How long does it take a Boeing to freeze?

When an aircraft flies through a cloud containing supercooled water droplets —liquid despite being below 0 °C— those droplets freeze on impact with the leading edge of the wings, the tailplane or the engine inlets. In severe conditions, a noticeable layer of ice can form in under a minute, and what matters isn't its weight but the way it distorts the wing's airfoil shape, which reduces lift and brings forward the stall. That's why the anti-ice system is switched on pre-emptively on entering favorable conditions, not after ice is actually seen.

The Boeing 737 bleeds hot air from the engine compressors and circulates it inside the leading edge; the 787, by contrast, did away with bleed air and heats that area with electric resistance heating blankets, one of the innovations of its electrical architecture. Engine inlets always carry their own dedicated system, and the pitot tubes and probes are heated independently, because a sensor blocked by ice is just as dangerous as ice on the wing.

Before taking off in snow, ice or freezing rain, the aircraft is sprayed with de-icing and anti-icing fluid. That protection isn't indefinite: it has a validity window, the holdover time, tabulated every winter by the FAA and Transport Canada, which depending on the fluid type and precipitation intensity ranges from a few minutes in heavy snowfall to more than an hour in mild conditions. If the aircraft doesn't take off within that window, the treatment must be repeated and the aircraft goes back to the queue.