Freezing contamination : aircraft icing

1. Ross Paulson photography Freezing contamination : aircraft icing Freezing contamination : aircraft icing Total duration (4 hours)

2. Contents (1) (Description of the hazard) A few definitions (5) Process of ice formation (6) An historic accident (7) Statistics on aircraft accidents related to icing (8) Costs induced by icing (9) General framework for this presentation (10) (Effects on aircrafts) First part : aviation and icing (11) Different types of accretion (12-15) Intensity of ice accretion (16-17) Consequences of accretion (18-19) Vulnerability factors examples (20-21) Specific vulnerabilities (22-24) Detection in flight (25) Removal of accretion : De-icing (26) Prevention of icing : Anti-icing (27) Certification and icing conditions (28) Marginal weather conditions (29) Conclusion of the first part : a list with meteorological objectives on icing (30)  Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

3. Contents (2) (Diagnosis, forecasting) Second part : meteorology and icing (31) Prognostic variables in the atmosphere Air temperature (including 0°C level definition) (32-35) Relative humidity (36-37) Liquid Water Content (38-43) Size of the elements (44-45) Vertical velocity (46-47) Vertical instability (48) Wind shear (48) Conclusions about icing potential (49) Favourable scenarios for icing Convective (50-52) Turbulent (53-55) Frontal (56-57) Orographic (58-60) Freezing fog (61) Remote sensing observation systems satellite observation (1.6-3.9-8.7-10.8 microns and composite imagery) (62-67) radar observation (68-69) Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

4. Contents (3) Forecasting techniques (70) Extrapolation of observational icing data (71-72) Looking for icing scenarios (73) Using numerical model outputs (74-75) Crossing observations with model outputs (data fusion, icing index...) (76-77) Towards a conclusion (78) Putting in place icing conditions forecasts (79-80) A transmission chain of adapted information (81-84) SIGMET, AIRMET, GAMET, SIGWX charts PIREPS Awareness of the icing phenomena (85-86) A few points on where to go with research and development (87) A few bibliography, internet ressources (88-89) List of key words (90) Case study (91-) (*) to read in the comment page Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

5. A few definitions Contamination, contaminant : general term used in aeronautics to identify the deposit of a “foreign” body onto a surface. For instance, water on a runway is a contaminant. Icing : contamination of an aircraft by ice. This contamination can happen both on surface and in the air. Accretion (of ice) : accumulation (of ice) on a certain part of the aircraft. Icing Conditions : state of the atmosphere favourable for ice accretion on an aircraft. Icing Potential : information about the state of the atmosphere giving (and graduating) the risk for icing. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

6. Process of ice formation Ice can form on objects in the atmosphere : • Through freezing of liquid water drops that were deposited in the form of droplets, drops, film or puddles on an object. The temperature of the object determines the solidification. In an atmosphere with a sub-zero temperature : • Through sublimation, water vapour transforms directly into ice. The amount of water is relatively small. • Through ending the state of supercooling of drops and droplets present in the atmosphere. Supercoooling is frequently present in clouds with negative temperature until –20°C. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

7. An historic accident • On 31 October 1994, a commuter departs from Indianapolis going to Chicago-O’Hare (USA) for a regular flight taking about 1 hour 5 minutes. At 50 Nm from Chicago, the airplane is put in a holding pattern at 10 000ft because of the heavy traffic. 30 minutes later the plane crashes in a field close to Roselawn, Indiana. The 68 passengers and crew die. • Conclusions of the investigations : • Holding pattern with liquid precipitation during 24 minutes. • Air temperature varying between –2°C and –4°C. • Total estimated ice accretion between 35 and 65 millimetres. • Flight above certification limits. • No action was taken by the crew in order to “de-ice” the aircraft. • Icing conditions were communicated in the Met Flight Folder handed to the crew, through the transmission of an AIRMET signalling precipitation with temperatures below freezing at the chosen flight level of the aircraft. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

8. Statistics on aircraft accidents related to icing • The Investment Analysis and Operations Research department of the FAA (Federal Aviation Administration) researched a little over 10 000 accidents between 1982 et 2001. • In 20% of the cases, meteorological factors were the main cause and within this about 25% of the accidents resulted in loss of human life. • Icing including flight in icing conditions, freezing fog and carburettor icing for light aviation represented about 10% of the meteorological factors causing these accidents. • 13% of the accidents due to icing resulted in loss of life, about 30 cases in 20 years. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

