Originally Posted by
12Watt Tim
Surely John Travolta is restricted SIC because it is a multi-crew aircraft and he doesn't hold an ATP.
Not likely as is other type ratings are for multicrew aircraft are not limited to SIC. In addition, there is to my knowledge no limit on the size of aircraft that can be operated under part 125 - and this part makes no mention of an ATPL requirement.
The OPs question will have different detail answers depending on the registration, but there are definitely private pilots in the States that operate as PIC 'learjet' sized turbine aircraft ( Harrison Ford is a PPL Citation Driver - PPL/ME/IR type rated that is)