Icing

www.gridline.co.ukwww.gridline.co.uk 1117 MAY 200217 MAY 2002

Icing.Icing.Factors affecting aeroplane Factors affecting aeroplane

performance.performance.

17 MAY 200217 MAY 2002 www.gridline.co.ukwww.gridline.co.uk 22

Introduction. Introduction. • Types of icing, atmospheric Types of icing, atmospheric

conditions where it can be conditions where it can be encountered and piston engine encountered and piston engine induction icing.induction icing.

• Applicable to Single and Multi Engine Applicable to Single and Multi Engine Piston aeroplanes.Piston aeroplanes.


Agenda.Agenda.• Basic causes.Basic causes.• Super cooled water droplets.Super cooled water droplets.• The effects of icing.The effects of icing.• Clear or glazed ice.Clear or glazed ice.• Rime ice.Rime ice.• Rain ice.Rain ice.• Pack snow.Pack snow.• Hoar frost.Hoar frost.• Factors affecting severity of icing.Factors affecting severity of icing.• Freezing level.Freezing level.• Piston engine induction icing.Piston engine induction icing.


Terms.Terms.• Cb Cumulonimbus.Cb Cumulonimbus.• Cu Cumulus.Cu Cumulus.• Ns Nimbostratus.Ns Nimbostratus.• Sc Stratocumulus.Sc Stratocumulus.• St Stratus.St Stratus.• Virga Rain that falls from the base of cloud Virga Rain that falls from the base of cloud

but evaporates before reaching the ground. but evaporates before reaching the ground. • AIC Aeronautical Information Circulars.AIC Aeronautical Information Circulars.• LASORS Licensing, Administration, LASORS Licensing, Administration,

Standardisation, Operating Requirements Standardisation, Operating Requirements and Safety.and Safety.

• AIS Aeronautical Information Service. AIS Aeronautical Information Service.


Introduction to icing and it’s Introduction to icing and it’s basic causes.basic causes.• Air frame icing can cause a serious loss Air frame icing can cause a serious loss

of aircraft performance resulting in an of aircraft performance resulting in an increase in fuel consumption and increase in fuel consumption and difficulty with aircraft control. Ice will difficulty with aircraft control. Ice will form on an airframe if there is:form on an airframe if there is:

• Water in a liquid state;Water in a liquid state;• Ambient air below 0Ambient air below 0°C(but see later);°C(but see later);• Airframe temperature below 0Airframe temperature below 0°C.°C.


Super cooled water Super cooled water droplets.droplets.• A super cooled water droplet is a droplet of A super cooled water droplet is a droplet of

water still in the liquid state although it’s water still in the liquid state although it’s temperature is below 0temperature is below 0°C;°C;

• If the super cooled water droplet contains a If the super cooled water droplet contains a freezing nucleus then the droplet will start to freezing nucleus then the droplet will start to freeze, as the numbers of freezing nuclei are freeze, as the numbers of freezing nuclei are small in the atmosphere super cooling is a small in the atmosphere super cooling is a frequent occurrence;frequent occurrence;

• Super cooled water droplets can exist in clouds Super cooled water droplets can exist in clouds at temperatures as low as –40at temperatures as low as –40°C.°C.


The effects of icing.The effects of icing.• AERODYNAMIC. Icing tends to form on AERODYNAMIC. Icing tends to form on

the leading edges. This spoils the the leading edges. This spoils the aerofoil resulting in aerofoil resulting in reduced lift, reduced lift, increased drag, increased weight, increased drag, increased weight, increased stalling speedincreased stalling speed and and increased fuel consumption.increased fuel consumption.

• WEIGHT. Ice can form at the rate of one WEIGHT. Ice can form at the rate of one inch in two minutes affecting mass and inch in two minutes affecting mass and balance. Ice can break away from balance. Ice can break away from propellers causing engine vibration.propellers causing engine vibration.


The effects of icing.The effects of icing.

• INSTRUMENTS. Blocked pitot and static INSTRUMENTS. Blocked pitot and static heads affect the ASI, VSI and Altimeterheads affect the ASI, VSI and Altimeter..

• GENERAL. Windscreens can be GENERAL. Windscreens can be obscured. A thin film of ice/frost obscured. A thin film of ice/frost increases skin friction. Ice on aerials increases skin friction. Ice on aerials causes static interference.causes static interference.


The effects of icing.The effects of icing.


Clear (Glaze) ice.Clear (Glaze) ice.• Large super cooled water droplets Large super cooled water droplets

in contact with the airframe start to in contact with the airframe start to freeze releasing freeze releasing latentlatent heat. This heat. This delays the process resulting in the delays the process resulting in the flow back of flow back of clear ice. clear ice. As well as As well as changing the aerofoil section this changing the aerofoil section this ice can jam controls;ice can jam controls;

• Clear ice forms in Ns, Cu and Cb at Clear ice forms in Ns, Cu and Cb at temperatures from 0 to -20temperatures from 0 to -20°C°C..


Rime iceRime ice• Small super cooled water droplets Small super cooled water droplets

freeze on impact with the airframe freeze on impact with the airframe sticking to form a white opaque sticking to form a white opaque deposit with a light texture;deposit with a light texture;

• Air intakes can be blocked;Air intakes can be blocked;• Rime ice forms in Ns, As, Ac, St and Rime ice forms in Ns, As, Ac, St and

the parts of heaped clouds where the parts of heaped clouds where super cooled water droplets are super cooled water droplets are small (0 to -40small (0 to -40°C)°C)..


