What does the unidentified aircraft, in Chapter 1, allude too?

Aside from illustrating quite a lot about how Jonas's community works by the way they respond to the sudden appearance of the unidentified aircraft, you could say that the flashback to the incident involving it alludes to several things. Let's consider them in turn.

First, the incident could allude to the general atmosphere of the community being a highly policed state.In general, if there are unidentified planes flying over you, you're in a state...

Aside from illustrating quite a lot about how Jonas's community works by the way they respond to the sudden appearance of the unidentified aircraft, you could say that the flashback to the incident involving it alludes to several things. Let's consider them in turn.

First, the incident could allude to the general atmosphere of the community being a highly policed state. In general, if there are unidentified planes flying over you, you're in a state of war. And that raises a big question in readers' minds, because aren't we told that the community is completely peaceful? Perhaps there's an unknown war that exists outside the safe bubble of the community. Or perhaps the community has less control over its citizens than we are led to believe.

Second, the incident could allude to the fact that although the community is highly regimented and utterly safe, the unexpected still occurs. This sets the scene for Jonas's unusual job assignment, as well as the highly irregular course of events that his life takes afterward.

Third, the incident could allude to the fact that, although people are taught to be precise and obedient in all ways in the community, people there (like the pilot flying the plane) do make mistakes. Or, perhaps they disobey and "fly" off in their own direction. In this case, the plane foreshadows Jonas's eventual break from the community.

Fourth, the incident could allude to the eventual ending of the story, when planes once again make an appearance as they search for the missing Jonas and Gabriel. Once more, the planes are an unwelcome source of fright for Jonas as he tries to avoid capture.

Fifth, because Jonas finds it especially unusual that the aircraft is a "single-pilot jet," the incident could allude to the fact that the community is not as united as it might seem: there could be a single person in the community who "flies alone" and exists outside the community's regimented order. In that case, the plane foreshadows the appearance of the Giver himself. Considering the Giver's conversation with Jonas in Chapter 14 about the aircraft incident, when we learn that the Giver was the one who prevented the community from shooting down the pilot, it's reasonable to believe that the aircraft and the Giver are thematically connected: that is, they both represent a deviation from the norm.