Collison due to no flight plan
 

https://www.euronews.com/2019/01/26/...e-italian-alps
This accident, happened more than two years ago at the Alps close to Mont Blanc, reminds us how important it is to file a flight plan when cross into foreign airspace even when it is very close to border. This ski area is frequented by pilots on both sides of the border, considered virtually as local flights for many years. So there was no radio contacts with FIS. My own experience is that radio contact with FIS there is very difficult due to terrain. So the only conclusion the Italian authority could reach was that the French plane pilot was responsible for the accident solely because he did not file flight plan to cross border even though the site was just a couple of hundred meters from the French side.

My memory is that the French pilot is serving a considerable prison sentence for this.
If he HAD filed a flight plan, would it have been known to the helicopter pilot?
If the helicopter had known his flight plan, would it have helped?
Only radio announcements on the same frequency, or ADS-B both out and in could have assisted "see and avoid".
And "see" has faults. I only avoided a Pa28 200' above my C172, both descending, our radar blips fused, thanks to the very agitated ATC we contacted for approach.
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You are quite right that if both had ADS-B on board, both side may have "see and avoid". If my memory is correct, in this case one was about to land and the other about to take off in poor visibility. None could have time to see the other and avoid. Yes all use mountain frequency when flying in that area, but people are usually on different tone. The French speak French and the Italians speak Italian. A/A communication has always been a problem there. A scapegoat has to be found, thus the flightplan.