What should be used when the wind is blowing less than 5 knots?

When the wind is blowing less than 5 knots, the term "calm wind" is used to describe the conditions. Calm winds refer to light air, which typically means that the effects of wind are minimal and aircraft operations can proceed without significant wind-related concerns.

In aviation, understanding wind conditions is crucial for safe takeoffs and landings. Light winds are generally less of a factor in flight operations, leading pilots and air traffic controllers to consider situations as “calm.” This is important for various reasons including performance calculations for aircraft and ensuring that pilot workload remains manageable during critical phases of flight.

In contrast, the other options do not specifically relate to indicating wind conditions below 5 knots. Taxiway clearance pertains to vehicle movement on the ground; visual flight rules are a set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going, not specifically tied to wind conditions; and standard wind patterns refer to expected wind behaviors, rather than the current state of wind being calm. Hence, referring to wind under these conditions as "calm wind" is the most accurate and relevant choice.