In less than three minutes, an intruder exploited a security gap at one of the nation’s busiest airports and stepped into the path of an airplane hurtling down a Colorado runway with 231 people aboard. The 41-year-old man slipped unnoticed past motion detectors in a remote corner of Denver International Airport, which sprawls across open plains and covers an area twice the size of Manhattan . He quickly scaled an 8-foot perimeter fence topped with barbed wire, then walked unobstructed onto the runway where he was fatally struck by a Frontier Airlines jet as it attempted to take off late Friday night. Surveillance video showed the man getting pulled into an aircraft engine that instantly burst into flames, forcing the pilot to abort the takeoff and evacuate the 224 passengers and seven crewmembers. Twelve people had minor injuries. Aviation and risk experts said the Denver runway collision represents a clear security failure.…