Why might the arrival of a jet be a terrifying experince for the entire community in "The Giver"?
The previous answer is quite good, but I am thinking that the arrival of a jet unnanounced would break the isolation that this community has chosen for itself. It is a "perfect" totally controlled utopian community. The arrival of outside influences in the form of people from another location would create total chaos for the people living in the "perfect" society.
I am thinking of the arrival of The Beatles to the U.S. in the 1960s. The influence and media coverage of these "outsiders" created problems for the parents of many young people in the United States because the craze and influence the Beatles created impelled many young people to rebel and try new and different things. Their influence allowed the other British rock bands even more influence on the American youth culture. This new style of music upset the status quo of American culture and society.
Imagine a time when planes, care, electricity, etc... didn't exist. If you've never seen or heard of such technology, and a big, noisy flying object (which you could never have imagined) arrives in your midst. Would you not be terrified? It is difficult for us to imagine because we are accustomed to these technologies. But, think of your great great grandparents and how they might have reacted to such a sight. They might think that is was some kind of beast meant to do them harm. Remember, that anytime we have been faced with the unknown, fear has been involved.