9. Costs caused by icing • On the aircraft: when designing aircraft with possibility to fly in icing conditions, it is necessary to include equipment preventing icing or de-icing equipment. • At the airports: specific means to de-ice surfaces have to be deployed. • The methods have a high “operational” cost. • In flight the possible pay-load and autonomy are reduced. (*). • On surface, the operational procedures are often time consuming and induce frequent delays on the bigger airports. (**). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

10. General framework for this presentation Atmospheric Icing Potential. The meteorologist puts forward a diagnostic on the icing potential of the atmosphere. He qualifies, localises and quantifies the phenomena using different parameters (temperature, LWC…). The feedback of the aviators is necessary to improve the forecasts. The information has to be adapted for use by the aviation community The user filters the information and the operator evaluates the risk. The “filter” consists of aircraft, pilot, ATC, airport officials or more in general terms “the aeronautical system”. The induced effects lead to operational decisions, otherwise the safety is compromised. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

11. First part : Aviation and icing Source NASA-Lewis Research Center Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

12. Different types of accretion (1) hoar frost source Transport Canada 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

13. Different types of accretion (2) rime ice Source Météo France Aspect : opaque and white, but rather fragile 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 must be removed. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

14. Different types of accretion (3) clear ice Source NASA-Lewis Research Centre Aspect : transparent, homogeneous et 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

15. Different types of accretion (4) Source NASA-Lewis Research Centre mixed ice 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

16. Intensity of ice accretion (1) severe : > 12g/cm²/hour light : > 1g/cm²/hour moderate : > 6g/cm²/hour Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

17. 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 Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

18. Consequences of accretion (1) • 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. • 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

19. Consequences of accretion (2) clean with contamination source NASA • The aerodynamic consequences: a major impact. Lift force (*) reduces considerably (20% - 30%) when modern wings get contaminated Moreover, forms of light icing have a similar effect as forms of severe icing. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

20. A vulnerability factor: the aerodynamic profile (1) Aerodynamic flux PROFILE Drop trajectory Collection zone The aerodynamic flux (flow of air along the surfaces of an aircraft) is modified by the aircraft shape. Also the collection efficiency, for a fixed size of cloud droplets, of a wing profile depends upon its form and thickness. In the same icing conditions, the resulting accretion and the effect on aerodynamics vary largely from one type of aircraft to another. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

21. Another vulnerability factor: Aerodynamic speed (2) 27 • t in °C 8 1 Aerodynamic speed in knots 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

22. Specific vulnerability of the turbo reactor : ingestion of ice during flight Source Pratt et Whitney Source Pratt et Whitney 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 during severe icing conditions. The ice that breaks off can fly straight into the engine. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

23. Specific vulnerability for light aviation: Formation of ice in a carburettor 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 The risk is at a maximum in saturated air with temperatures between +5 and +15°C. 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

24. Specific vulnerability for light aviation:flight in clouds under Instrument Flight Rules (IFR) Frank Jansen photography If thissheet of stratocumulus has a temperature below freezing, it would not be a good idea to level out at its level! Light aircraft flying IFR and which do not have 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. (*) Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

25. Detection in flight Visual indications for ice accretion (*) Source ATR Electronic ice detector (**) Source ATR Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

26. Removal of accretion: de-icing Black surface can be deformed by pneumatic systems Source ATR De-icing is the process whereby a “system” removes icing 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 equip light aircraft and turboprops. The downside is that these systems can be ineffective in exceptional icing conditions. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

27. Prevention of icing: anti-icing Source I.A.E Anti-icing is a system which prevents icing from forming. 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 is that these systems consume a lot of energy. Their use will imply cost penalties. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

28. Certification in icing conditions accretion severe 12 g/cm²/hour measured liquid water content moderate 6 g/cm²/hour The conditions which cause the extreme observations have to be specified. light 1 g/cm²/hour icing potential median volume diameter Aeronautical authorities impose exploitation rules during icing conditions. More particularly, certain standards have been defined to certify an aircraft. In this way, the properties of a de-icing system are pointed out. These standards have been defined on the 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

29. 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. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

30. Conclusion of the first part : a list with meteorological objectives on icing • 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. • Achieve 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) • 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. • Deepen theoretical knowledge about the subject in extreme cases through experiments. This, in order to refine the current standards. • Develop training for the “aeronautical actors” on the subject: • To allow them to correctly interpret the information • To familiarize them with the methods and classical scenarios we use • To create awareness about the need of feedback. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

31. Second part : meteorology and icing LFMM SIGMET 3 VALID 111145/111545 LFML- SEV ICE OBS LFMM FIR MAINLY ALPS RHONE VAL CORSICA AND MAR= PIREP AIRMET On the topic of contamination by icing, the actions of an aviation forecaster must correspond to the aeronautical objectives which we have defined in part one. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

