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.