Green’s Dictionary of Slang

(US milit.) the airplane that took time-served soldiers back from Vietnam to ‘the World’, i.e. the USA.

1967		Tucson Dly Citizen (AZ) 27 Apr. 14/2: Soon Johnson will ‘fly the freedom bird home’.
1969		Jrnl & Courier (Lafayette, IN) 22 Dec. 14/4: Hiill, a Trans World Airlines (TWA) stewardess 10 years, flies with the ‘Freedom Bird,’ a plane which carriies men to and from the Vietnam war.
2011		T. Pluck ‘Freedom Bird’ in Life During Wartime (2018) 3: Martins had missed the freedom bird by a cunt hair.