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

FREEZING CONTAMINATION : AIRCRAFT ICING

EFFECTS ON AIRCRAFT

Different types of accretionIntensity of ice accretionConsequences of accretionVulnerability factors examplesSpecific vulnerabilitiesDetection in flightRemoval of accretion : De-icingPrevention of icing : Anti-icingCertification and icing conditionsMarginal weather conditions


Source NASA-Lewis Research Center


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 can occur without clouds.Effects : Even if the amount of deposited material is low, it can be significant under certain conditions.

hoar frost

source Transport Canada



Aspect : Opaque and white, but rather fragile and brittle. Formation conditions : On a cold surface in a homogeneous cloud environment (T<<0°C). The supercooled cloud droplets rapidly freeze resulting in entrapped bubbles of air.Effects : Forms on leading edges. Rime ice is always significant and needs to be removed.

rime ice

Source Météo France



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 environment with a temperature close to 0°C. 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



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


Intensity of ice accretion (1)

light : > 1g/cm²/hour

moderate : > 6g/cm²/hour

severe :> 12g/cm²/hour


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 change heading or altitude

Severe icing : icing conditions which force the crew to immediately change heading or altitude


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 rupture of 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.


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


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


Aerodynamic speed in knots

∆ t i

n °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.


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 contaminated with 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


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 saturated air with temperatures between +5 and +15°C.

This is a diagram which allows us to determine the risk for carburettor icing in function of the temperature and dew point (*).

source Royal Australian Air Force


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 0°C. (*)

If this sheet of stratocumulus has a temperature below freezing, it would not be a good idea to level out at this level!

Frank Jansen photography


Detection in flight

Visual indications for ice accretion (*)

Source ATR

Electronic ice detector (**)

Source ATR


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 be deformed by pneumatic

systems

Source ATR


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


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 (red circle), which are rarely encountered are not taken into account.

mea

sure

d liq

uid

wat

er c

onte

nt

median volume diameter

accretion

severe12 g/cm²/hour

moderate6 g/cm²/hour

light1 g/cm²/hour

icing potential

The conditions which cause the extreme observations have to be specified.


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.


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 limit of 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.


Forward to: prognostic variables in the atmosphere

Notes for teachers

http://www.caem.wmo.int/_pdf/icing/icing_03_prognostic_variables.pdf

http://www.caem.wmo.int/_pdf/icing/icing_02_teacher_notes.pdf

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