Air traffic control made “‘numerous’ attempts to contact a De Havilland pilot from Chatteris who forced a helicopter with the Duchess of Cornwall on board to veer to avoid a possible collision.
"The pilot noted that this was his eighth flight of the day," an air safety investigation report noted.
The pilot told the inquiry he had been distracted by "unrelated domestic concerns" and had forgotten to establish contact with Lakenheath air traffic control.
The Sikorsky S76 from the Royal Flight was taking the Duchess back to her home in Gloucestershire after last year's Sandringham Flower Show.
The S76 came within 200ft of the de Havilland DHC6 aircraft carrying parachutists, which had just taken off from an airfield at Chatteris, as it flew a prearranged flight plan south of Wisbech at around 12.20pm on July 25 last year.
A report by the UK Airprox Board says an air traffic controller at RAF Marham "perceived the severity of the incident as high". The Royal pilot described the risk as "medium". The de Havilland pilot assessed the risk of collision as 'low'.
A supervisor at RAF Marham told the inquiry that she had seen the para-dropping aircraft get airborne from SkyDive Chatteris and take up a north-northeasterly profile. As the flight wore on she called RAF Lakenheath to find out if the Chatteris pilot had been in touch and to get him to call her once he'd landed.
"His response was that he was aware of the S76 inbound earlier on in the day and the reason for him delaying his call to Lakenheath was due to him being distracted by the passengers in the back of the aircraft," says the report.
The report says that the de Havilland pilot said he that when he saw the helicopter he recognised it immediately and initiated a turn to the right to increase separation "whilst simultaneously calling Lakenheath. He found that they had been calling him blind for several minutes, attempting to warn him that he was heading towards the S76".
The inquiry board looked at video evidence and listened to sound recordings, one of which included the comment from the Marham supervisor to a Lakenheath controller.
"Ok, if you can keep trying to call him because he's now about to burst the bubble; he's within five miles and three thousand feet unless co-ordinated".
In their summary the board noted that there was no formal requirement for the Chatteris aircraft to call Lakenheath by a certain point or altitude but agreed it was probably best if they did.
The board says it was "heartened to learn" the Chatteris parachuting school had addressed the situation and was agreeing procedures about contact with Lakenheath once their plane reached an agreed height.
They praised the Lakenheath controller for his repeated efforts to contact the DHC pilot.
"Members agreed that the aircraft were approaching about head-on and that ultimately both pilots were required to turn right, which they did," says the report.
However the DHC6 pilot was not aware of the S76 until a relatively late stage and so was unable to apply an early avoiding turn.
But they felt that the information possessed by the S76 pilot could have meant him taking action early on and he had "essentially flown into contact with the DHC6".
However in retrospect it was felt that the S76 pilot had taken some action and so it was unfair to characterise the incident as flying into conflict".
They summed it up as a conflict in airspace noting that the DHC6 pilot did not change to the Lakenheath frequency at the usual height and that the S76 pilot "did not act in a timely manner on the traffic information passed by Marham."
Airprox board members concluded that the regulations governing separation zones around Royal flights "introduced more ambiguity and complexity than clarity". They are recommending the rules are reviewed by the Civil Aviation Authority (CAA).
During the same flight, the Royal helicopter had to take evasive action to avoid a glider as it neared the Duchess's home at Highgrove House.
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