In The Giver, what was the unidentified aircraft at the beginning of the story?

The unidentified aircraft that flew over the community was a jet plane.

One day a “needle-nosed single-pilot jet” flies over the community.  This causes fear in Jonas and everyone else, because planes are not allowed to fly over the community.  The Speaker orders everyone to stop what they are doing and seek shelter immediately. 

He had seen it both times. Squinting toward the sky, he had seen the sleek jet, almost a blur at its high speed, go past, and a second later heard the blast of sound that followed. Then one more time, a moment later, from the opposite direction, the same plane. (Ch. 1) 

The pilot-in-training is released for his error.  He flew over the community by accident.  The community is very intolerant of error, especially when it causes fear or discomfort for everyone.  The Elders considered shooting the plane down, but The Giver urged them to be cautious and wait for more information. 

The plane might have been a search plane.  When Jonas and Gabe escape, they are dogged by planes who are searching for them, looking for their heat signature.  Jonas is terrified of getting caught.  He uses the memories to hide their body heat, making them both cold. 

He knew they were search planes. They flew so low that they woke him with the noise of their engines, and sometimes, looking out and up fearfully from the hiding places, he could almost see the faces of the searchers. (Ch. 21) 

Eventually, the planes give up when they do not find Jonas and Gabe.  The plane Jonas remembers in the beginning of the book foreshadows this later trouble.  Jonas is convinced that he will be released if the search planes find him.  He has already broken too many rules and caused too much trouble.