UK accident investigators have determined that visibility limitations and an absence of suitable warning systems contributed to the fatal collision between a helicopter and a light fixed-wing aircraft in which four people died.
The two aircraft came down on 17 November 2017 after a Cessna 152 descended into a Helicopteres Guimbal Cabri G2 at around 1,400ft while both were flying on the same track, says the Air Accidents Investigation Branch (AAIB).
Examination of the wreckage showed initial impact marks on the Cessna's right wing consistent with being struck by the helicopter's main rotor.
However, a "single rotor blade" subsequently sliced through the upper half of the 152's aft fuselage, precipitating the detachment of its empennage.
Damage caused to the Guimbal, which included the loss of a main rotor blade, "would have rendered the helicopter uncontrollable", says the AAIB.
Although weather conditions were clear with good visibility, the relative positioning of both aircraft meant that the Cabri remained obscured in the Cessna's blind spot.
"The angle at which the aircraft were closing was such that neither was in the field of view of the other, until perhaps a few moments before the collision," the report says.
Post-accident modelling based on radar data showed that when the 152 was 1,950ft above the helicopter, it was only 0.5nm (0.9km) behind it.
The AAIB calculated that it would have needed to be 1.9nm to the rear "for the pilot to have had any opportunity of detecting a possible conflict without additional manoeuvring".
However, even if the Cessna pilots "did bring the helicopter into their field of view", the chances of either occupant seeing it would have been "limited" given the lack of conspicuity markings and "uniform" colour of its main rotor blades.
Although the Cabri was fitted with an ADS-B Out transponder – which allowed the transmission of position and altitude data – neither aircraft was equipped with the corresponding receiver.
Work is ongoing, led by the UK Civil Aviation Authority, to promote the development and use of electronic conspicuity devices, adds the AAIB.
Both aircraft – operating training flights with an instructor and student pilot on board – had departed from Wycombe airfield at around 11:45 and, while their initial courses were different, the two ended flying on the same track on a heading of 340°.
The Cessna climbed to 4,130ft, before descending at a rate of 1,230ft/min (6.25m/s) at a ground speed of between 79-85kt (146-157km/h), while the helicopter maintained an altitude of 1,380-1,550ft at 60-73kt.
The aircraft came down near Waddesdon, around 16 miles (26km) north of their departure point in central England.
Since the accident, the flying school that operated the Cessna has instructed pilots to regularly change their heading during a prolonged descent to "clear the airspace below".