Argentinian and French authorities investigating the 10 March mid-air helicopter collision that killed eight French nationals – including three star athletes – and both Argentine pilots will have the unusual advantage of video footage of the incident.

Mobile phone footage shown byFrance24, theBBCand other broadcasters shows clearly the moment of mid-air impact between two Airbus Helicopters AS350s flying over the Argentine province of La Rioja, northwest of Buenos Aires. The accident happened during filming of French reality television programme Dropped forTF1, which may have higher-quality images.

Flightglobal operations and safety editor David Learmount, reviewing the video, observes that when the two helicopters first appear in the video footage, they appear to be flying a parallel course almost exactly abeam each other. Their tail booms are also more or less parallel, indicating the same heading. But just before the collision, the aircraft on the right turns slightly left and the small distance between the two is eliminated in a second.

If aircraft are flying in formation the only safe way for it to be done is for one of them to be the agreed leader, who determines the flight trajectory of the formation, and the other or others simply concentrate on formating on the leader. There may have been no such agreement in this case, notes Learmount.

Another factor that may have applied to this accident is that helicopter pilots sit on the right of the cockpit, so in this case the pilot of the aircraft on the right (closer to the video camera) may not have been able to see the helicopter on his left until it was too late. This could have been a case of temporary unsighting, or some form of distraction. The pilot in the helicopter on the left would have had a good view of the proceedings, but was given insufficient time to act.

Victims include yachtswoman Florence Arthaud, Olympic swimmer Camille Muffat and Olympic boxer Alexis Vastine; one of the pilots, Juan Carlos Castillo, is understood to have been formerly with the Argentinian air force.

France’s BEA investigation agency is assisting its Argentininian counterpart, JIAAC, in the investigation.