48 CFR §47.403-3

1. (a)Agencies shall disallow expenditures for U.S. Government-financed commercial international air transportation on foreign-flag air carriers unless there is attached to the appropriate voucher a memorandum adequately explaining why service by U.S.-flag air carriers was not available, or why it was necessary to use foreign-flag air carriers.
2. (b)When the travel is by indirect route or the traveler otherwise fails to use available U.S.-flag air carrier service, the amount to be disallowed against the traveler is based on the loss of revenues suffered by U.S.-flag air carriers as determined under the following formula, which is prescribed and more fully explained in 56 Comp. Gen. 209 (1977):
3. (c)The justification requirement is satisfied by the contractor's use of a statement similar to the one contained in the clause at 52.247-63, Preference for U.S.-Flag Air Carriers. (See 47.405.)