My Nephew Screams WIth Fear When A Plane Fly's Near The House - Please Help?

Hi and thanks for taking the time to try and help, my nephew is 18 months old and I am looking after him due to her mother being incarcerated, the problem is we live near an airport and when a plane flies over the house (its not loud as by the time it gets here its really high in the sky) its enough that you can still hear it, my nephew screams with fear and runs and hides. This is becoming a problem as there would be at least 30 planes a day to fly over. I dont understand what is so scary and what to do to help, i have tried holding him but he completely freaks out and screams and cries. Any istance would be helpful. Thanks