What is the maximum speed during takeoff at which a pilot must begin the first action to stop the aircraft?

The maximum speed during takeoff at which a pilot must begin the first action to stop the aircraft is V1. This speed represents a critical decision point during the takeoff roll. Below V1, the pilot has the capability to safely abort the takeoff, and actions should be initiated immediately if necessary, such as applying brakes to stop the aircraft.

V1 is specifically calculated based on factors such as aircraft weight, environmental conditions, runway length, and other relevant variables. Once the aircraft reaches V1, it is no longer safe to stop because the distance required to bring the aircraft to a stop would exceed the remaining runway.

Understanding the role of V1 is essential for flight safety, as it dictates the point of no return in the takeoff sequence, guiding pilots in their decision-making process during critical phases of flight. Other speeds such as V2, Vr, and VREF serve different purposes related to takeoff and climb performance but do not define the threshold for initiating a stop.