@TeachMe
As far as I know the reason for that is that average sand only melts at temperature's above 1300/1400 degrees celcius , and vulcanis ashes usually around 500/600 celcius
Average TIT is around 850 celcius , so big engine trouble
Sand usually gives a lot of 'mechanical'damage to windscreens , a/c packs etc. , but the real killer is vulcanic ash , even in less dense properties.