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8 November 2018, 11:40 | Updated: 8 November 2018, 11:43
Two trainee pilots involved in a fatal mid-air crash in Buckinghamshire only had "limited" opportunity to see each other, an investigation has found.
Four people were killed when the accident happened above the Waddesdon Estate on November 17 last year.
Both two-seater aircraft - a Cessna 152 plane and Guimbal Cabri G2 helicopter - were engaged in training flights with instructors at the time of the crash.
The collision happened while the light plane was embarking on a prolonged descent from above the helicopter.
A report by the Air Accidents Investigation Branch (AAIB) stated:
"The opportunity for the occupants of either aircraft to see the other was limited because, although they were in proximity for some time, they were both following a similar track and were not in each other's field of view."
Neither aircraft was receiving an air traffic control service.
Such "see and avoid" flying has "well-known limitations", according to the AAIB.
Investigators noted that pilots of planes such as the Cessna 152 must make a series of shallow turns to see into blind spots, while it would have been "impractical" for the helicopter pilot to search the area behind him.
The flying club which operated the light plane has issued a notice to instructors highlighting the importance of maintaining an effective lookout.
The warning also stressed the need to change the direction of a plane during a long descent to check the area ahead was clear.
Work to develop electronic devices which signal the presence of an aircraft to other airspace users is being led by the Civil Aviation Authority.
The victims on board the helicopter were Vietnamese pilot Thanh Nguyen, 32, and veteran instructor Michael Green, 74.
Those killed on the plane were 18-year-old student Saavan Mundae and instructor Jaspal Bahra, 27.
The crash happened over woodland near the Waddesdon Estate, the former country seat of the Rothschild banking dynasty.
Both aircraft came from the Wycombe airstrip known as Booker Airfield, which is about 20 miles from the crash site.