No problem DeltaMD11!
The beancounters have calculated the entire predicted lifespan of the aircraft way before the contracts are signed. SQ
, for example, have a high fleet turnaround, so their beancounters calculate how much it will take to pay back the aircraft (plus a profit of course) in that time. Other companies do the same, with longer planning horizons. This principle is of course valid for any investment.
Of course, nothing ever turns out the way you think it will, and all of a sudden fuel costs go up/down, the economy changes, Mr. O'Leary makes an appearance on CNN, the weather is unusually cold, etc. And that's where hedging comes in. To guard yourself agains, for example, rising fuel prices, an airline will buy fuel options and be able to buy fuel at a predetermined price in a couple of years. This way the beancounters, who like certainty, can sleep at night.
Now, the airframer has probably guaranteed a certain performance level over a certain period, so the operator already knows today how much each plane will cost for every year in the next couple of decades. The further in the future, the fuzzier the forecast, but there is a lot of history to be drawn on and these guys are, like insurance actuators, good at probabilities and statistics.
If I were a shareholder in NW
today, I would definitely not want them to buy new planes to replace the DC-9's, since that would decrease my projected dividends in the next decade, or in other words decrease the profits of the company.
Ok, I (and others on the board) could probably go on about this sort of thing for ages. Buy yourself a copy of Brealey/Myers "Principles of Corporate Finance", the quintessential textbook on investment, for (much) more detail.