Recent aircraft incidents and accidents have highlighted the existence of icing cloud characteristics beyond the actual certification envelope (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).
Most problematic 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). Because of that, international airworthiness authorities intended to jointly develop and issue updated regulations for the certification envelope. These SLD conditions are very troublesome, and their physics is not yet fully understood.
The rules require aircraft manufacturers to demonstrate that their product can safely operate in SLD environments. To do so, they are requested to demonstrate that. Compliance has typically involved actual flight into natural icing conditions. But SLD icing conditions occur less frequently than the current icing specifications (see JAR/FAR Appendix C). Thus, it will be difficult and expensive to demonstrate compliance by natural icing flights alone. Therefore, the use of so-called engineering tools (icing tunnels, tankers & computer codes) is promoted.
The objectives of this project were as follows:
- To reduce aircraft development cost by improving tools and methods for aircraft design and certification in an icing environment;
- The development and validation of Means of Compliance and tools for aircraft icing certification. This research activity will also have a direct impact on aircraft safety, allowing future aircraft to be designed safer with respect to the icing and the SLD environment.
Compare ice accretion obtained in icing wind tunnel test to icing accumulated on a specific test article installed on an aircraft flying in icing conditions. The comparison will be made, and a critical review of all obtained results will be performed.
The most important project achievements are:
- A website in which the results of EXTICE are presented. This website contributes to the dissemination of information that was gathered during the course of the project.
- A comprehensive literature review of the state of knowledge on super-cooled large drops. This presented a comprehensive summary of SLD (Super-cooled Large Droplets) cloud conditions and appropriate ranges for various parameters to be used during experiments.
- Basic SLD related experiments (splash, break-up) have been completed and have added significantly to the preexisting knowledge base, especially of direct relevance to aircraft icing.
- Empirical expressions were determined for the deposited mass ratio. A deterministic model of splashing up to the point of breakup was developed and a new SLD splashing model was formulated for wet surface drop impact.
- All key experimental programs have been completed: the 2D and 3D icing wind tunnel tests and the Flight test program in March 2012.
- A wide range of droplet trajectory and ice shape prediction codes have been successively Modified. In particular, both drop splashing and rebound phenomena have been incorporated in the various codes.
- Comparisons between numerical simulation and 2D data obtained in a French icing facility (DGA) have been successfully completed. Both Lagrangian and Eulerian methodologies for calculating drop trajectories were employed in EXTICE with similar results.
- At higher Mach numbers, the models failed to accurately predict leading edge ice accretion amounts and the losses due to drop splashing seem to have been underestimated. This was particularly true at warm temperatures and it was not known whether this was a SLD specific feature or a general limitation in the icing prediction codes. This would suggest that further work on ice accretion code modeling is required in the future.
- The 3D experimental data were only gathered during the final stages of the project and only a partial numerical-experimental validation has been performed.
- While ice shape predictions provided good results for the cold cases, at warmer temperatures none of the codes utilized were able to demonstrate an ability to predict the feathers and scallops observed in the experiments.
- It appears that the application of 2D techniques to the 3D cases were relatively successful, leading to the conclusion that the 3D experiments did not lead to strongly 3D effe
The developed models have failed to accurately predict leading edge ice accretion amounts. Also the losses due to drop splashing seem to have been underestimated. Thus, further work on ice accretion code modeling is required in the future.
Innovating for the future (technology and behaviour): A European Transport Research and Innovation Policy
No, not yet. Further work on ice accretion code modeling is required.