14 CFR §271.8

1. (a)The subsidy rate generally will be set for a 2-year period, or two consecutive 1-year periods. The Department may set the rate for a shorter period in the following situations:
   1. (1)A commuter air carrier is replacing a larger certificated carrier at the eligible place;
   2. (2)Traffic at the eligible place has substantially decreased;
   3. (3)The Department considers the cost or revenue projections of the carrier for the second year to be unrealistic;
   4. (4)It is likely that there will be changes in the eligible place essential air service level; or
   5. (5)The uncertainties of the market or other circumstances warrant a shorter rate period.
2. (b)The subsidy rate established for a carrier under this part will not be changed during the rate period unless an adjustment is required in the public interest.
3. (c)At the end of the rate period, the carrier will not have a continuing right to receive subsidy for providing essential air service at the eligible place.