Icing 02 Effects

7/27/2019 Icing 02 Effects

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TET1 aircraft icing- 12/2005- V1 1+WMO

FREEZING CONTAMINATION : AIRCRAFT ICING

EFFECTS ON AIRCRAFT

Different types of accretion

Intensity of ice accretion

Consequences of accretion

Vulnerability factors examples

Specific vulnerabilities

Detection in flight

Removal of accretion : De-icing

Prevention of icing : Anti-icing

Certification and icing conditions

Marginal weather conditions


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Source NASA-Lewis Research Center


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Different types of accretion (1)

Aspect : Crystalline in the form of scales, needles or feathers.

Formation conditions : Sublimation of water vapour into ice. This deposit canoccur without clouds.

Effects : Even if the amount of deposited material is low, it can be significant

under certain conditions.

hoar frost

source Transport Canada


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Aspect : Opaque and white, but rather fragile and brittle.

Formation conditions : On a cold surface in a homogeneous cloud environment

(T


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Aspect : Transparent, homogeneous and smooth, very compact. Its specific mass is

close to the one of pure ice.

Formation conditions : On a cold surface in a homogeneous cloud environmentwith a temperature close to 0C. The supercooled cloud droplets are present in

large quantities and spread out before they slowly freeze.

Effects : Develops in cones on the leading edges and is very significant. Should be

prevented from forming.

clear ice

Source NASA-Lewis Research Centre


6/22



Aspect : Mix of clear ice, hoar frost and rime ice. Whitish and brittle.Formation conditions : On a cold surface in a heterogeneous cloud environment

where the temperature and cloud drops sizes fluctuate (*).

Effects : Similar to rime ice.

mixed ice

Image source: NASA-Lewis Research Centre


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Intensity of ice accretion (1)

light :

> 1g/cm/hour

moderate :

> 6g/cm/hour

severe :

> 12g/cm/hour


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Intensity of ice accretion (2)

Light Icing : does not pose any specific restraints on the behaviour of the

aircraft

Moderate icing : icing conditions may cause the crew to changeheading or altitude

Severe icing : icing conditions which force the crew to immediately

change heading or altitude


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Consequences of accretion (1)

The accumulation of ice represents an increase in mass and leads to the

modification of the longitudinal equilibrium of the aircraft. The effect is relatively

small on larger aircraft.

Icing on tubes and antennas disturbs their operation and can lead to the ruptureof elements.

Icing on the windshield reduces the visibility.

Means of propulsion (motors, propellers, fans, rotors) are also vulnerable to ice

accretion. Their efficiency is reduced and they can stop functioning altogether.


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Consequences of accretion (2)

Lift force (*) reduces considerably

(20% - 30%) when modern wings

get contaminated.

Moreover, forms of light icing have

a similar effect to forms of severe

icing.

clean

with contamination

The aerodynamic consequences:

a major impact.

source NASA


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PROFILE

A vulnerability factor: the aerodynamic profile (1)

In the same icing conditions, the resulting accretion and the effect on

aerodynamics vary largely from one type of aircraft to another.

The aerodynamic flux (flow of air along the surfaces of an aircraft) is modified by

the shape of its profile. Also the collection efficiency, for a fixed size of cloud

droplets, of a wing profile depends upon its form and thickness.

Drop trajectory

Aerodynamic flux

Collection zone


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Aerodynamic speed in knots

tin

C

Another vulnerability factor : Aerodynamic speed (2)

1

8

27

Difference in air temperature and temperature of the point of impact (t) in the

lower layers of the atmosphere .

Fast aircraft are less vulnerable to ice accretion.


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Specific vulnerability of the turbo reactor : ingestion of ice during flight

A turbo reactor in operation can shut down or be destroyed by ingestion of a mass

of ice.

Two possible scenarios:

Accumulation on the landing gear while taxiing-out in an area contaminatedwith frozen snow. The ice lumps will become projectiles on the initial

acceleration of the aircraft. Late use of de-icing or anti-icing equipment flight during severe icing

conditions. The ice that breaks off can fly straight into the engine.

Images source: Pratt and Whitney


14/22


Specific vulnerability for light aviation: Formation of ice in a carburettor

The carburettors in light aircraft are prone to formation of ice. This icing occurs in the

part of the carburettor where the pressure decreases (temperature drops) and where the

fuel vaporises (temperature decreases even further). When the icing is important

enough, the engine stalls.

