If you saw a truck spraying fluid on the plane before departure, it’s de icing or anti icing. Totally normal, especially at Heathrow in colder or damp conditions.
Even if there’s no visible snow, frost or ice can form on the wings and control surfaces, and that can seriously affect lift. The spray removes any ice and leaves a protective coating so more doesn’t form while you wait to take off.
It can happen on chilly mornings, after rain, or when temps are hovering around freezing. Nothing to worry about, just part of standard winter ops.
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