Two trainee pilots, involved in a fatal mid-air crash in Buckinghamshire, had "limited" opportunity to see each other.
That's according to the findings of the Air Accident Investigation Branch, based in Farnborough.
Four people died in the crash which happened on the 17th November last year.
A light aircraft and a helicopter collided above Waddesdon.
The Cessna 152 was conducting a sustained descent while the helicopter was maintaining a height of around 1,500ft until the point of the collision.
Neither the plane or the helicopter were receiving an air traffic control service.
Two student pilots, Nguyen Thanh Trung who was 32, and 18-year-old Saavan Mundae died in the crash along with instructors Michael Green, 74 and Jaspal Bahra, 27.
The report by the Air Accidents Investigation Branch said:
"Although the occupants of both aircraft were in proximity for some time, they were both following a similar track and were not in each other's field of view."
The flying club which operated the plane has since highlighted the importance of maintaining an effective lookout to instructors.