09/01/2014 08:43 GMT | Updated 25/01/2014 16:01 GMT

Prince Of Wales And Duchess Of Cornwall's Emergency Helicopter Landing Details Revealed

An emergency landing drama in which a helicopter carrying the Prince of Wales and the Duchess of Cornwall started veering to one side has been described by air accident investigators.

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 camilla

A royal source described it as "quite a hairy incident"

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.

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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 which 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."

The AAIB went on: "A detailed metallurgical analysis of the ball by the (USA) National Transportation Safety Board is continuing. However, the most likely cause of the ball fracture was an anomaly in its heat treatment process during manufacture."

The report said that it was on the approach to Denham that the pilot noticed the aircraft to be "out of balance".

Realising they had a yaw control problem, both pilots conducted system checks and further diagnosis before declaring a Pan.

The report said the Pedal Damper Trim Actutator (PDTA) - the part in which the ball had split - had been replaced after the Denham incident and the helicopter had subsequently "flown without further incident".

After an overhaul, the PDTA had been fitted in the 2009-manufactured Sikorsky helicopter at the end of February 2013 and had totted up about 73 flying hours since installation. A new steel ball had been fitted prior to this PDTA fitting, the AAIB said.

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".

On the day of the incident, a spokesman for Charles and Camilla had said that the helicopter had "diverted" to Denham.

The Hay engagement was meant to be a day-long one, with the planned leisurely tour having to be carried out more quickly following the couple's late arrival.

However, today's AAIB report made no mention of a diversion and described the pilot becoming aware of the yawing problem "during approach and descent to Denham Aerodrome".

The report also made no mention of the nature of the flight, nor of the status of the passengers, merely saying the flight was a "private" one.

Once at Hay, the royal couple were reported to be looking relaxed and smiling, with neither alluding to their in-flight drama.

''They were unflappable despite what they went through. If anyone else had gone through what they did they would have cancelled their day,'' said one pensioner at Hay.