Boeing this month discovered the latest in a long line of glitches on its KC-46 aircraft: Some trim hanging down above the over-wing emergency exit doors prevents them from opening. Though the KC-46 is designed as a troop transport as well as an air-to-air refueling aircraft, Boeing somehow missed this basic exit flaw in the tanker’s emergency egress system. Every commercial airplane Boeing designs, including the 767 that is the basic airframe for the KC-46, is tested during certification to make sure all passengers can evacuate in an emergency within 90 seconds. For this defect-plagued milita…