The plane is believed to have come down on private land less than two miles from Prestwick International Airport.
The A719 was closed near the B739 for several hours in the aftermath of the crash. It reopened shortly after 19:30.
An AAIB spokesperson said: “We have sent a multi-disciplinary team of AAIB inspectors to Prestwick to begin investigating an accident involving a light aircraft.”
Police Scotland said its inquiries were ongoing.
