Koreea Bertelsmann, 2003
Buku Referensi
edited by Gerald W. Whitted
Pan American World Airways, 1971
Buku Referensi
edited by Gerald W. Whitted
Pan American World Airways, 1963
Buku Referensi
APA Publications , 2007
Buku Referensi
edited by Gerald W. Whitted
Pan American Airways, 1963
Buku Teks SO