The U.S. Air Force recently had to cancel ones of its Red Flag training exercises because there were not enough tankers available. The air force holds 4-5 of these exercises a year, to maintain fighter pilot skills, and sometimes to complete the training of pilots before they go overseas. The training includes aerial refueling.
The problem here is that the air force fleet of KC-135 tankers are very old. An unpredictable, and growing, number of them are unavailable because of maintenance problems. Earlier this year, the air force disclosed that, on average, 20 percent of its 415 aging KC-135 tankers are in the shop for long term maintenance. Last year alone, these aircraft (plus 59 larger and less elderly KC-10s) transferred some 455,000 tons of fuel in 82,000 refueling operations (5.4 tons per transfer). This has been a strain on these aging aircraft. In the last four years, a quarter of these aircraft have been grounded just because of age-related issues, and the number of these cases is growing. Maintainers are not only worked hard, but have to keep coming up with new solutions for unforeseen problems.
Not surprisingly, the air force asserts that, for some of the possible war scenarios (Korea, Taiwan/China), there would not be enough KC-135s to go around. This is largely because of the need to refuel navy aircraft. The navy can work around that, to a certain extent, by using U.S. Marine Corps KC-130 tankers and navy F-18 fighters equipped for the task. But this means the navy has fewer resources. The air force wants a new tanker, but has been unable to get it done.
Meanwhile, the air force pays close attention to what happens to elderly commercial air freight aircraft. That's because, while nearly half of U.S. Air Force aircraft are over twenty years old, over eighty percent of those tankers are. The oldest are the KC-135s. Operating aircraft this old is unexplored territory, because this is the first time in history that so many large, and fast, aircraft have gone on flying for so long. The commercial freighters fly more frequently than their military counterparts, putting more strain on them, and forcing their operators to develop new maintenance techniques the air force can use. The KC-135s are the oldest transports the air force is still using, and keeping them working is proving to an expensive and challenging effort.
The basic problem is that, despite constant maintenance and careful monitoring unexpected failures still occur with elderly aircraft. Nothing that is cause for alarm. Older aircraft are grounded if any unexpected failure seems imminent. While that just about eliminates these aircraft having fatal failures while in the air, it also makes older aircraft less available for service. But so far, it's been more cost effective to keep the old birds flying, than to buy new ones. Even growing fuel economy problems can be solved by installing new (more fuel efficient) engines. For the tankers, advanced age have made replacement a necessity, not an option.
Meanwhile, after over a decade of effort, the U.S. Air Force is still struggling to select a new aerial tanker. The air force procurement has been cursed with corruption, incompetence, meddling politicians and litigious suppliers, all combining to prevent the acquisition of a new tanker, to be called the KC-45A. The contract to build 179 KC-45As is worth about $35 billion (about $196 million per aircraft). Whoever wins, the air force won't begin receiving KC-45As for another 4-5 years, and all of them wont be delivered for another twenty years. At that point the last of the KC-135s, after 80 years of service, will be retired. That process won't be pretty, and it's already gotten quite ugly as the air force struggles to keep half century old transports flying.
The four engine KC-135 carries 90 tons of fuel and can transfer up to 68 tons per sortie. Typically, aerial tankers have to service B-52s, which carry over 140 tons of jet fuel, and fighters like the F-15 (over five tons). The KC-135 has long made itself useful carrying cargo and passengers, as well as fuel, and both leading candidates to replace it (the KC-767 and KC-30) have more capacity for this, with the KC-30 having a decisive edge.
The KC-767 was developed partly because it is about the same size as the KC-135 (wingspan is 50.3 meters/156 feet, 7 percent more than the KC-135). Thus the 767 could use the same basing and repair facilities as the 135. That turned out to not be critical factor. Moreover, Airbus has been developing the KC-30 for several years, and the first entered service with Australia last year. This annoys American tanker crews, as they struggle to operate their elderly KC-135s, and know that they will have to continue doing so for another two decades.