BigKahunaBurger---
See you are from the UK and therefore most likely know nothing about US tax regulations.
So, some enlightenment will be provided, FOC.
If an airline purchases a new aircraft, it is assigned a depreciation schedule and over a period of years (for example 15) it is depreciated to a residual value. The entire depreciation is decuctable for tax purposes, directly reducing taxable income. Meanwhile, the airline benefits from the new equipment (less maintenance, lower fuel burn, generally lower operating costs).
Expensive new aircraft purchased equals large tax benefits.
At the end of the depreciation period, a new aircraft is purchased and the cycle repeats.
From a financial point of view, this is quite advantageous. This requires an airline with a rather good credit rating.
Leasing is even better (in some cases) allowing the airline to keep its fleet relatively young while at the same time benefiting from lower operating costs of new(er) equipment while retaining the benefits of deducting the full lease amount from taxable income. Large leasing companies such as International Lease-Finance and GE Capital make buckets of money in the process, a win-win situation.
I have kept this relatively simple so that even you might hope to understand.