Ice Protection

Mod 11.12 issue 1 Page 1

ICE FORMATION,

CLASSIFICATION AND DETECTION INTRODUCTION

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

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

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

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 Pack Snow. Dependent on the circumstances, variations of these forms of icing can occur and two different types of icing may appear simultaneously on parts of the aircraft.

Ice in the atmosphere is caused by coldness acting on moisture in the air. Water occurs 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 drops occur only in rain and in between these are the drops making drizzle. Icing consists of crystals, their size and density being dependent on the temperature and the type of water in the atmosphere from which they form. Snowflakes are produced when a number of these crystals stick together or, in very cold regions, by small individual crystals.

AREAS TO BE PROTECTED

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

a. all aerofoil leading edges

b. engine air intakes (including carburettor intakes)

c. windscreens

d. propellers

e. pitot static pressure heads


Icing – Areas to be ProtectedFigure 1

EFFECTS ON AIRCRAFT

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

a. Decrease in Lift

This may occur due to changes in wing section resulting in loss of streamlined flow 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 decrease due to alteration of the blade profile and increased blade thickness. Vibration may also occur due to uneven distribution of ice along the blades.

Gas Turbine engines may also be affected by ice on the engine intake, causing disturbance of the airflow to the compressor. Furthermore, ice breaking away from the intake, may be ingested by the engine causing severe damage to the compressor blades and other regions within the engine.


e. Inaccuracy of Pitot Static Instruments

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

f. Loss of Inherent Stability

This may occur due to displacement of the centre of gravity caused by the weight of 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).

ANTI-ICING AND DE-ICING SYSTEMS

INTRODUCTION

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

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.

ANTI-ICING SYSTEM

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

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


FLUID SYSTEMS

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

WINDSCREEN PROTECTION

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

Fluid storage tank

Hand operated or electrically driven pump

Supply pipelines

Spray tubes


The diagram illustrates a typical aircraft system in which the fluid is supplied to the spray tubes by two electrically driven pumps.

Typical Fluid De-icing SystemFigure 7

This design enables the system to be operated using either of the two pumps, or both pumps, according to the severity of the icing.


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

Windscreen Auxiliary De-icing SystemFigure 8


AEROFOIL SYSTEMS

The fluids used for aerofoil ice protection are all GLYCOL based and have properties of low freezing point, non-corrosive, low toxicity and low volatility. They have a detrimental effect on some windscreen sealing compounds and cause 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 failure warning light and tank contents indicator.

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

Fluid is supplied to the pump by gravity feed from the tank and is then directed under pressure to the distributors on the aerofoil leading edges. After an initial 'flood' period, during which the pump runs continuously to prime the pipelines and wet the leading edge, the system is then controlled by a cyclic timer which turns the 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 the primary and secondary channels, by a central web. The outer part of the channel is closed by a porous metal spreader through which the de-icing fluid seeps to wet the outer surface. The primary and secondary feed channels are interconnected by flow control tubes to ensure an even spread of fluid over the outer surface.

The strips are let into the leading edge so that the porous element is flush with the surface of the leading edge curvature. This type of distributor is rarely used and 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 the metering tube that calibrates the flow rate into a cavity between the plastic sheet and a back-plate. This cavity remains filled when the system is operating and the fluid seeps through the porous stainless steel outer panel. The airflow then directs the fluid over the aerofoil.


The outer panel is usually made of stainless steel mesh although a new technique of laser drilling of stainless steel sheet is appearing on some new aircraft.

Fluid De-icing System with Distribution PanelsFigure 9


PUMP TANK

MAIN FEED

FILTER VENT

DISTRIBUTOR

PANELS

DISTRIBUTOR

PANELS

GALLEY

PIPES

When 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 and causing blockage of the metering tubes, porous surfaces and pipelines.

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

Section of a TKS Distribution PanelFigure 10

PROPELLER SYSTEMS

It is necessary to de-ice the propeller blade root and a section of the propeller blade to prevent the build up which could change the blade profile and upset the aerodynamic characteristics of the propeller. Uneven ice build up will also introduce imbalance of the propeller and cause vibration. The leading edge of the 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 pipeline from a slinger ring on the spinner back plate. From the cuff the fluid is spread along the leading edge of the blade by centrifugal force.


Fluid is fed into the ‘slinger ring’ from a fixed pipe on the front of the engine.

Propeller ‘Slinger Ring’ De-IcingFigure 11

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 deflation of rubber tubes on aerofoil leading edges. The system is employed in certain types of piston engine and twin turbo-propeller aircraft. The number of components comprising a system and the method of applying the operating principle will vary but a typical arrangement is shown.

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


Depending 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 to bleed off accumulations of static electricity.

Pneumatic De-Icing BootsFigure 12

AIR SUPPLIES

The tubes in the overshoes are inflated by air from the pressure side of an engine driver 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 operating sequence, and whenever the system is switched off, the boots are deflated by vacuum derived from the vacuum pump or from the venturi section of an ejector nozzle in systems using the engine compressor tapping.

DISTRIBUTION

The method of distributing air supplies to the boots depends on the system required 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


OPERATION

When the system is switched on, pressure is admitted to the boot sections to inflate groups of tubes in sequence. The inflator weakens the bond between ice and the boot surfaces and cracks the ice that is carried away by the airflow. At the end of the inflation stage of the operating sequence, the air in the tubes is vented to atmosphere through the distributor and the tubes are fully deflated by the vacuum source. The inflation and deflation cycle is repeated whilst the system is switched on. When the system is switched off, vacuum is supplied continually to all tubes of the overshoes to hold the tubes flat against the leading edges thus minimising aerodynamic drag.