32. Prognostic variables of the icing potential in the atmosphere: air temperature (1) The air temperature is an excellent parameter to approach the phenomena. In fact : • Water only freezes when its temperature reaches 0°C or lower. This way one can eliminate all areas with positive temperatures (*). • Supercooled water cannot exist at temperatures below –40°C. Moreover, because all ice nuclei are active below –35°C, one can consider the icing potential to be nil below –35°C. • Only a small amount of aerosols make up the ice nuclei between 0°C et –12/-15°C. Condensed water therefore exists mainly in supercooled water. The levels with high icing potential in clouds, are situated in this temperature range. • Taking into account the existing relation between air temperature and its capacity to hold water vapour(**), which in a lifting mode can condensate into supercooled droplets, one can state that the icing potential will decrease with the potential temperature, all things being equal. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

33. Prognostic variables of the icing potential in the atmosphere: air temperature (2) The temperature defines for the major part the composition of a cloud. This useful division has been confirmed by in-flight observations of icing occurrences: few occurrences at temperatures below -20°C (*). Most occurrences were reported at temperatures between -3°C and -7°C (**). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

34. Prognostic variables of the icing potential in the atmosphere: air temperature (3) Cross-section across a warm front (distance of the cross 750 km) with a temperature field (blue) and wet-bulb potential temperature in red (*) 0°C level Considering the numerical model point of view; temperature has the advantage that it is one of the parameters which is routinely calculated and easyly accessible. Anyway, its spatial coverage has to be detailed enough in order to exactly isolate the areas with risk for icing and, particularly, the levels with icing potential in the vertical plane. Fine mesh models (mesoscale) are necessary to deliver this necessary precision. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

35. Prognostic variables of the icing potential in the atmosphere: air temperature and the 0°C-isotherm (4) This Météo France product represents the freezing levels in flight levels. The temperatures are plotted if they are sub-zero on surface level. The level(s) of the 0°C-isotherm, more or less determines the lower limit of the icing volumes. On a local level and for very short range forecasting, a representative sounding will help to determine the corresponding level(s). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

36. Prognostic variables of the icing potential in the atmosphere: relative humidity (1) Cross section of a warm front (distance of the cross 750 km) with a humidity field (pink) and the wet-bulb potential temperature. (**) Saturation of air with icing potential can be represented by the relative humidity of the air. (*). If it is described correctly, it is also a parameter which can eliminate areas without icing potential. Form the viewpoint of numerical models, relative humidity is also a frequently calculated parameter. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

37. Prognostic variables of the icing potential in the atmosphere: relative humidity (2) It is then quite natural that we associate icing potential to a cross reference temperature/relative humidity (Hu). In this way, algorithms are based upon a climatological study.Icing potential is at its maximum when you have high relative humidity combined with the most favourable ranges of temperatures. The determination firstly results in black and white (icing or not). It is more difficult to justify the determination of intensity solely based upon one cross-referenced criterion (*). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

38. Prognostic variables of the icing potential in the atmosphere: Liquid Water Content (1) • Liquid Water Content (supercooled) or LWC expresses the amount of condensed liquid water present in a volume of one cubic meter moist air (generally air inside a cloud) of which the temperature is negative. • It takes a significant value in a saturated environment. • The content of liquid water is a parameter that gives an excellent indication of the icing potential. It is expressed in grams per cubic meter (*). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

39. Prognostic variables of the icing potential in the atmosphere: Liquid Water Content (2) Amounts of condensed water present in the atmosphere : • Stable clouds in layer (St, As, Ns) : from 0,1 to 0,5 g/m3 • Unstable clouds (Cu, Ac, Cb) : from 1 to 5 g/m3 (up to 15 g/m3 in certain unstable clouds in the tropics) Remark : stratocumulus is a cloud that possess both characteristics, stable on a large scale and unstable on a smaller scale. Their water content therefore varies a lot. • Mist and Fog : from 0,1 to 2 g/m3 • Larger amounts are present in precipitation areas. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

