraft.

Fluid De-icing System with Distribution Panels

Figure 9

PUMP TANK

MAIN

FEED

FILTER VENT

DISTRIBUTOR

PANELS

DISTRIBUTOR

GALLEY

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PROTECTIONengineeringukWhen a system is to be out of service, or unused for an extended period of time,it should be functioned periodically to prevent the fluid from crystallising andcausing blockage of the metering tubes, porous surfaces and pipelines.

Distributors should be cleaned periodically by washing with a jet of water sprayedon to the distributor at an angle.

Section of a TKS Distribution PanelFigure 10

12.11.3 PROPELLER SYSTEMS

It is necessary to de-ice the propeller blade root and a section of the propellerblade to prevent the build up which could change the blade profile and upset theaerodynamic characteristics of the propeller. Uneven ice build up will alsointroduce imbalance of the propeller and cause vibration. The leading edge ofthe propeller blade is therefore de-iced and the ice is shed by centrifugal force.

The blade root has a rubber cuff into which the de-icing fluid is fed by a pipelinefrom a slinger ring on the spinner back plate. From the cuff the fluid is spreadalong the leading edge of the blade by centrifugal force.

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PROTECTIONengineeringukFluid is fed into the slinger ring from a fixed pipe on the front of the engine.

Propeller Slinger Ring De-IcingFigure 11

12.12 PNEUMATIC SYSTEMS

Pneumatic (or mechanical) systems are used for de-icing only, It is not possible

to prevent ice formation and works on the principle of cyclic inflation and deflationof rubber tubes on aerofoil leading edges. The system is employed in certaintypes of piston engine and twin turbo-propeller aircraft. The number ofcomponents comprising a system and the method of applying the operatingprinciple will vary but a typical arrangement is shown.

The de-icer boots (or overshoes) consist of layers of natural rubber andrubberised fabric between which are disposed flat inflatable tubes closed at theends. They are fitted in sections along the leading edges of wing, verticalstabilisers and horizontal stabilisers. The tubes may be laid spanwise, chordwiseor a combination of each method. The tubes are made of rubberised fabricvulcanised inside the rubber layers and are connected to the air supply by shortlengths of flexible hose secured by hose clips.

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PROTECTIONengineeringukDepending on the type specified, a boot may be attached to the leading edge

either by screw fasteners or by cementing them directly to the leading edge skin.

The external surfaces of the boots are coated with a film of conductive material tobleed off accumulations of static electricity.

Pneumatic De-Icing BootsFigure 12

12.12.1 AIR SUPPLIES

The tubes in the overshoes are inflated by air from the pressure side of an enginedriver vacuum pump or, in some types of turbo-propeller aircraft, from a tapping

on the engine compressor. At the end of the inflated stage of the operatingsequence, and whenever the system is switched off, the boots are deflated byvacuum derived from the vacuum pump or from the venturi section of an ejectornozzle in systems using the engine compressor tapping.

12.12.2 DISTRIBUTION

The method of distributing air supplies to the boots depends on the systemrequired for a particular type of aircraft. In general three methods are in use:

shuttle valves controlled by a separate solenoid valve

individual solenoid valves direct air to each boot motor driven valves

12.12.3 CONTROLS AND INDICATION

The controls and indication required for the operation of a system will depend onthe type of aircraft and on the particular arrangement of the system. In a typicalsystem a main ON-OFF switch, pressure and vacuum gauges or indicating lightsform part of the controlling section.

Pressure and vacuum is applied to the boots in an alternating, timed sequenceand the methods adopted usually vary with the methods of air distribution. Inmost installations, however, timing control is affected by an electronic device.

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Pneumatic De-icing System LayoutFigure 13

12.12.4 OPERATION

When the system is switched on, pressure is admitted to the boot sections toinflate groups of tubes in sequence. The inflator weakens the bond between iceand the boot surfaces and cracks the ice that is carried away by the airflow. Atthe end of the inflation stage of the operating sequence, the air in the tubes isvented to atmosphere through the distributor and the tubes are fully deflated bythe vacuum source. The inflation and deflation cycle is repeated whilst thesystem is switched on. When the system is switched off, vacuum is suppliedcontinually to all tubes of the overshoes to hold the tubes flat against the leadingedges thus minimising aerodynamic drag.

