Given the task definition and input, reply with output. In this task, you will be given a passage to read. A fill in the blank question will be given to you. Your answer should fit the blank appropriately.

Under CAA and EASA rules , all aircraft are required to meet certain standards of airworthiness to fly safely and legally . Aircraft that meet these standards are issued with a Certificate of Airworthiness . However , British @-@ registered aircraft which are excluded from the scope of EASA , and which cannot satisfy the requirements for the issue of a Certificate of Airworthiness , may be issued with a Permit to Fly . This allows them to fly in UK airspace subject to certain limitations , for example being restricted to day @-@ time flights under visual flight rules only . A number of organisations ( e.g. the British Microlight Aircraft Association and the Light Aircraft Association ) have obtained a standing over @-@ flight permission for Permit to Fly aircraft within their area of interest with some European countries , notably France . Permits are typically issued to vintage and historic aircraft , amateur built aircraft , and microlights . fill in the blank : Under CAA and EASA rules , all aircraft are required to meet __________________ standards of airworthiness to fly safely and legally .
certain