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.