The Sikorsky S-76C helicopter, with the royal pair and four other passengers aboard, began to veer – or yaw – to the right as it approached Denham aerodrome in Buckinghamshire, a report by the Air Accidents Investigation Branch (AAIB) said.
Yaw control to the left was also limited and with various attempts to correct the problem having no effect, the 53-year-old captain declared a Pan emergency, a less serious declaration than a Mayday.
Charles and Camilla, who were heading to an engagement at the Hay-on-Wye Festival in Wales, and the other passengers were briefed on the situation.
They were also told that the two-man crew would carry out a running landing at Denham – a different type of landing that can be used if an aircraft has a problem.
The AAIB report said: “The crew performed an uneventful running landing and ground-taxied to dispersal with the yaw control abnormality still apparent.
“The aircraft was shut down and the crew and passengers vacated the aircraft.”
The incident, described at the time by a royal source as “quite a hairy incident”, took place at 9.30am on May 23, last year.
The royal couple travelled on to Hay by car, arriving around three hours behind schedule.
The AAIB said the fault was later traced to the splitting of a metal ball within a system that helps control yaw. This had caused an internal leakage of hydraulic pressure which led to a restriction of the helicopter’s yaw control system.
The report said the ball was “most likely to have fractured during the flight from London to Denham”.