Helicopter and plane in near-miss over Maidford
Pilots in an RAF helicopter and a light aircraft failed to spot each other until it was almost too late, according to a near-miss report.
The Chinook came within 600 ft (183m) of the light aircraft over Maidford in Northamptonshire.
The UK Airprox Board, which investigates near-misses, said the pilots were flying "at the same height and were head-on".
It concluded that "safety margins had been much reduced below the normal".
The Chinook pilot reported the incident to the board, which happened at 17:50 BST on 20 August last year.
'Broke hard left'
He told investigators that he was flying at 1,800 ft (549m) when he noticed a blue and white plane, which was a Grumman AA5, flying close to him on his left-hand side.
The AA5 pilot told investigators he suddenly saw "two small lights dead ahead", so he "broke hard left" into clear airspace, but the Chinook took the same course of action.
The board said the AA5 pilot "could have done more to ensure greater separation as they approached head-on", while the Chinook crew did not spot the other plane until it was almost too late, possibly due to "worsening conditions".
Both pilots shared an equal responsibility "for collision avoidance" and "not to operate in such proximity to other aircraft", it concluded.