By Guy Taylor The crisis engulfing Boeing has deepened after a major US airline found loose bolts on a number of 737 Max-9 jets, which have been grounded for inspection. In a statement, United Airlines said it had found instances that “appear to relate to installation issues in the door plug,” including bolts that needed “additional tightening. These findings will be remedied by our Tech Ops team to safely return the aircraft to service.” Boeing is under mounting pressure after an Alaska Airlines flight was forced into an emergency landing after a section of the aircraft’s fuselage blew out 16…