During what phase of flight preparation is de-icing most critical?

De-icing is most critical just prior to taxiing for takeoff because this is the phase when ice accumulation on an aircraft can lead to significant performance issues. Ice and snow can affect the aerodynamics of the wings and other surfaces, potentially causing diminished lift, increased drag, and altered handling characteristics.

Ensuring that the aircraft is free of ice and snow before taxiing is essential because any accumulation during taxi can continue to pose risks during takeoff. The process of de-icing typically involves applying heated fluids designed to remove ice and prevent further accumulation, thus ensuring that the aircraft is in optimal condition for safe operation during departure.

In comparison, while de-icing may also be important before landing, during flight, or after arriving at the gate, those phases do not present the same immediate performance and safety concerns as the period right before takeoff. The aircraft needs to be fully cleared of ice to adhere to safety standards and regulations, making this timing crucial for effective de-icing.