Pneumatic De-Icing Boots - OperationFigure 14

THERMAL (HOT AIR) SYSTEM

The thermal (hot air) system fitted to aerofoils for the purpose of preventing the formation of ice employs heated air ducted span-wise along the inside of the leading edge of the aerofoil and distributed between double thickness skins. Entry to the leading edge is made at the stagnation point where maximum temperature is required. The hot air then flows back chord-wise through a series of 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 leading edges, but in de-icing systems it is usual to supply more intensely heated air for shorter periods on a cyclic basis.

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

ELECTRICAL ICE PROTECTION SYSTEN

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

HEATER MAT

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

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


Electrical Anti-Icing Heater Mat Figure 1

Anti-icing

Anti-iced areas have their heat supplied continuously, the heating intensity being graded such that under operating conditions no ice formation occurs. The heat is regulated by means of either a sensing element embedded in the mat and an associated thermal controller or a surface mounted thermostatic switch which is pre-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 cyclic switch. The detailed design of the cycling switch depends upon the loading and type of power supply, e.g. dc or 3-phase ac. Its operation is controlled either by timed impulses from a pulse generator or by an electronic device built into the switch.

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

At a relatively high ambient temperature the atmospheric water content, and consequently the rate of icing, is likely to be high but only a comparatively short heating period will be required to shed the ice. At very low temperatures the atmospheric water content and rate of icing are lower and longer heating periods are 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, as estimated from indications of ambient air temperature, or by an automatic control system in which the ON:OFF periods are varied by signals derived from an ambient air temperature probe, working in conjunction with either an ice detector or a rate of icing indicator.

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


RAIN REPELLANT AND RAIN REMOVAL

WINDSCREEN CLEARING SYSTEMS

Vision through windscreens may become obscured by factors other than ice and misting. For example, rain, dust, dirt and flies can impair vision to an extent where methods of clearing the screens must be provided to enable safe ground manoeuvring, take off and landing. Windscreen clearing systems may be considered 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.


WINDSCREEN WIPER SYSTEMS

ELECTRICAL SYSTEM

In this type of system the wiper blades are driven by an electric motor(s) taking their 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 is maintained through one of the screens in case one system should fail.

The following diagram shows a typical electrical wiper and installation. An electrically operated wiper is installed on each windscreen panel. Each wiper is driven 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 the assembly provides an attachment for the wiper arms.

Electric Windshield Wiper SystemFigure 25


The wiper is controlled by setting the wiper control switch to the desired wiper speed. When the "high" position is selected, relays 1 and 2 are energised. With both relays energised, fields 1 and 2 are energised in parallel. The circuit is completed and the motors operate at an approximate speed of 250 strokes/minute. When the "low" position is selected, relay 1 is energised. This causes fields 1 and 2 to be energised in series. The motor then operates at approximately 160 strokes/minute. Setting the switch to the OFF position allows the relay contacts to return to their normal positions. However, the wiper motor will continue to run until the wiper arm reaches the "park" position. When both relays are open and the park switch is closed, the excitation of the motor is reversed. This causes the motor to move off the lower edge of the windscreen, opening the cam operated park switch. This de-energises the motor and releases the brake solenoid applying the brake. This ensures that the motor will not coast and re-close the park switch.

Windshield Wiper Circuit DiagramFigure 26


The 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 is preferred as it provides a greater swept surface, but the operating mechanism is more complex.

Windshield Wiper Swept AreasFigure 27

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 two independently operated motors powered from each hydraulic system with control valves operated from a selector on the flight deck


Figure 28

WINDSCREEN WIPER SERVICING

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

Inspection

a. Examine the system for cleanliness, security, damage, connections and locking

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

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

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


Operational Check

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

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 Maintenance Manual. This is carried out by attaching a spring balance to the wiper arm at its point of attachment to the wiper blade and lifting at an angle of 90º. If the tension is not within the required limits, the spring may be adjusted by the appropriate 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 involve re-positioning the operating arm on the drive spindle. Where a parallel motion bar is used, the length of the tie rod may be altered to vary the angle of sweep.

c. Proper parking of the wipers are essential to ensure that they do not obscure vision. If the wipers do not park as they should, they should be adjusted by the 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).


PNEUMATIC RAIN REMOVAL SYSTEMS

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

Pneumatic Rain Removal SystemFigure 29

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

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.


The 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 pump or by pressurising the top of the reservoir with compressor bleed air via a pressure 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 of the reservoir and checks for security, leaks and damage.

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


RAIN REPELLANT

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

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

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

Examples of rain repellent systems are shown.

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


Combined Windscreen Wash And Rain Repellent SystemFigure 31


The system shown below is a rain repellent only system and uses a disposable pressurised canister.

Rain repellent SystemFigure 32

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

The system may be used with, or without wipers, depending on the aircraft speed, but it is normally used to supplement the wipers in heavy rain at low altitude 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 used as this will increase the smearing. The screen should be washed with clean water immediately. The windscreen wash system, if fitted, may be used.

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


Date post:	12-Jan-2016
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Ice Protection

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