I'm thinking of kit/owner-built aeroplanes. They can fly like regular (type-certified) aeroplanes, but have to be placarded (e.g. https://www.aircraftspruce.com

I'm thinking of kit/owner-built aeroplanes. They can fly like regular (type-certified) aeroplanes, but have to be placarded (e.g. https://www.aircraftspruce.com/catalog/cspages/passgrwarn.php https://www.aircraftspruce.com/catalog/cspages/passgrwarn.ph...) https://www.faa.gov/aircraft/air_cert/airworthiness_certification/sp_awcert/experiment/expt_operating https://www.faa.gov/aircraft/air_cert/airworthiness_certific...