Why was Jonas frightened when he saw the plane?

Jonas was frightened when he saw the plane because it represented an intrusion into his carefully controlled and predictable life within the community. The sight of the plane flying overhead reminded him of the possibility of danger and conflict, which were emotions and experiences he had been sheltered from. Additionally, the plane's presence made him acutely aware of the limitations imposed on his society regarding freedom and choice. Thus, the encounter triggered a sense of fear and uncertainty about the world beyond his community.