The board gave the incident last September the highest risk rating - A - which is described as “aircraft proximity in which serious risk of collision has existed.”
It concluded the near miss was caused by a late sighting by the pilot of a Sikorsky S92 helicopter and the pilot of a Eurocopter EC135 helicopter failing to see the other aircraft.
The incident happened as the Sikorsky pilot was descending to land at Scatsta airport, nine miles away, when he saw the other helicopter 200ft below, flying in the opposite direction.
The pilot took a sharp left-hand avoiding turn, but the board said this probably “did not measurably increase the separation between the two helicopters.
“The minimum separation was recorded as 100ft vertical and 0.1 nautical mile [600ft] horizontal.”
The report said: “The board agreed unanimously that, because separation had been reduced to the minimum and chance had played a major part in events, nothing more could have been done to improve matters and the risk was assessed as Category A.”
The board said it was also at a loss to explain why the Sikorsky pilot had not received a warning from its collision avoidance system of the other helicopter getting close .
The Eurocopter pilot, who was flying from Tingwall airport on Shetland to the northern tip of the islands, did not see the other helicopter.