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EXTreme ICing Environment

Tags: Air

State of the Art - Background

Recent aircraft incidents and accidents have highlighted the existence of icing cloud characteristics beyond the actual certification envelope currently defined by the JAR/FAR Appendix C, which accounts for an icing envelope characterised by water droplet diameters up to 50 μm (so-called cloud droplets). The main concern is the presence of super-cooled large droplets (SLD) such as freezing drizzle, in the range of 40-400 μm, or freezing rain, with droplet diameters beyond 400 μm. The presence of SLD was also confirmed in Europe by the European funded project EURICE. The main results raised within the EURICE project was that, while the existence of SLD has been proved, means of compliance and engineering tools to accurately simulate these conditions are lacking and existing measures must be improved and/or new techniques developed.

International airworthiness authorities, namely the Federal Aviation Administration (FAA), Transport Canada (TC), and the European Aviation Safety Agency (EASA) are intending to jointly develop and issue updated regulations for certification in SLD: 'Appendix X'.

If implemented, the proposed new rules will require aircraft manufacturers to demonstrate that their product can safely operate in SLD environments. To do so, they will be requested to demonstrate that specific capabilities comply with the new regulation.


At the present time, certification authorities rely primarily on flight-test data for icing certification. Unfortunately flight tests in icing conditions are costly and difficult to achieve. If standard icing conditions are not easy to meet during an icing flight campaign, flight tests in extreme icing conditions, such as Super Cooled droplet (SLD) conditions are still more troublesome. Advantages over the present situation could be achieved by performing part of the certification process through a combination of wind-tunnel testing and numerical simulation; however these approaches must be proven reliable and trustworthy. Indeed, to cover the SLD envelope, there exists a need to extend and improve existing wind-tunnel techniques and numerical simulation tools.

The objectives of this proposal are twofold:

- to reduce aircraft development cost by improving tools and methods for aircraft design and certification in an icing environment;

- to improve safety by providing more reliable icing simulation tools.

Description of Work

Compliance has typically involved actual flights into natural icing conditions, as well as the use of engineering simulations of the natural environment provided by experimental means, icing tunnels and tankers, and analytical methods, namely ice prediction computer codes.

In an effort to improve the reliability of simulations and to prove their accuracy, the methodology chosen here is one integrating basic experiments, wind-tunnel testing and flight testing.

The basic experiments are planned to improve the knowledge of SLD physics. Results from these experiments can be used to define a single SLD droplet basic mathematical model that can be implemented in ice accretion numerical simulation tools. Icing wind-tunnel tests on 'industrial components', such as a wing or an airfoil, will be necessary since they will be used both to validate and to improve numerical tools by identifying the best approach to be used for ice accretion simulation.

Finally, within the EXTICE project, it is planned to compare ice accretion obtained in an icing wind-tunnel test to icing accumulated on a specific test article installed on an aircraft flying in icing conditions. The comparison will be performed by using the same numerical code for the simulation of icing wind-tunnel tests and of in-flight icing, and performing a critical review of all the obtained results.

Expected Results

The expected top-level results will be a fundamental knowledge of the SLD ice accretion environment analysis and the development of European theoretical and experimental capabilities, the so-called engineering tools, to model accurately the SLD encounter effect on aircraft in order to comply with the new icing certification rules. At the same time a deeper knowledge of SLD impact on aircraft will be obtained and countermeasures from SLD conditions will be investigated.

Therefore the EXTICE project will contribute to AAT.2007.4.1.1 Design Systems and Tools by improving tools for aircraft design since the improved icing simulation tools developed within the EXTICE project will allow for a more effective design process by reducing required wind-tunnel and flight-test costs. The results will also have an impact on AAT.2007.3.3.2 System and Equipment as knowledge of accumulated ice shapes within the flight envelope will be increased and reliable tools for ice shape simulation will be developed within the EXTICE project.

Concluding the EXTICE project will allow:

a) a decrease in time and costs for aircraft design and certification;

b) an increase in aircraft safety by providing industries with more reliable icing simulation tools.