What must be compared against CFL when changing runways?

The correct choice is the available runway length, as it is crucial to ensure that the new runway can safely accommodate the aircraft's takeoff or landing requirements. The Comparison to the Critical Field Length (CFL) is essential because CFL is the maximum distance an aircraft can operate safely on a given runway under specified conditions.

When changing runways, evaluating the available runway length allows pilots to determine whether the new runway meets the necessary performance criteria for the aircraft at its current weight and environmental conditions. If the available runoff is shorter than the CFL, it can lead to an unsafe situation where the aircraft might not be able to take off or land effectively.

While factors like aircraft weight, weather conditions, and speed of the aircraft are certainly important to overall operational safety and performance, they are used in conjunction with available runway length to make an informed decision about runway changes. The critical aspect here revolves around ensuring that the available runway meets or exceeds the CFL for a safe operation.