You may use Government aircraft for travel only if you have authorization from an executive agency under the rules specified in this part (except with regard to travel under §§ 301-70.808 and 301-70.910). Because the taxpayers should pay no more than necessary for your transportation, generally you may travel on Government aircraft only when a Government aircraft is the most cost-effective mode of travel.
41 C.F.R. §301-10.260