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Pneumatic De-Icing Boots - OperationFigure 14

12.13 THERMAL (HOT AIR) SYSTEM

The thermal (hot air) system fitted to aerofoils for the purpose of preventing theformation of ice employs heated air ducted span-wise along the inside of theleading edge of the aerofoil and distributed between double thickness skins.Entry to the leading edge is made at the stagnation point where maximumtemperature is required. The hot air then flows back chord-wise through a seriesof corrugations into the main aerofoil section to suitable exhaust points.

Thermal (Hot Air) de-Icing SystemFigure 15

In anti-icing systems a continuous supply of heated air is fed to the leadingedges, but in de-icing systems it is usual to supply more intensely heated air forshorter periods on a cyclic basis.

Hot gas may be derived from heat exchangers around exhausts, independentcombustion heaters or direct tappings from turbine engine compressors.

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12.13.1 EXHAUST GAS HEATING SYSTEM

The following diagram illustrates the principle of a thermal system using exhaustgases to heat ambient air.

Ambient air enters an intake formed on one side of the engine nacelle and isducted to pass through tubes of a heat exchanger. The exhaust gases from thejet pipe are partially diverted by electrically actuated flaps to flow between thetubes of the heat exchanger before discharging to atmosphere.

The heated air from the heat exchanger passes to a duct containing an

electrically operated hot air valve before passing to the leading edges.In the event of failure of the gas flap in the open position, an emergency manualoverride facility is provided to close the hot air valve and open an actuatoroperated spill valve to direct the hot air overboard.

The gas flap actuator and the hot air valve actuator are electrically interlocked insuch a way that the hot air valve must be fully open before the gas flap opens.Conversely, the gas flap must be fully closed before the hot air valve closes. Thisarrangement, controlled by the limit switches in the actuators, preventsoverheating of the heat exchanger.

Temperature control is automatic with a standby 'manual' facility. A control unit,

in conjunction with 'normal' control and 'overheat' thermistors, provides automaticcontrol and overheat protection. An overheat control unit, in conjunction with an'override' thermistor and flame-stat provides a final overheat protection system.

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Exhaust Gas Heating systemFigure 16

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PROTECTIONengineeringuk12.13.2 HOT AIR BLEED SYSTEM

In this system, air is bled from a late stage of the gas turbine engine compressorbefore being distributed to aerofoil leading edges in the same manner as theexhaust system. The system may be used for anti-icing or de-icing purposes onwing and tail leading edges. It may also be used for ice protection of engineintakes

In principle, the system works by either maintaining the temperature of the skinabove that at which ice occurs or by raising the skin temperature to melt the iceafter it has formed. On aircraft with engines mounted on the rear fuselage,distribution of air along the wing leading edges may be graded to give a higher

intensity of heating for the inboard section. This is to prevent the shedding of iceaccretions into the engine intakes of a size that could result in hazards to theengine.

The following diagram illustrates, in schematic form, a thermal system for a fourengine aircraft.

In operation, anti-icing shut off valves on each engine open to supply air to theleading edge ducting at temperatures of about 200C. Wing and fuselage crossover ducts ensure a supply to all surfaces in the event of an engine shut down inflight.

Hot Bleed Air Anti-icing SystemFigure17

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On some installations, air temperature in the ducting may be controlled by mixingcompressor bleed air with ram air admitted to the system by a cold air controlvalve.

Hot and Ram Air MixingFigure 18

When initially switched on, hot air is fed undiluted into the cold leading edgeducting. Temperature sensors in the leading edge monitor the temperature riseand progressively open and close the cold air valve via an inching unit to controlthe skin temperature. In the event of failure of the:

temperature sensor to control the temperature of the leading edge

cold air valve

or blockage of the ram air inlet, the overhead sensor will control the temperatureby regulation of the hot air valve.

Note: Temperature regulation may also be achieved by controlling the position ofthe hot air valve.

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12.14 ELECTRICAL ICE PROTECTION SYSTEN

Electrical heater elements are attached to the outer surface of the area to beprotected. There are two methods; these being the heater mat and spray mat.

