The U.S. National Transportation Safety Board is investigating a near miss between a Frontier Airlines Airbus A319 and a Swearingen Metroliner, operated by Key Lime Air, which had inadvertently entered the runway to which the Frontier aircraft was approaching. The move comes after a series of runway incursions across the country. The two aircraft came within 50 feet of one another, said the Board. The Frontier aircraft broke out of the clouds and executed a missed approach after spotting the turboprop on the runway. The Airport Movement Area Safety System (AMASS) alerted the control tower personnel of the situation at the same time the Frontier crew saw the Metroliner on the runway. Weather at the time of the incident was one-half mile visibility, ceiling 600 feet overcast, snow and mist.The incident raises new questions about the relevancy of a Federal Aviation Administration anti-collision system that sounded in the air traffic control tower at about the time the Frontier pilots saw the other plane. A solution to runway incursions ison the Board’s 10 Most Wanted List which calls for a cockpit alert system. Currently, unless they see the danger themselves, pilots must rely on tower guidance to help them avoid such situations. The investigation is the fourth launched by the board recently. The Board said there are hundreds of runway incursions annually, most involving small planes. The safety board launched an investigation in November of three incidents involving commercial airliners that used wrong runways at US airports.