When is an acceleration check required?

An acceleration check is required whenever the S1 speed is less than Vrot. S1 is the minimum safety speed for the aircraft during takeoff, while Vrot is the speed at which the aircraft can safely lift off the ground. If S1 is below Vrot, it indicates that the aircraft may not have sufficient speed for a safe takeoff, and the pilot must conduct an acceleration check to ensure that the aircraft can reach Vrot in a safe manner. This check assesses the aircraft's performance and confirms that it can indeed achieve the necessary speed for a safe takeoff.

In contrast, the other options do not correctly identify conditions for requiring an acceleration check. For instance, when Vr is less than Vrot, it does not necessarily affect the need for an acceleration check. Similarly, being at cruising altitude or initiating a descent involves different flight parameters that do not directly relate to the acceleration check in relation to takeoff speeds.