<h1>IndiGo cancels 80 flights due to dense fog; low-visibility rules require trained crews and CAT IIIB aircraft for landings.</h1> Operators cancelled flights and issued advisories amid dense fog within the recognised Fog Window, invoking low-visibility procedures. Carriers are required to roster pilots trained for low-visibility operations and to deploy a CAT IIIB-compliant aircraft fleet for such landings, while airports operating under Category III conditions may sequence movements and impose constraints that can result in delays and cancellations.