By Alwyn Scott and Joseph Menn NEW YORK (Reuters) – A common design problem in the U.S. air traffic control system made it possible for a U-2 spy plane to spark a computer glitch that recently grounded or delayed hundreds of Los Angeles area flights, according to an inside account and security experts. As aircraft flew through the region, the $2.4 billion system made by Lockheed Martin Corp, cycled off and on trying to fix the error, triggered by a lack of altitude information in the U-2’s flight plan, according to the sources, who were not authorized to speak publicly about the incident. Lockheed Martin said it conducts \”robust testing\” on all its systems and referred further questions about the En Route Automation Modernization (ERAM) system to the Federal Aviation Administration. The FAA later set the system to require altitudes for every flight plan and added memory to the system, which should prevent such problems in the future, Brown said.