Aircraft Icing: Droplet Impact, Post-Impact Motion, Ice Accretion, and Mitigation Strategies

Aircraft icing couples droplet transport, impact, surface-water motion, nucleation, and freezing. This narrative review examines how these processes produce aircraft hazards and constrain ice-protection choices. Targeted literature searches support comparisons of laboratory conditions, measurement methods, validation evidence, and limits of transfer to flight. Particular attention is given to pre-impact deformation and breakup, local impact conditions, and the transport of unfrozen water as films, rivulets, and droplets over an airfoil. We link wettability to contact-line retention and heterogeneous nucleation while distinguishing the water contact angle from the ice-nucleus contact angle in classical nucleation theory. The review relates these mechanisms to structural, ground, induction-system, instrument, and ice-crystal icing. Established thermal, pneumatic, and fluid-based systems are distinguished from developing ultrasonic and passive-surface approaches. Sensing and machine learning are treated as facilitating technologies for diagnosis and control, rather than independent ice-removal mechanisms. The synthesis identifies local water retention, freezing fraction, runback, and removal performance as complementary evaluation outcomes. Remaining needs include measurements under representative impact and airflow conditions, uncertainty reporting, coating durability, and validation of integrated protection systems.