Prince Charles has "concerns" about Prince William flying his family by helicopter.

Prince Charles and Prince William have reportedly not always seen eye to eye. In an excerpt from his new book, A Biography of the Future Queen (an excerpt from: A Biography of the Future Queen), royal expert Robert Jobson states that Prince Charles has had disagreements with his son, Prince William, about flying the family by helicopter.

"One of the reasons for the recent disagreement is Prince William's stubborn rejection of his father's advice on the succession issue," Jobson explains (via the Daily Mail). Importantly, Prince Charles feels that William feels it is unsafe to allow his family, including Prince George, the second in line to the throne, on the plane. Jobson noted, "Earlier this year, the King expressed concern about his son." But the prince did not always take that concern well.

According to Jobson, William flat out refused to stop flying the helicopter with his family on board. As a result, the King is said to have resorted to drastic measures. He insisted that Prince Charles sign a formal document acknowledging the danger and accepting full responsibility for his actions."

Prince William is a very experienced helicopter pilot, having completed a search and rescue training course at the Royal Air Force (RAF) Valley in Anglesey, Wales, in 2010. However, Prince Charles is not the first royal to express concern about Prince William piloting his wife and son in a helicopter.

"William upset the late Queen when he disobeyed her request that he not pilot the 115 miles from Kensington Palace to his home Anmer Hall in Norfolk," Jobson wrote in his new book. One aide said that the Queen was troubled by the 1967 crash that killed the captain of the Queen's Flight (though no royals were on board) and "did not refrain."

William does not want to stop flying helicopters anytime soon. Of course, that could all change once his son Prince George turns 12, the Mirror reports, as both men are important heirs to the British throne, and William and George will likely not be allowed to fly together

in order to protect the line of succession.