The airlines' executives contend that this mammoth fleet will be able to deliver on the promise of a single U.S. carrier capable of connecting the world. But all those 1,097 aircraft could be a mammoth headache to manage, as well.
Between them, Delta and Northwest fly more than a dozen models of airplanes, with only the Boeing 757 and the regional jet CRJ-200 in common. The aircraft potpourri - Airbuses, Boeings, old McDonnell-Douglases and new Embraers - runs counter to the streamlined fleet many experts contend is necessary to keep maintenance issues to a minimum and the greatest number of planes in the air instead of the hangar.
''The fleets don't fit together well,'' said Ernie Arvai, an aviation consultant with the Arvai Group. ''It's incompatibility multiplied.'' Each make of aircraft requires its own aircrew training, spare parts, maintenance training and manuals.
Delta CEO Richard Anderson last week argued that the diversity of aircraft is a strength, not a weakness.
''The fleet integration actually works out quite well because we don't have common fleet types. . . . We don't have to sit down and merge a lot of maintenance programs,'' Anderson told Wall Street analysts.
''That's the first time I've heard that rationale,'' said Scott Hamilton, a Seattle-based aviation industry consultant with the Leeham Co. ''I thought that's what consolidation is all about.''
Hamilton said the merged airline over time will need to streamline its fleet and make it more homogenous in order to achieve the economies of scale and performance needed to reach management's goal of $1 billion in cost savings.
But passengers eager to see something other than an aged Northwest DC-9 or a legacy Delta MD-88 pull up to the gate for their next flight might have to wait for some time.
The two principal manufacturers used by Delta and Northwest - Boeing and Airbus - are booked with orders for several years.
There's a four- to five-year backlog for the Boeing 777, a wide-body that is fairly new to the Delta fleet, Hamilton said. Airbus is back-ordered until 2012.
Northwest is scheduled to be the first U.S. carrier to get the widely anticipated Boeing 787 Dreamliner, but Boeing repeatedly has pushed back the delivery time, now into 2009.
''For the short term, they have to fly with what they have,'' Hamilton said.
Mechanics say that a properly maintained airplane can fly safely for decades.
But safety is one thing; cost is another. Older planes guzzle fuel and are expensive to inspect, maintain and update.
By eventually retiring the oldest workhorses, such as the DC-9s, the merged airlines can eliminate their worst fuel guzzlers.
Earlier this year, Northwest said it would ground about a third of its 103 DC-9s, which average more than 30 years of service.
By parking some of those planes, Northwest has lowered its average fleet age to less than 19 years, but it still has the oldest fleet among the six largest U.S. carriers, according to Airsafe.com, a Web site that collects aviation data.
Delta spokesman Kent Landers said it's too early in the merger process to say just how many planes would be retired, but at least some DC-9s would continue flying because they have roughly 100 seats, a good size for some routes.
The immediate goal is not simply to retire planes, but to have ''an aircraft to fit every market so that we can most efficiently match customer demand,'' he said.
The problem of aging aircraft is common throughout the industry. U.S. airlines were due to start making big investments in modern aircraft as the new century dawned, only to run up against a recession and the 9/11 terrorist attacks involving four airliners.
The subsequent industry downturn put carriers into a financial tailspin, ultimately driving Delta, Northwest and others into bankruptcy.
* Continental: 10.1 years.
* U.S. Airways: 12.2
* United: 12.7
* Delta: 13.8
* American: 14.7
* Northwest: 18.5