From personal experience, the AIRPROX board is reluctant to rate incidents "A", where there is no solid radar evidence to corroborate a pilot report.
This incident reminds me of one I submitted some years ago. Another helicopter pilot who claimed to have seen us after being alerted to our presence by an A/G operator, for unknown reasons failed to fly in the direction and routing he had said he was actually flying and shortly afterwards flew directly over us (only just) from our 6 to 12 o'clock in a steep descent. I was hovering into wind at about 1200 feet. He missed us by possibly ten metres at most as he suddenly appeared from behind and above, with almost nil vertical separation. The tail of his aircraft almost passed through our rotor disc as he descended through our level on the same heading as us. Strangely, there is a common factor here - his transponder appears to have been switched off before we got close but came on without Mode C as he flew onwards at low level.
This pilot either did it deliberately (which is what he admitted in his report due to "late sighting") and therefore lied on the radio that he'd seen us and was then so incompetent he didn't realise how close we'd actually come to having a mid air collision.
It was as close as I ever want to get to another helicopter; it still makes me shiver to think about it (even though as "lsh" might recall, I've flown quite lot of pre-briefed very close formation in my time). However, the incident was formally classified merely as a "B" risk (i.e. safety not assured).
Last edited by ShyTorque; 22nd Nov 2018 at 19:26.