▁Limited ▁Zone ▁( TL Z ). ▁Air space ▁categor ised ▁as ▁T L Z ▁is ▁similar ▁to ▁air space ▁experien cing ▁severe ▁weather ▁in ▁that ▁the ▁restrictions ▁are ▁expected ▁to ▁be ▁of ▁a ▁short ▁duration ; ▁however , ▁the ▁key ▁difference ▁with ▁T L Z ▁air space ▁is ▁that ▁air lines ▁must ▁produce ▁certific ates ▁of ▁comp liance ▁in ▁order ▁for ▁their ▁aircraft ▁to ▁enter ▁these ▁areas . ▁Fly be ▁was ▁the ▁first ▁air line ▁to ▁conform ▁to ▁these ▁reg ulations ▁and ▁their ▁aircraft ▁will ▁be ▁permitted ▁to ▁enter ▁air space ▁in ▁which ▁the ▁ash ▁density ▁is ▁between ▁ 2   m g ▁and ▁ 4   m g ▁per ▁cub ic ▁met re . ▁ ▁Air space ▁restrictions ▁in ▁Europe ▁ ▁Under ▁international ▁reg ulations , ▁all ▁scheduled ▁passenger ▁fl ights ▁operate ▁under ▁the ▁instrument ▁flight ▁rules ▁( IF R ) ▁which ▁requires ▁clear ance ▁to ▁be ▁obtained ▁from ▁air ▁traffic ▁control . ▁From ▁ 1 4 ▁April ▁on wards , ▁the ▁controlled ▁air space ▁was ▁closed ▁in ▁several ▁European ▁countries ▁and ▁no ▁IF R ▁clear ances ▁were ▁granted . ▁The ▁air space ▁of ▁I cel and ▁itself ▁was ▁hardly ▁dis rupted ▁in ▁this ▁period ▁( although ▁a ▁change ▁in ▁the ▁wind ▁direction ▁on ▁ 2 3 ▁April ▁caused ▁the ▁main ▁air port ▁to ▁be ▁closed ). ▁ ▁By ▁ 2 3 ▁April , ▁the ▁restrictions ▁were ▁lifted ▁over ▁much ▁of ▁Europe ▁through ▁the ▁introduction ▁of ▁new ▁gu idel ines ▁on ▁vol can ic ▁ash ▁density . ▁After ▁this ▁date ▁there ▁were