front page - tuprints - Technische Universität Darmstadt

1 Introduction
1.1 Motivation
Aircraft icing presents a serious hazard for flight at subsonic speeds in conditions
of visible moisture and temperatures below freezing. Ice accretion on wing surfaces
modifies the profile of an airfoil, increasing drag while decreasing the lift.
Icing on ailerons can affect the roll control, and ice accumulation on the horizontal
stabilizers might cause tail stall. Ice can also cause engine stoppage by either icing
up the carburetor or, in the case of a fuel-injected engine, blocking the engine’s air
source [141].
Many aircraft have been lost owing to ice accumulation, for example some 20
accidents where icing was a contributing factor is summarized by Valarezo in 1993
[149]. The protection of aircraft from the adverse effects of ice accretion has been
a crucial design problem since the very early years of flight. The Federal Aviation
Administration (FAA) requires an airplane manufacturer to demonstrate that its
aircraft can fly safely in icing conditions as defined by the so-called icing envelopes
in the FAA’s Federal Airworthiness Regulations (FAR) Part 25, Appendix C [1].
However more recent accidents, especially the one in 1994 reported by the National
Transportation Safety Board Aviation Accident Report [82, 120], have highlighted
the existence of icing cloud characteristics beyond the actual certification
envelope of the Appendix C, which accounts for an icing envelope characterized by
water droplet diameters up to 50 µm (so called cloud droplet). Multiple meteorological
investigations documented the existence of the Supercooled Large Droplets
(SLD) in the range of 100 µm to 400 µm.
International airworthiness authorities, the FAA, Transport Canada (TC), and the
European Aviation Safety Agency (EASA) are intending to jointly develop and issue
updated regulations for certification in SLD environments based on investigations
of consultative expert panels coming from research establishments, industry and
national aviation regulatory bodies: the “Appendix X” [84]. 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.
Demonstration of the anti-icing capability is conducted by both experimental and
numerical engineering tools. In the 1940s and 1950s, significant experimental and
1