Does anyone know the reasoning behind using "not above" rather than "or below"?
Where I come from the standrad phraseology seems to be "1500 feet or below", but in the UK it's "not above 1500 feet". I'm just thinking that, from my untrained point of view, it seems more logical to use "or below" since the risks of missing part of the transmission are less. E.g.
Controller transmits "1500 feet or below", pilot hears, "1500 feet", in that case the only think that you miss is that the pilot maintains 1500, rather than the block of 1500 or below.
But if the controller transmits "not above 1500 feet", and the pilot hears that as "above 1500 feet", the meaing of the transmission is reversed.
Now granted with proper readbacks and hearbacks the above should never be able to happen, but I'm just trying to see if there's any advantage to using "not above" rather than "or below" style phraseology. Once again I'm not an ATCO or a pilot (even though I hope I will one day make it to become a validated ATCO), but from my perspective it seems more safe to use a positive "or below" clearane rather than a "negative" "not above" clearance.