From AIN dated 10 Jan 19:
Unauthorized air charter—often called gray charter and even “Part 134 1/2” operations—has long been illegal. Yet some aircraft operators still flout or inadvertently violate the FARs involving proper on-demand or charter operations. Continuing to operate such disguised charters with impunity or in obscurity might be short-lived, however, as the FAA has recently ramped up its investigations and enforcement actions against such offenders.....
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Gray Charter Violations. In a December 4 press release, the FAA announced a proposed $624,000 civil penalty against
Steele Aviation of Beverly Hills, California, for allegedly conducting 16 customer-carrying jet flights “for hire” when the company did not have the air carrier certificate required for these operations and allegedly used unqualified pilots. The case illustrates how the FAA will pursue charter operations that appear normal but blatantly fail to qualify for charter services under Parts 135 and 119..