12.14.1 HEATER MAT

This type of element consists of two thin layers of rubber or PTFE sandwiching aheater element. Each mat is moulded to fit snugly over the section to beprotected. Heater elements differ in design, construction and materials accordingto their purpose and environment. The latest mats have elements made from arange of alloys woven in continuous filament glass yarn.

The diagram below shows the application of a heater element to the air intake ofa turbo-prop engine.

Electrical Anti-Icing Heater MatFigure 19

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12.14.2 SPRAY MAT

This type of element is so called because it is sprayed directly on to the surfaceto be protected. The technique was developed by the Napier Company toprovide a lightweight system for use on aerofoils and is ideally suited forapplication to compound curves.

A base insulator is brushed directly on to the airframe and is composed basicallyof synthetic resin. The insulator is normally about 0.03 inches thick although insome cases this may vary. The heater element, made of either aluminium orKumanol (copper manganese alloy) is sprayed on to the base insulation using a

flame spraying technique.

The insulation is of the same material as the base insulation and about 0.01inches thick. Finally, a protective coating is used where the heater requires extraprotection from mechanical damage, eg on leading edges. This protectivecoating known as 'stoneguard' consists of stainless alloy particles bonded withsynthetic resin.

Napier Type Anti-Icing Spraymat

Figure 20

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PROTECTIONengineeringukThe system layout shows the distribution and heating elements on the leading

edges of an aircraft tailplane and fin.

Distribution of Heating ElementsFigure 21

Some of the elements are supplied continuously with electrical power (anti-icing)whilst others are supplied intermittently on a cyclic basis (de-icing). Areasprovided with continuous anti-icing heating are situated immediately in front of

areas on which limited ice formation is tolerable but which require de-icing by thecyclic application of heat. Heating of these areas is rapid in order to breakadhesion as quickly as possible, allowing the detached ice to be blown away bythe airflow. To ensure a clean breakaway of the ice, the cyclically heated areasare separated by continuously heated 'breaker' strips.

A system requiring different intensifies of anti-icing and cyclic de-icing wouldrequire one or more cyclic switches, temperature sensing elements andtemperature control units. In general, control methods may be classified as anti-icing and de-icing.

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PROTECTIONengineeringukAnti-icing

Anti-iced areas have their heat supplied continuously, the heating intensity beinggraded such that under operating conditions no ice formation occurs. The heat isregulated by means of either a sensing element embedded in the mat and anassociated thermal controller or a surface mounted thermostatic switch which ispre-set to give cut-in and cut-out temperature levels.

Cyclic De-icing

Cyclic de-icing areas are usually arranged in groups being connected to a cyclicswitch. The detailed design of the cycling switch depends upon the loading andtype of power supply, e.g. dc or 3-phase ac. Its operation is controlled either bytimed impulses from a pulse generator or by an electronic device built into theswitch.

The timed impulses are set to the appropriate rate for the range of ambienttemperatures likely to be encountered.

At a relatively high ambient temperature the atmospheric water content, andconsequently the rate of icing, is likely to be high but only a comparatively shortheating period will be required to shed the ice. At very low temperatures theatmospheric water content and rate of icing are lower and longer heating periodsare required. The ratio of time ON to time OFF, however, remains unchanged.The typical ratio is 1:10. Setting of the pulse generator may be manual, asestimated from indications of ambient air temperature, or by an automatic controlsystem in which the ON:OFF periods are varied by signals derived from anambient air temperature probe, working in conjunction with either an ice detectoror a rate of icing indicator.

The source of power may be dc, single phase ac or 3-phase ac. In a 3-phasesystem the heated areas are arranged so as to obtain balanced loading ofphases for both anti-icing or de-icing circuits, if possible. De-icing heaters areconnected in such a manner that, as far as practicable, current requirements areconstant. To achieve this the OFF period for certain areas is made to coincide

with the ON period for others.

