14 CFR § 241.2-4 - Sec. 2-4 Accounting period.
(b) Each air carrier shall keep its financial accounts and records on a full accrual basis for each quarter so that all transactions, as nearly as may reasonably be ascertained, shall be fully reflected in the air carrier's books for the quarter in which revenues have been earned and the costs attaching to the revenues so earned in each quarter have been incurred independently of the incidence of sales or purchases and settlement with debtors or creditors.
(c) Expenditures incurred during the current accounting year which demonstrably benefit operations to be performed during subsequent accounting years to a significant extent shall be deferred and amortized to the period in which the related operations are performed when of sufficient magnitude to distort the accounting results of the year in which incurred.
(d) Expenditures charged directly or amortized to operations within one accounting year shall not be reversed in a subsequent accounting year and reamortized or charged directly against operations of subsequent years except that retroactive adjustments are permitted where necessary to conform with adjustments required by the DOT for ratemaking purposes.