Why is Jonas frightened by the plane flying overhead?

Jonas is frightened by the plane flying overhead because it represents a threat to the carefully controlled environment of his community. In his world, such aircraft are associated with the potential for conflict and violence, which contrasts sharply with the peace and order that the community maintains. The noise and presence of the plane evoke feelings of fear and uncertainty in Jonas, as he is aware that it signifies a breach of the safety that his society strives to uphold. This moment also foreshadows the deeper complexities and dangers of the world beyond his sheltered existence.