Rain ice.Rain ice.• This is the most dangerous form of icing. It This is the most dangerous form of icing. It

occurs in rain which becomes super cooled occurs in rain which becomes super cooled by falling from an inversion into air below by falling from an inversion into air below 00°C;°C;

• Rain ice builds up very quickly and a Rain ice builds up very quickly and a pilot’s action should be to turn onto a pilot’s action should be to turn onto a reciprocal heading immediately;reciprocal heading immediately;

• Rain ice occurs ahead of a warm or Rain ice occurs ahead of a warm or occluded front at low level during occluded front at low level during moderate to continuous rain at about 1 moderate to continuous rain at about 1 000 feet;000 feet;

• It is rare in the UK, but common in Central It is rare in the UK, but common in Central Europe and North America. Europe and North America.


Formation of Super cooled Formation of Super cooled water droplet water droplet


Pack snow.Pack snow.• This is a combination of super This is a combination of super

cooled water droplets and snow. It cooled water droplets and snow. It can block air intakes and other can block air intakes and other aircraft openings.aircraft openings.


Hoar frost.Hoar frost.• Hoar frost is a Hoar frost is a white crystal depositwhite crystal deposit

which appears similar to frost on the which appears similar to frost on the ground;ground;

• It forms when the airframe temperature It forms when the airframe temperature is below 0is below 0°C and when clear ambient °C and when clear ambient air temperatures drop to saturation air temperatures drop to saturation level;level;

• Water vapour in contact with the Water vapour in contact with the airframe is converted to ice crystals airframe is converted to ice crystals without becoming liquid i.e. without becoming liquid i.e. sublimating;sublimating;

• It must be cleared before take-off. It must be cleared before take-off.


Windscreen icing. Windscreen icing.


Super cooled water Super cooled water droplets.droplets.


Iced Wing. Iced Wing.


Severity of icing.• The size of the super cooled water droplet The size of the super cooled water droplet

is dependant on the cloud type and is dependant on the cloud type and temperature;temperature;

• There is always a greater concentration of There is always a greater concentration of droplets at the base of the cloud where it droplets at the base of the cloud where it is warmestis warmest. Icing severity tends to be: . Icing severity tends to be: Severe in Cu and Cb, moderate to severe Severe in Cu and Cb, moderate to severe in Ns and light to moderate in Sc;in Ns and light to moderate in Sc;

• The orographic effect of a range of hills The orographic effect of a range of hills increases uplift in cloud producing a increases uplift in cloud producing a greater concentration and size of super greater concentration and size of super cooled water droplet. cooled water droplet.


Freezing Level.• The height where the ambient The height where the ambient

temperature is zero is called the temperature is zero is called the freezing level;freezing level;

• Given in area forecasts as the Given in area forecasts as the Zero Zero degree isotherm;degree isotherm;

• With an inversion two freezing levels With an inversion two freezing levels are possible;are possible;

• In the south of the UK the average In the south of the UK the average freezing level in February is 3 000 freezing level in February is 3 000 feet. feet.


Piston Engine Induction Icing.• Impact icing. Impact icing. Intake areas can be Intake areas can be

blocked by blocked by snow, pack snowsnow, pack snow or or super super cooled water droplets;cooled water droplets;

• Fuel icing. Fuel icing. Water in fuel freezing in the Water in fuel freezing in the bends of induction piping;bends of induction piping;

• Carburettor icing. Carburettor icing. This is caused by This is caused by the sudden temperature drop as the sudden temperature drop as latent latent heatheat is absorbed when fuel evaporates is absorbed when fuel evaporates and the temperature drop due to the and the temperature drop due to the adiabatic expansionadiabatic expansion of the air as it of the air as it passes through the venturi. passes through the venturi.


Piston Engine Induction Icing.


Carburettor Icing Notes:• More common withMore common with Mogas; Mogas;• Greater problem with low power settings;Greater problem with low power settings;• Occurs in Occurs in clear airclear air;;• More of a hazard on More of a hazard on warm, humid warm, humid

summersummer days; days;• High humidity indicators are: Wet ground, High humidity indicators are: Wet ground,

dew with light winds, poor visibility near dew with light winds, poor visibility near water in mornings and evenings, flying water in mornings and evenings, flying below or between cloud layers, below or between cloud layers, precipitation, fog, cloud and clear air just precipitation, fog, cloud and clear air just after fog has dispersed. after fog has dispersed.


Carburettor Icing Conditions.


Summary.Summary.• Icing can be hazardous to aviation.Icing can be hazardous to aviation.• Clear ice is a major hazard to flight safety.Clear ice is a major hazard to flight safety.• Rime ice is the most common form of icing.Rime ice is the most common form of icing.• Frost remaining on the wings is dangerous, Frost remaining on the wings is dangerous,

especially during take-off.especially during take-off.• When the air is moist carburettor icing can When the air is moist carburettor icing can

form in temperatures as high as 25form in temperatures as high as 25°C!°C!• An ice laden aeroplane may be completely An ice laden aeroplane may be completely

incapable of flight.incapable of flight.


Where to Get More Where to Get More Information.Information.• Web search engines for “Icing”; Web search engines for “Icing”; • Trevor Thom Air Pilot’s Manual 2-Air Law and Trevor Thom Air Pilot’s Manual 2-Air Law and

Meteorology;Meteorology;• LASORS 2002;LASORS 2002;• General aviation safety sense leaflets 3B General aviation safety sense leaflets 3B

Winter flying and 14 Piston Engine Icing;Winter flying and 14 Piston Engine Icing;• AICs 93(P8)/00, 98(P136)/97 and AICs 93(P8)/00, 98(P136)/97 and

145(P161)/97;145(P161)/97;• www.ais.org.ukwww.ais.org.uk

Documents

Icing