government aircraft for ALL travel...including "Personal" (e.g. Vacation) trips...plus, as mentioned by another poster...they reimburse the government...from the linked article..
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The Attorney General and the FBI Director are required to reimburse the government for any travel made aboard government aircraft for personal or political reasons.
Other agencies are allowed to designate "required use" travelers on a trip-by-trip basis.
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Link: https://www.thoughtco.com/who-flies-on-the-taxpayers-dime-3321451