Cause Of Charles' Chopper Emergency Revealed The cause of a helicopter's "hairy" emergency landing involving Prince Charles and the Duchess of Cornwall has been published by air accident investigators. The incident in May last year occurred as the royal couple and four other passengers were en-route to the Hay-on-Wye literature festival in Wales. The report said it was on the approach to Denham that the pilot noticed the aircraft was "out of balance." The Sikorsky S-76C helicopter began to veer - or "yaw" - with little or no control to the right as it approached Denham aerodrome in Buckinghamshire shortly after taking off, the report said. With the heir to the throne, his wife Camilla and the other passengers briefed about the situation, the 53-year-old captain attempted to correct the problem before declaring a 'Pan' emergency, a less serious emergency than a Mayday. The Duchess of Cornwall pictured at the book festival The two-man crew subsequently carried out a "running" landing and ground-taxied before "the aircraft was shut down and the crew and passengers vacated the aircraft." At the time a royal source described the moment as "quite a hairy incident." The Air Accident Investigation Bureau said the fault was later traced to the splitting of a steel ball within the system helping to control the "Yaw" system, in turn causing a leak in hydraulic pressure. The part in which the ball had split was later replaced and the helicopter had subsequently "flown without further incident." But the report indicated the part had only been fitted two months before the incident and had totted up just 73 flying hours. The AAIB said Sikorsky was in the process of issuing a safety advisory and a flight manual revision "to inform operators of the symptoms of a PDTA fault and actions to be taken by the crew." After their delayed arrival at the Hay festival, the royal couple appeared relaxed, with neither alluding to their in-flight drama.