You are so right that very few understand Ac drilling... your good self included.
AC works much like RC, except the drill blade operates more like a vacuum cutting tool than high PSI energy hammer tool. Both use double-tube rods to inject the air between the inner and outer tubes, and retrieve the sample uncontaminated up the inner tube to the cyclone.
Me thinks you may be confusing AC with RAB drilling, which uses a single steel tube rod to inject air to the drill bit and returns sample up the hole where it can collect material from all the way up hole and become contaminated... or maybe you simply don;t know what you're talking about? IMO, AC delivers a better sample in sands and clays because of the cleaner cutting only action of the bit and neat vaccum extraction of sample only from inside the bit due to the less aggressive method. Too much indurated hard rock or water, then RC comes into it's own with the ability to cut through and keep water out with higher air pressure...
The downgrade of shifting to RC drilling is due to geological interp... The most economic Nb deposits are and probably will continue to be high grade supergene blankets on top of the harder, less weathered karst to fresh rock carb. Supergene weathering will increase Nb grades by between 4-10x due to removal of highly weatherable and mobile Ca-Mg minerals and other elements in the primary carb rock, concentrating less mobile Nb minerals many times over. As soon as you start drilling RC deeper than highly concentrated Nb clays and in soft transitional zone the odds of hitting thick continuous high grades below low-grade supergene blanket drops to very low...
In short, if there is no HG supergene blanket in lateritic clays the odds of HG primary orebody sitting durectly under it is very low... and AC drilling is perfect for sampling soft clays like weathered carbonatite.
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