Boeing’s safety culture and manufacturing quality, both at the center of a full-blown crisis following a January mid-air panel blowout, faced scrutiny in two U.S. Senate hearings.   Boeing has been grappling with a safety crisis after the door plug panel blew off an Alaska Airlines flight that took off from Portland, Oregon, on Jan. 5. The planemaker has undergone a management shakeup, U.S. regulators have put curbs on its production, and deliveries fell by half in March.   Testimony at the U.S. Senate permanent subcommittee on investigations raised questions about missing records surrounding the panel, along with production concerns over two separate Boeing widebody jets.   Former Boeing engineer Ed Pierson said he turned over records, sent to him from an internal whistleblower, to the FBI that he said provided information about the plug.   Boeing has said it believed that required documents detailing the removal of the door plug were never created.   Boeing directed questions to the ...