Charles and Camilla were 15 minutes into their flight from London to the Hay Literary Festival in Powys when a "technical fault" aboard the chopper forced the pilot to divert the flight to Denham Aerodrome in Buckinghamshire, according to the Press Association.
"The pilot carried out a controlled emergency landing after diverting to the airport," a spokesman for the royal couple said.
An investigation has been opened into what caused the mid-air problem aboard the helicopter, which had as many as five people, in addition to Charles and Camilla, aboard.
Once safely on the ground, the prince and the duchess gamely boarded cars to continue on to Hay-on-Wye as scheduled.
Despite arriving three hours late, Charles and Camilla were greeted by a crowd of several hundred people at the festival, an annual gathering of authors, politicians and celebrities.
"They were unflappable despite what they went through. If anyone else had gone through what they did they would have canceled their day," one attendee told the Press Association.
Prince Charles was later scheduled to attend the Welsh National Opera's opening night of Lohengrin at the Wales Millennium Center in Cardiff, according to the BBC.