United States. Government Accountability Office. Federal Aviation Administration: Efforts to Hire, Staff, and Train Air Traffic Controllers Are Generally on Track, but Challenges Remain, text, June 11, 2008; (https://digital.library.unt.edu/ark:/67531/metadc294667/m1/1/: accessed December 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .