Originally Posted by Baradium

I'd like to give this a go.

These are all hypothetical numbers.

Let's say an MD-88 costs 10 cents per seat mile in fuel and 10 in maintenance.

Meanwhile AMR is operating on the same route a A319 that costs on average 6 cents per seat mile in fuel, 5 in maintenance and 9 in lease payments.

In this example the costs per seat mile end up being the same (although I think the numbers skew so that with fuel prices like they have been the -88 is cheaper).

If both carriers park the associated aircraft DL now has a zero cost aircraft while AMR has to make payments for the leases as if it was flying a full schedule, reducing the cost advantages of parking it. It might cost the same amount to fly either one, but the cost advantages to not fly it are smaller.

Something no one has mentioned either is that leased aircraft are required to be maintained in airworthy condition for the duration of the lease per contract terms so, unlike an -88 where you can park them as checks become due, the leased aircraft cannot be parked with maintenance outstanding. Many of the lease terms actually require the aircraft be kept in active service as part of this requirement as well. I remember one of the majors (I think it was American) being sued at one point for parking leased aircraft.

What I'm getting at is it's not just a cost thing for operating the aircraft as there are additional rules at play with a leased or mortgaged aircraft that they don't have to worry about one they own outright.