[krisoft]

"§ 830.2

Aircraft accident means an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight and all such persons have disembarked, and in which any person suffers death or serious injury, or in which the aircraft receives substantial damage. "

https://www.law.cornell.edu/cfr/text/49/830.2

Later on the same page you can see how they define "substantial damage" and "civil aircraft" too.