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12.14.3 WINDSCREEN ANTI-ICING

The windscreens and other criticalwindows in the cockpit (e.g. directvision windows, sliding side windows)of high performance pressurisedaircraft are complicated and expensive

items of the airframe structure as theyare designed to withstand varying airpressure loads, possible shock loadsdue to impact of birds and hailstorms,and thermal stresses due to ambienttemperature changes. In all cases, alaminated form of construction is used,similar to that shown

Typical Laminated Glass WindscreenFigure 22

Laminated glass panels were conceived in order to impart shatter proofcharacteristics to the glass. Such panels are produced by interposing sheets ofclear vinyl plastic (polyvinyl Butyral) between layers of preformed and pre-tempered glass plies. The vinyl and glass plies are then bonded by the

application of pressure and heat.

Since the desired bird-proof characteristics of a windscreen depend to a largedegree, on the plasticity of the vinyl, it therefore follows that it also depends uponits temperature. The optimum temperature range for maximum energyabsorption by the vinyl is between 27C and 49C and the electrically heatedwindscreen panel assemblies are normally maintained within these limits. Belowthis range the bird-proof characteristics decline rapidly and depending upon theactual configuration, a panel's impact resistance can be reduced by 30% to 50%when still at quite a moderate temperature of 16C.

Electric heating of a windscreen therefore is an important factor in maintaining theoptimum bird-proof characteristics.

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PROTECTIONengineeringukThe heating element is an extremely thin transparent conductive coating which is

'floated' on to the inside surface of the outer glass ply; this being normally thinnerin section allows a more rapid heat conduction. The coating may be a tin oxide ora gold film depending on a particular manufacturer's design.

The conductive coating is heated by alternating current supplied to busbars at theedges of the windscreen panel. The power required for heating varies accordingto the size of the panel and the heat required to suit the operating conditions.

Windscreen Temperature ControlFigure 23

The circuit of a typical windscreen de-icing system embodies a controlling device,

the function of which is to maintain a constant temperature at the windscreen andalso to prevent over-heating of the vinyl inter-layer(s). The controlling device isconnected to temperature-sensing elements embedded in the windscreen. Thereare two methods of temperature sensing commonly in use. One of these utilisesa grid in which the resistance of the grid varies directly and linearly withtemperature. The other uses a thermistor, in which the resistance of thethermistor varies inversely and exponentially with temperature.

The number of sensing elements employed depends on the system and circuitdesign requirements. A system of warning lights and/or indicators also forms partof the control circuit and provides visual indications of circuit operating conditions,e.g. 'normal', 'off' or 'overheat'.

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PROTECTIONengineeringukWhen the electrical power is applied, the conductive coating heats the glass.

When it attains a temperature predetermined for normal operation the change inresistance of the appropriate sensing element causes the controlling device toisolate the heating power supply. When the glass has cooled through a certainrange of temperature, power is again applied and the cycle is repeated. In theevent of a failure of the controller, the glass temperature will rise until the settingof the overheat system sensing element is attained. At this setting an overheatcontrol circuit cuts off the heating power supply and illuminates a warning light.The power is restored again and the warning light extinguished when the glasshas cooled through a specific temperature range.

12.15WINDSCREEN CABIN WINDOW DE-MISTING SYSTEMS

Glass is a very poor conductor of heat and at altitude the low atmospherictemperature will maintain the inside of the windscreens and cabin windows at lowtemperature resulting in condensation on the inner surface and obscured vision.

Windscreens are normally kept mist free by blowing hot air, from the airconditioning system, across the inner surface of the glass. In addition, demistingof some windscreens and, usually, all cabin windows is achieved by usingwindows of "dry air sandwich" construction.

This is rather like double-glazing with outer and inner layers of glass sandwichinga layer of dry air between them.

The outer layer of glass is of thick laminate construction (glass and vinyl) to givethe necessary impact and shatterproof qualities. The inner layer of glass is muchthinner allowing it to be warmed by the cabin air temperature, thus preventingcondensation.

The air sandwich is kept dry to prevent internal condensation of the outer glass,by one of two methods:

During manufacture the two layers of glass are hermetically sealed with dry airbetween them.

The space between glass layers is vented to the cabin to allow the pressure in

the air space to equalise with cabin pressure. Venting takes place through adesiccant unit that absorbs moisture from the air during the venting process tomaintain the dry air sandwich.

