Cleve Kimmel first proposed an inerting system to passenger airlines in the early 1960s. His proposed system for passenger aircraft would have used nitrogen. However, the US Federal Aviation Administration (FAA) refused to consider Kimmel's system after the airlines complained it was impractical. Indeed, early versions of Kimmel's system weighed 2,000 pounds—which would have probably made an aircraft too heavy to fly with passengers on it. However, the FAA did almost no research into making fuel tanks inert for 40 years, even in the face of several catastrophic fuel tank explosions. Instead, the FAA focused on keeping ignition sources out of the fuel tanks.