No, a sonic boom does not occur when a plane slows down to less than the speed of sound. A sonic boom occurs when an object traveling faster than the speed of sound creates a shock wave in the air. This shock wave is what produces the loud, booming sound that we hear.

When a plane slows down to less than the speed of sound, it is no longer creating a shock wave, so there is no sonic boom.

However, if a plane is traveling at supersonic speed and then makes a sharp turn, it can create a sonic boom. This is because the plane is still traveling faster than the speed of sound in some parts of the turn.

Sonic booms can be disruptive and even harmful, so planes typically avoid flying at supersonic speeds over land. However, they are sometimes used for military purposes, such as to break through enemy defenses.