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PROTECTIONengineeringukOn some larger aircraft the fixed cabin windows are interconnected to a commondesiccant unit whilst escape windows have their own integral unit. The diagramshows typical fixed window and escape hatch desiccant systems.

The desiccant used is Silica Gel crystals which are blue in colour but graduallychange to pink or white as they absorb moisture. Frequent checks must be madeon the state of the desiccant which must be replaced when it begins to turn pink.Failure to take this action may result in condensation within the dry air sandwichwhich may involve lengthy rectification to dry out the sandwich or may require thewindscreen/window to be replaced.

Cabin Window Desiccant SystemFigure 24

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PROTECTIONengineeringuk12.16 RAIN REPELLANT AND RAIN REMOVAL

12.17 WINDSCREEN CLEARING SYSTEMS

Vision through windscreens may become obscured by factors other than ice andmisting. For example, rain, dust, dirt and flies can impair vision to an extentwhere methods of clearing the screens must be provided to enable safe groundmanoeuvring, take off and landing. Windscreen clearing systems may beconsidered under the following headings:

a. Rain clearing systems which can be further broken down into

a. windscreen wipers

b. pneumatic rain removal

c. rain repellent

d. windscreen washing

Windscreen washing systems.

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PROTECTIONengineeringuk12.18WINDSCREEN WIPER SYSTEMS

12.18.1 ELECTRICAL SYSTEM

In this type of system the wiper blades are driven by an electric motor(s) takingtheir power from the aircraft electrical system. Sometimes the pilot's and co-pilot's wipers are operated by separate motors to ensure that clear vision ismaintained through one of the screens in case one system should fail.

The following diagram shows a typical electrical wiper and installation. Anelectrically operated wiper is installed on each windscreen panel. Each wiper isdriven by a motor-converter assembly that converts the rotary motion of the motor

to reciprocating motion to operate the wiper arm. A shaft protruding from theassembly provides an attachment for the wiper arms.

Electric Windshield Wiper SystemFigure 25

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PROTECTIONengineeringukThe path swept by the wiper blade may clear an arc as shown in the diagram on

the left, or in a parallel motion as shown on the right. The parallel motion ispreferred as it provides a greater swept surface, but the operating mechanism ismore complex.

Windshield Wiper Swept AreasFigure 27

12.18.2 ELECTRO-HYDRAULIC SYSTEM

Older aircraft employed hydraulic motors instead of electric motors to drive the

wiper blades. A typical example is shown in the figure below. It consists of twoindependently operated motors powered from each hydraulic system with controlvalves operated from a selector on the flight deck

Figure 28

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12.18.3 WINDSCREEN WIPER SERVICING

Servicing of the windscreen wiper systems consists of inspection, operationalchecks, adjustments and fault finding.

Inspection

a. Examine the system for cleanliness, security, damage, connections andlocking

b. Examine blades for security, damage and contamination. Blades should bereplaced at regular intervals.

c. Check level of fluid in pump reservoir (electro-pneumatic system)

d. Examine hydraulic pipes for leakage and electrical cables for deteriorationand chafing

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Operational Check

Before carrying out an operational check, the following precautions must betaken:

a. Ensure that the windscreen is free of foreign matter

b. Ensure that the blade is secure and undamaged

During the check ensure that the windscreen is kept wet with water.

NEVER operate the windscreen wipers on a dry screen. It may cause scratches.

Adjustments

The following adjustments may be made:

a. Blade tension should be adjusted to the value stated in the MaintenanceManual. This is carried out by attaching a spring balance to the wiper arm atits point of attachment to the wiper blade and lifting at an angle of 90. If thetension is not within the required limits, the spring may be adjusted by theappropriate pressure adjusting screw.

b. Blade angle should be adjusted to ensure that the blade does not strike the

windscreen frame. This would cause rapid blade damage. This may involvere-positioning the operating arm on the drive spindle. Where a parallel motionbar is used, the length of the tie rod may be altered to vary the angle ofsweep.

c. Proper parking of the wipers are essential to ensure that they do not obscurevision. If the wipers do not park as they should, they should be adjusted bythe method laid down in the Maintenance Manual.

Trouble shooting may be carried out using charts in the Maintenance Manual(Chapter 30-42-0 in the ATA100 Scheme).

