Why do de-icing methods typically require the airplane to be sprayed twice?

The reason the second application of de-icing fluid is typically more viscous is that it is designed to adhere longer to the aircraft's surface and provide continued protection against ice formation until takeoff. The first application of de-icing fluid effectively melts any existing ice or snow, ensuring that the surface is clean before the aircraft takes off.

Once the surface is clear, the second application, which often utilizes a more viscous fluid, serves to create a protective layer that remains in place, preventing new ice from forming and allowing for safe operations during taxiing and takeoff. This is crucial, as any residual ice can significantly impact the aircraft's performance and safety.

In contrast, while the first application may focus on immediate de-icing, the second application is about ensuring prolonged protection, which is essential in maintaining operational safety in cold weather conditions. The viscosity of the fluid is a key factor in achieving this continued protection.