Certification of systems that are critical to the safety of flight has been the focus of several recently concluded National Transportation Safety Board accident investigations of transport-category airplanes: USAir flight 427 in 1999; TWA flight 800 in 2000; Alaska Airlines flight 261 in 2002; and American Airlines flight 587 in 2004. Each of these investigations raised questions about the certification process used by the FAA to determine compliance with airworthiness standards.