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PROTECTIONengineeringuk12.19 PNEUMATIC RAIN REMOVAL SYSTEMS

Windscreen wipers suffer from two basic problems. One is that at speed theaerodynamic forces tend to reduce the blade pressure on the screen and causeineffective wiping. The other problem is to achieve blade oscillation rates that arehigh enough to clear the screen during heavy rain.

Pneumatic Rain Removal SystemFigure 29

Pneumatic rain clearance systems overcome these problems by using highpressure bleed air from the gas turbine engine and blowing it over the face of thewindscreen from ducts mounted at the base of the screen. The air blast forms abarrier that prevents the rain spots from striking the screen.

12.20 WINDSCREEN WASHING SYSTEM

A windscreen washing system allows a spray of fluid (usually de-icing fluid, e.g.Kilfrost), to be directed on to the windscreens to enable the windscreen wider to

clear dust and dirt from dry windscreens in flight or on the ground.

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PROTECTIONengineeringukThe fluid is contained in a reservoir and sprayed on to the screen through

nozzles. The fluid may be directed to the nozzles by an electrically driven pumpor by pressurising the top of the reservoir with compressor bleed air via apressure reducing valve.

An example of an electrically driven system is shown.

Electrically Driven Windscreen Wash SystemFigure 30

Servicing of the system involves functionally testing the system, replenishment ofthe reservoir and checks for security, leaks and damage.

The system may be used in flight and on the ground.

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PROTECTIONengineeringuk12.21 RAIN REPELLANT

When water is poured onto clear glass it spreads evenly to form a thin film. Evenwhen the glass is tilted at an angle and subjected to an air stream, the glass willremain wetted and reduce vision. However, when the glass is treated with certainchemicals (typically silicone based), the water film will break up and form beadsof water, leaving the glass dry between the beads. The water can now be readilyremoved.

This principle is used on some aircraft for removing rain from windscreens.

The chemical is stored in pressurised, disposable cans and is discharged on tothe windscreen through propelling nozzles.

Examples of rain repellent systems are shown.

The following system shows a combined rain repellent and windscreen washingsystem.

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Combined Windscreen Wash And Rain Repellent SystemFigure 31

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PROTECTIONengineeringukThe system shown below is a rain repellent only system and uses a disposablepressurised canister.

Rain repellent SystemFigure 32

The system is operated by a push button which causes the relevant solenoidvalve to open. Fluid from the container is discharged onto the windscreen for aperiod of about 5 seconds under the control of a time delay unit. About 5cc offluid is used with each discharge from the container which holds approximately 50cc. The solenoid will be de-energised and the button must be re-selected for afurther application. The fluid is spread over the screen by the rain which acts asa carrier.

The system may be used with, or without wipers, depending on the aircraftspeed, but it is normally used to supplement the wipers in heavy rain at lowaltitude where airspeeds are low.

It is essential that the system is not operated on dry windscreens because:

heavy undiluted repellent will cause smearing

the repellent may form globules and distort vision

If the system is inadvertently operated, the windscreen wipers must not be usedas this will increase the smearing. The screen should be washed with cleanwater immediately. The windscreen wash system, if fitted, may be used.

Rain repellent residues can cause staining or minor corrosion of the aircraft skin.

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PROTECTIONengineeringuk12.22 DRAIN MAST HEATING

On many large aircraft, the water supply and water drain lines are electricallyheated to prevent ice formation. Power is normally supplied via the AC bus lineand is available both on the ground and in flight.

12.23 WATER SUPPLY AND DRAIN LINES

Heater tapes and blankets are wrapped around some water supply and drainlines, the temperature being controlled by thermostats. In a typical aircraft(Boeing 757), the thermostats control the heating, to open when the temperatureexceeds 15.5C and closes when the temperature drops to 7.2C. Heatinggaskets may be installed on the ends of toilet drain pipes.

12.24 DRAIN MASTS

Drain masts are heated to allow in-flight drainage without freezing. Drain mastheating is controlled by an air/ground relay. Low heat is supplied on the groundand high heat in flight.

Figure 37 overleaf illustrates some of the heating methods used.

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Waste Water Heater ComponentsFigure 33

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