Four people died at the scene, where a helicopter collided with a Cesna 152 light aircraft.
Nguyen Thanh Trung, 32, from Vietnam, and Capt Mike Green, 74, of Berkshire, who were in the helicopter, died from multiple injuries in the crash near Waddesdon Manor, Buckinghamshire.
Saavan Mundae, 18, and Jaspal Bahra, 27, both from London, died from head injuries in the 17 November accident. They were both in a two seater Cesna 152 light aircraft.
Both had taken off from Wycombe Air Park, also known as Booker Airfield, about 20 miles away from the crash site.
At the time of the accident Captain Green, of Long Lane, Newbury, was in the helicopter giving a lesson to Vietnamese military officer Capt Trung.
Mr Bahra, a flight instructor from Tokyngton Avenue, Wembley, was giving a flying lesson to Mr Mundae, a student from Kingwood Court, Richmond.
There was a pre-inquest review in May 2018, but this could not be concluded until the Air Accident Investigation Branch (AAIB) report had been finished.
The families of Jaspal Bara, Mike Green, and representatives from the Nygen family were present.
On the first day, the inquest heard a testimony from Dr Kolar who said the four men died of multiple injuries, suffering widespread non-survivable blunt force injuries.Dr Kolar said there were levels of 24 per cent carbon monoxide in the body of Jaspal Bahra. He said a normal level would be 3 per cent, with it being 10 per cent in smokers. He said he could not tell the source of the reading from the result.
Bucks coroner Crispin Blunt is presiding over the inquest.
Today (Wednesday May 1) the inquest jury heard testimony from the Air Accident Investigation Bureau (AAIB) report.
During the morning of the day of the accident, the instructor Mr Bahra completed a training flight in G-WACG, the Cesna 152, with another student.
The accident occurred on the second flight of the day.
One witness, about 0.5m to the south-west of the collision, saw the two aircraft immediately before the collision in close proximity, estimated at around 20m apart.
He described both as flying fairly low and observed that “the plane was gliding down slightly” and the helicopter “was directly underneath the plane and seemed to be rising underneath it.”
Both aircraft were based at Wycombe Air Park/Booker (WAP).
Stewart Wayne from the Civil Aviation Authority gave evidence at the inquest. He is an experienced former Air Force pilot who has experience flying at low level in Europe, the Middle East, America and Canada.
He said: “The incident occurred in category G airspace. In class G airspace, aircraft may fly when and where they like, subject to a set of simple rules.
“Although there is no legal requirement to do so, many pilots notify Air Traffic Control of their presence and intentions and pilots take full responsibility for their own safety, although they can ask for help.
“G is the least regulated airspace in the U.K. A Boeing 747 or a hang-glider could operate in this area if they so wished. Every aircraft can use this space.
“Once they left the airfield, there was no requirement for them to have any equipment switched on. No GPS, satellites, etc. The captain is responsible for the safety of their passengers. They must maintain a good lookout to ensure they 'see and avoid' potential collisions.”
As neither aircraft was electronically conspicuous to the other, the inquest heard that the only available method of collision avoidance between the two aircraft was ‘see and avoid.’
There are considerable and well understood limitations to ‘see and avoid’ and there was no evidence to suggest that the occupants of either aircraft had seen each other in time to avoid the collision.
Stewart Wayne added: “Blind-spots could well have been a contributing factor. The scan technique was was perhaps not carried out as stringently as it should have been. It's like being in a car with a pillar. You need to regularly check your blind spots.”
The opportunity for the two aircrafts to 'see and avoid' each other was very limited, coroner Crispin Blunt said after examining the geometry of the flight paths.
Stewart Wayne agreed with this assertion.
The legal representative of Mr Trung asked Mr Wayne about weather conditions, which he confirmed as very good.
The case continues at Beaconsfield Coroners Court.