40. Prognostic variables of the icing potential in the atmosphere: Liquid Water Content (3) Stable clouds in layers: • They have a reduced water content : only 15 to 20% of these clouds contain more than 0,3 g/m3 and only a few of them more than 1 g/m3. • The strongest concentration of liquid water can be found in the temperature range between -5° and -10°C. • In these clouds, which are considered to be homogeneous in the horizontal plane, a big relative variability in water content can be observed. • Water distribution in the vertical plane is irregular. Certain cases have the maximum at the bottom of the cloud, while others have their maximum in the upper part. • There is an interesting correlation (*) between the mean extreme values of water content and the flight distance over which the measurement is done. • Between –5°C and –10°C : over 20 km never more than 1 g/m3 ,over 100 km never more than 0.7 g/m3 ,over 200 km never more than 0.5 g/m3 . Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

41. Prognostic variables of the icing potential in the atmosphere: Liquid Water Content (4) 2 g/m3 Measurement of LWC through a Cumulus Congestus field-TCU- at a temperature of –10°C (campaign “ LANDES-FRONTS 84 ”). Unstable clouds : • have a liquid water content that frequently surpasses 1 g/m3. • in these clouds, which are in general considered to be heterogeneous, a large variability in water content is observed. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

42. Prognostic variables of the icing potential in the atmosphere: Liquid Water Content (5) (**) It is possible to establish an intensity scale solely based upon the liquid water content – LWC (*). Light icing potential : LWC < 0,6 g/m3 moderate icing potential : 0,6g/m3 < LWC < 1,2 g/m3 severe icing potential : LWC > 1,2 g/m3 Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

43. Prognostic variables of the icing potential in the atmosphere: Liquid Water Content (6) Numerical model output depicting LWC LWC is depicted as an output of an experimental high resolution model (Meteo France MESO-NH) in a big convection system (*). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

44. Prognostic variables of the icing potential in the atmosphere: the size of the elements (1) Measure of Mean Diametric Volume (MDV) of cloud droplets in a cumulus congestus field -TCU- at a temperature of –10°C (campaign “ LANDES-FRONTS 84 ”). The dimensions of the cloud droplets are in general less than 40 microns. The dimension of cloud droplets determine partially the ice accretion : collection, shape and contaminated zone (*). The mean dimension of these droplets (**) therefore have to be combined with temperature and liquid water content in order to produce quantitative and qualitative forecasts of ice accretion. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

45. Prognostic variables of the icing potential in the atmosphere: the size of the elements (2) The big droplets (SLD) Starting from 40 microns (*) a droplet becomes a large droplet (SLD – Supercooled Large Droplet). Beyond 200 microns they become drizzle and rain drops or supercooled precipitation (FZDZ and FZRA). Taking into account the major consequences related to a contamination by these big elements of supercooled water, we have to discriminate between SLD scenarios and others (**). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

46. Prognostic variables of the icing potential in the atmosphere: Vertical Velocity (1) Sometimes vertical velocity is used as a complementary parameter to discriminate icing conditions, especially when you do not have a model prediction of LWC at your disposal. The idea is that the water present in the clouds originates from the base of the atmosphere and that it has been brought there by vertical movements. All things being equal, the more intense these currents are, the more important the upward flux of water is (*). The advantage is that numerical models predict vertical velocity. Experience shows however that a good discrimination (**) of icing layers require a mesoscale description, which is a scale at which the operational models are not always very efficient. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

47. Prognostic variables of the icing potential in the atmosphere: Vertical Velocity (2) A cold front reaches Europe. In the north-west of Spain, significant icing conditions were reported. At the same time, elevated icing index values (8 to 9 on a scale of 10) indicate a high risk of icing (*). The vertical velocity field is here combined with an icing index T/Hu. The icing index (pink isolines) is “filtered” by the vertical velocity with values below –0.2 hPa/s in the red areas. Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

48. Prognostic variables of the icing potential in the atmosphere: Vertical Instability – Wind shear Vertical instability Taking into account the importance of the stable or unstable nature of a cloud with respect to its LWC, it is clear that anticipating this characteristic is part of the forecasting icing conditions process when one does not have a numerical model output of LWC at its disposal (*). Wind Shear We will see that wind shear can favour conditions with presence of SLD (**). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

49. Conclusion about Icing Potential Situations with severe icing are equivalent to a high LWC or high amount of large droplets or drops (lots of SLD). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing

50. Favourable scenarios for icing : convective (1) An unstable scenario (convective) This corresponds to the possibility of an intermittent, but important, vertical convective transport of water vapour from the lower layers towards a level where the conditions favour the condensation of vapour into liquid supercooled water. This can be observed in all situations leading to the presence of low or medium level unstable clouds. The vertical temperature profile serves to determine the freezing levels (down to -20°C) and in some cases the icing potential, which is strongest at the base of the negative layer within the cloud (*). Aviation meteorology : significant phenomena - freezing contamination- aircraft icing