Supercooled large droplets (SLD) are a particularly insidious form of water when it comes to air travel. They remain liquid at temperatures below 0 °C but in an unstable state. They are also less likely to be deflected by airflow around the aircraft. When an SLD encounters an aerofoil surface, the portion touching the aerofoil freezes, but the rest remains liquid. It is blown back over the surface, freezing as the first contact point did until the whole drop has frozen, creating a smooth layer of ice over the surface that is very difficult to remove. The EU-funded SENS4ICE project is developing a novel sensor system to detect ice formation before it becomes dangerous to flight. It leverages a combination of direct and indirect techniques including monitoring both ice accretion on the airframe and atmospheric conditions.