▁rose ▁above ▁zero , ▁then ▁the ▁air space ▁was ▁considered ▁unsafe ▁and ▁was ▁consequ ently ▁closed . ▁ ▁The ▁April ▁er u ption ▁of ▁E y ja f j alla jö k ull ▁caused ▁enough ▁economic ▁difficulties ▁that ▁aircraft ▁manufact ur ers ▁were ▁forced ▁to ▁define ▁specific ▁limits ▁on ▁how ▁much ▁ash ▁is ▁considered ▁acceptable ▁for ▁a ▁jet ▁engine ▁to ▁ing est ▁without ▁damage . ▁On ▁ 2 1 ▁April , ▁the ▁C AA ▁in ▁conj unction ▁with ▁engine ▁manufact ur ers , ▁set ▁new ▁gu idel ines ▁which ▁allowed ▁aircraft ▁to ▁fly ▁when ▁there ▁are ▁levels ▁of ▁vol can ic ▁ash ▁between ▁ 2 0 0 ▁and ▁ 2 0 0 0 ▁micro gram mes ▁of ▁ash ▁per ▁cub ic ▁met re . ▁These ▁levels ▁were ▁declared ▁by ▁govern ments , ▁aircraft ▁manufact ur ers , ▁and ▁air lines ▁not ▁to ▁have ▁safety ▁imp lications ▁if ▁appropriate ▁maintenance ▁and ▁ash ▁ins pection ▁procedures ▁were ▁followed . ▁ ▁From ▁no on ▁ 1 8 ▁May , ▁the ▁C AA ▁further ▁re vised ▁the ▁safe ▁limit ▁up wards ▁to ▁ 4 ▁mill igr ams ▁per ▁cub ic ▁met re ▁of ▁air ▁space . ▁Any ▁air space ▁in ▁which ▁the ▁ash ▁density ▁exceed s ▁this ▁limit ▁is ▁categor ised ▁as ▁a ▁no ▁fly ▁zone . ▁ ▁To ▁minim ise ▁the ▁level ▁of ▁further ▁dis ruption ▁that ▁this ▁and ▁other ▁vol can ic ▁er u ptions ▁could ▁cause , ▁the ▁C AA ▁announced ▁the ▁creation ▁of ▁a ▁new ▁category ▁of ▁restricted ▁air space ▁called ▁a ▁Time