The cost of an airframe is one of the most important factors for freight companies. In addition the performance characteristics of an airplane come into the picture as well. One of the main reasons that the MD
-11 became such a good freighter is because it was so cheap to acquire. There was a flood of relatively new (less than 10 years) MD
-11s in the market, which was great for cargo carriers.
The 772LR would be a good candidate for a freighter based on specifications, but in practicality, it probably won't amount to much. The 772LR is the most expensive plane that Boeing offers with a list price of around $200 million, which puts it out of the reach of most cargo carriers (and airlines for that matter). And unlike the MD
-11 and 747, the 772LR probably will not be produced in quantities that will cause it to have a low resale value down the road. Basing a fleet of freighters on the 772LR is prohibitively expensive, especially when 747Fs can be had for less and are a superior plane for cargo at the moment and will be available widely on the used market.
Cargo carriers like FedEx are basing their longterm plans on the A380F. They are spending massive amounts of capital for it, and they expect it to be in their fleet for the long term because it is a dedicated freighter and probably will be the cheapest to operate cargo plane. That leaves the majority of their international cargo based on the MD
-11 and A380. The A340 and 777 probably won't be able to penetrate the stronghold because they were designed with passengers in mind. However other carriers (especially Asian ones) are basing their fleets on the 747F. This will probably be a long term solution because 744s are entering the used market, and their values are going down.
This leaves the smaller markets needing a longterm solution. The A300F and 767F are the basis of DHL and UPS. They are cheap and efficient. The 7E7 might be able to penetrate the market if its production costs are as low as Boeing is hinting at. The 7E7 might be able to capture some of the new cargo plane market, but probably the 767s, which will be inundating the market with the advent of the 7E7 and A330, will be basis for the smaller cargo needs.
Cargo conversions will be the way to go in the future. Airlines are having a relatively high turn over rate with their planes. You are hard pressed to find a widebody plane that has been in service for over 20 years now with a major airline. If the trend for shorter life spans continues, there will be plenty of planes on the cargo side to take over DC-8s in the future. A300s and 767s along with the larger A380, 747 and MD
-11 will probably hold the majority of the cargo market in the next 20-30 years.
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