By Valerie Insinna and Tim Hepher WASHINGTON (Reuters) – A mid-air blowout has put jetmaker Boeing in the exact place investors and management hoped it would avoid – back in the regulatory crosshairs just as it was awaiting approval of new models of its best-selling MAX jet. Investigators say it is too early to determine what caused a so-called door plug to fall off from the side of an aircraft operated by one of Boeing’s most loyal customers, Alaska Airlines, on Friday with 171 passengers on board. The Federal Aviation Administration (FAA) said on Sunday that 171 Boeing MAX 9 airplanes BA.N w…