It seems to be fairly logical in my mind that if you take ten airplanes flying from A to B (eastbound, let’s say) and tell them they must fly a semi circular level, the majority of UK GA traffic will choose 3500’. If the majority is all at the same level, then surely that increases the risk of collision? If you take the same 10 and tell them to fly at whatever level they want (and stipulate over 3000’, to keep it more fair) then you will get all sorts of levels being flown likely from between 3000’ and 4000’ feet, and if everyone is at a different level, surely then the risk is reduced?
Your logic and mine are very different. Given your scenario If I was flying eastbound at 3800 ft and IFR traffic was travelling west at 4000 ft I might be a little wary given the chances of both of us being plus or minus 100’.
But as has been stated this is perfectly legal in the UK it is not in France so perhaps we feel safer with what we know.
I keep it simple by flying a glider. Continuously going up or down, never in level cruise so not applicable to me ;)
Xtophe wrote:
I keep it simple by flying a glider. Continuously going up or down, never in level cruise so not applicable to me ;)
Cheater! 
(couldn’t resist…
gallois wrote:
But as has been stated this is perfectly legal in the UK it is not in France so perhaps we feel safer with what we know.
Since there seems to be no increased en-route occurrences of mid-air collisions in the UK vs France, I would wager that neither procedure is safer than the other …
Exactly. The topic tends to get people worked up. But in the end, as with many other things in GA, there is no data; no science to prove which is safer than the other, even though many think they know it.
I would suggest that the reason it makes little difference to mid-airs is because most GA happens at levels which are too low to be applicable to semicircular numbers.
For example, in the UK, the vast majority of traffic is at/below 2000ft. And nearly all traffic I have seen in France was also very low down.
Lack of data does NOT mean that both approaches are equally safe in principle, saying that is a logical fallacy…
Peter wrote:
I would suggest that the reason it makes little difference to mid-airs is because most GA happens at levels which are too low to be applicable to semicircular numbers.
I’d rather suggest that the reason is that enroute mid airs in GA are an extremely rare event – so there is simply only very little difference to make even if something could eliminate enroute midairs completely.
Alpha_Floor wrote:
Lack of data does NOT mean that both approaches are equally safe in principle, saying that is a logical fallacy…
Right! But it does mean, that it simply doesn’t matter which of the two approaches is safer in principle because there is not much you can improve.
The current practice of flying semi circular altitudes is extremely safe (in the ballpark of less than one accident in Europe every 10 years). In the absence of good data, why risk this extreme level of safety we already established by changing the rules that perhaps theoretically could improve this safety a bit at the risk of lowering it significantly.
Or to put it simpler: There is much more to loose by changing semicircular rules than to win.
Alpha_Floor wrote:
Lack of data does NOT mean that both approaches are equally safe in principle, saying that is a logical fallacy…
But we do have data, don’t we? Running quickly through the AAIB and BEA reports, I found from the AAIB the latest mid-air report occurred in June 2019 and nothing for 2020 or 2019 from the BEA. There’s an old report from BEA on mid-air occurrences that has 1.7 mid-airs per year in the interval 1989-1999. Since we hardly seem to have any mid-air collisions at all anyway, a statement “behaviour X causes more mid-airs than behaviour Y” simply has no basis.
