Mayday! Mayday! “Will & Grace” star Eric McCormack pitched such an out-of-control hissy fit aboard a plane that the incident was reported to the FAA, sources say.

The 44-year-old erupted after the crew told him that a booster seat he brought onboard for his 4-year-old son, Finnigan, was too large, violated regulations and had to be stowed with the luggage.

The actor was taking an Alaska Airlines flight from Los Angeles to Vancouver when crew members barred the non-regulation booster.

“Eric screeched like a banshee when he didn’t get his way,” said the insider. “He yelled at crew members – telling them his family had used the seat on many flights in the past,” said the insider. “But the price tag was still on it. And when an attendant turned over the seat, a warning label said, ‘Not to be used on aircraft!'”

But the actor’s outburst lasted so long that the plane was already taxiing toward the runway – and it was too late to remove the seat.

“The violation of airline regulations resulted in the FAA being notified,” said the insider. “The booster seat battle turned what should have been a routine flight into an uproar.”

A representative for Eric told The ENQUIRER: “The McCormacks followed the regulations with very little fuss, but still believe their son would be safer in his car seat. They did not jeopardize the safety of any member of the airline crew.”