As an accounting major and someone that has spent the past 5 years working for a global accounting firm, the way that the income statement is calculated drives me mad. I've gotten used to it, and used it to my advantage from a tax perspective, but really, there should be a balance sheet and income statement. The purchase of a plane should NOT trigger a huge paper loss for that quarter - rather its an asset (cash) thats being traded for another asset (an airplane). The true expense of owning the airplane, in theory, should be its ratable decline of value over time (which the game does keep track of). When a plane is sold, the only profit/loss on its sale should be what it fetches over/under fair market "value" according to the game. Leasing should be similar - the 4 month prepayment of a lease is really an asset that is consumed over the first 4 months of use until the payments kick in.
Back to RB211TriStar's point - the game seems to compute your profit/loss based on a statement of cash flows rather than any tax/accrual standard of accounting.
I would like to see the financial side of things revamped, and would be more than happy to assist with the accounting concepts involved to make it correct.