Buttuh BOOM!
Got it:
It was a two part Disney movie special aired in 1975. The title is "The Sky's the Limit". Cast included Pat O'Brien and Huntz Hall.
A young boy that has to spend the summer at his grandfathers fathers farm. The boy isn't very happy about that until he discovers that his grandfather has an old airplane hanging in the barn. The two, together with the help of a few friends from his grandfather, restore the plane back to flying condition. During the maiden flight, the grandfather and the boy are being chased by the local sheriff, the boy's father and others. They can end their flight and it's discovered that the grandfathers pilots licence was signed by one of the Wright brothers.
Hot damn. Huntz Hall? I gotta get see one.