Both planes were registered in America, which means they cannot be flown commercially
From report on BBC website referring to a/c used on some of the flights listed:
'Both planes were registered in America, which means they cannot be flown commercially in Europe without the express permission of US regulator, the Federal Aviation Authority, and the CAA.
The flights were listed on flight plans, as non-commercial.
BBC Wales has discovered that eight days after the plane carrying Sala crashed, one of these planes was de-registered in the US and re-registered as a Guernsey-based plane.
It was then placed on the company's AOC.'