Because helicopters are dangerous, the Queen asked William to refrain from flying, especially in bad weather.
The Queen is concerned that all five of them flying together could be hazardous and result in a constitutional crisis.
William's grandmother advised him to "stop flying himself," no matter how good a pilot he was.
Although William is an excellent pilot, the Queen believes that severe weather and accidents can occur at any time.
William and Kate preserve the monarchy, but the Queen worried about their safety.