why might the arrival of a jet be a terrifying experience for the community

In this novel, "The Giver," the community is isolated and they have no memories they have not be given. All of the memories are stored in the mind of The Giver. At one time in the long ago past the community found memories of things such as war, death, and fear too hard to bear, so they gave away all their memories to one man.

The reason the jet would frighten the people is because they didn't really understand what the jet was and it might mean that their way of life was being threatened. It is at the beginning of this experience we meet Jonas, the protagonist of this story and the eventual receiver of Memories.

"At any one moment individuals must be prepared to respond to announcements made through the ubiquitous loudspeakers. A day's holiday is proclaimed and everyone takes a rest from work. There is no opportunity for planning an individual vacation. There is nowhere to go, not much else to do. The community can only be successful in curtailing individual freedom by severely limiting the choices available to individuals. Hence, the community is paranoid in its insularity. Hence, the panic at the beginning of the novel, when an unidentified plane flies overhead."

The jet flying over the community is scary because they never saw a aircraft before.