Simplify the following text for a Grade 17 native English-speaking person.
Original: In accordance with the Convention on International Civil Aviation all aircraft must be registered with a national authority ( such as the Federal Aviation Administration or Transport Canada ) , and furthermore , they must carry proof of this registration in the form of a legal document called a Certificate of Registration at all times when in operation .
Simplified: In accordance with the Convention on International Civil Aviation all aircraft must be registered with a national aviation authority and they must carry proof of this registration in the form of a legal document called a Certificate of Registration at all times when in operation .