Solution

Since the speed of the plane exceeds the speed of sound, a spherical wave emitted at a later time is not contained in another spherical wave emitted at an earlier time. As a result, an envelope (i.e. shock wave) of these spherical waves is formed in the shape of a cone. In the figure, the shock wave (red) reaches an observer at $A$ as a loud popping noise before a spherical wave (blue) emitted from $B$ arrives as the noise of the engines a short moment later, by which time the shock wave (blue) has already passed the observer.