The risk is at a

maximum in saturatedair with temperatures

between +5 and +15C.

This is a diagram which

allows us to determine

the risk for carburettor

icing in function of the

temperature and dewpoint (*).

source Royal Australian Air Force


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Specific vulnerability for light aviation: flight in clouds under Instrument Flight Rules (IFR).

Light aircraft flying IFR and which are not equipped with effective de-icing

equipment are particularly vulnerable in icing conditions.

It is therefore very useful for the pilot to know the lowest flight level (altitude

or pressure) where the temperature is below 0C. (*)

If this sheet of stratocumulus

has a temperature belowfreezing, it would not be a

good idea to level out at this

level!Frank Jansen photography


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Detection in flight

Visual indications for ice accretion (*)

Source ATR

Electronic ice detector (**)

Source ATR


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Removal of accretion: de-icing

De-icing is the process whereby a system removes ice after it has formed on the

aircraft.

If the type of icing is not too solid and the intensity of the phenomena is moderate,

the pilot can remove the icing by mechanical means.

The advantage of these systems is that they use little energy.

This is why they are mounted on light aircraft and turboprops.

The downside is that these systems can be ineffective in exceptional icing

conditions.

Black surface can bedeformed by pneumatic

systems

Source ATR


18/22


Prevention of icing: anti-icing

Anti-icing is a system which prevents icing to form.

The most widely used technique is to heat the elements or surfaces prone to icing.

Advantage: the aircraft will be well protected in almost all icing conditions if they

are anticipated.

Disadvantage: these systems are high energy consumers. Their use will imply

cost penalties.

Source I.A.E


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Certification in icing conditions

Aeronautical authorities impose exploitation rules during icing conditions. More

particularly, certain standards haven been defined to certify an aircraft, for

example the properties of a de-icing system will be defined.

These standards have been

defined on a basis of special

studies conducted in the real

atmosphere (here CASP II in

Canada in 1992). Very

extreme situations (redcircle), which are rarely

encountered are not taken into

account.

measuredliquidwatercontent

median volume diameter

accretion

severe12 g/cm/hour

moderate6 g/cm/hour

light1 g/cm/hour

icing potential

The conditions which cause the

extreme observations have to

be specified.


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Marginal weather conditions

Experience shows that atmospheric conditions bordering icing conditions have

not been evaluated enough.

Actually, these situations correspond to an intense phenomena that is easy to

observe or detect and which warrants an immediate and effective reaction

from the aeronautical actor.

When the atmospheric parameters oscillate around icing conditions or when the

conditions are out of the regional or seasonal mean, traps ( real atmospheric

ambushes) will develop.

Early detection can easily be done by an aeronautical meteorologist.


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1) Deliver accurate forecasts within aeronautical range ( 24 hours) , adapted to

the local and seasonal context. Determine the first freezing level and obtain

good scores in forecasting the presence or absence of potential icing conditions

on a certain level. These points are operationally important.

2) Arrive to a good detection of extreme conditions corresponding to case of

observed severe icing. This is an important point for certain categories of public

transport aircraft. (commuters)

3) Underline the marginal situations (ambushes) which are bordering on the limitof unpredictable, in order to create a permanent state of vigilance amongst the

aeronautical operators. This point is important for air safety in general.

4) Deepen theoretical knowledge about the subject in extreme case through

experiments. This in order to refine the current standards.

5) Develop training for the aeronautical actors on the subject: To allow him to correctly interpret the information To familiarize him with the methods and classic scenarios we use.

To create awareness about the need of feedback.

Conclusion of the first part : a list with meteorological objectives on icing.


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Forward to: prognostic variables in the atmosphere

Notes for teachers
http://www.caem.wmo.int/_pdf/icing/icing_03_prognostic_variables.pdfhttp://www.caem.wmo.int/_pdf/icing/icing_02_teacher_notes.pdfhttp://www.caem.wmo.int/_pdf/icing/icing_02_teacher_notes.pdfhttp://www.caem.wmo.int/_pdf/icing/icing_03_prognostic_variables.pdf

Icing 02 Effects

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