Illegal charter / grey charter
Illegal or grey charter is carriage for hire on an aircraft or by an operator not certificated to do it, typically a privately operated Part 91 or non-AOC jet whose owner or a middleman takes money for flights. It is often dressed up as a dry lease, a cost-sharing arrangement or membership of a 'club'. The danger is not only legal: the pilots may not be trained to commercial standards, the maintenance programme may be lighter, and the insurance will usually be void for a commercial flight, leaving passengers with no cover in an accident. Protect yourself by asking for the operator's certificate number and confirming the tail number is on it.