500 feet clearance below e.g. an A380 on approach doesn't sound like much.
Isn't that what you have now? Aircraft fly the current STAR at 3000ft. Its all just 500ft lower than it was before....not an issue, just need to be aware that the airspace has dropped 500ft
So why is gbas accuracy so bad that increased tolerances are needed?
Umm...it has nothing to do with the accuracy of the system or tolerances. The tolerances are still the same, just lower. It has everything to do with the fact that the airspace to the south of YMML was originally designed for non precision approach procedures. Introduction of GLS, upgrades RW 34 to a precision approach runway and this requires different airspace architecture for containment. It has to do with the fact that enough airspace must be provided for intercept of the localiser before intercept of the glide path. This is not a constraint for NPA.
Alpha