Despite numerous attempts to replace it, the 64 year old American U-2 strategic reconnaissance aircraft remains in service. Before September 11, 2001 it seemed certain that the U2 would soon be retired. But because of Islamic terrorism, a resurgent China and more aggressive Russia, Iran and North Korea, the U-2 found itself more in demand than ever. The decade after 2001 was the busiest ever. Because of the spy satellite quality sensors carried by U-2s, and a limited number of spy satellites in space, there was always more demand for U-2 sorties than could be provided. For example, in 2009 two 41 year old U-2s achieved a record 25,000 hours in the air. One of these aircraft had made three belly (landing gear up) landings, requiring extensive rebuilding after each incident.
With a range of over 11,000 kilometers, the 18 ton U-2s typically fly missions 12 hours long. It carries a 2.3 ton payload at altitudes of up to, well exact ceiling has never been revealed but it is believed to be at least 23,000 meters (77,000 feet). Apparently it is high enough because at that altitude the U-2 can operate like a spy satellite and look sideways deep into hostile territory. This annoys Russia, North Korea, Iran and China no end but that was what the U-2 was designed to do.
All remaining U-2s have been upgraded to the same current (and regularly raised) standard, so they can be kept in service until a satisfactory replacement is found. That effort has been underway for decades but so far it was always found cheaper to upgrade or otherwise improve the U2 rather than spend a lot more money on a replacement. Currently there are two potential replacements. The one closest to actual service is a special strategic recon version of the 13 ton RQ-4 Global Hawk UAV. This was a great concept but the modified RQ-4 is not yet completely debugged and that will take a while because the continuing U2 upgrades mean the RQ-4 has to be reexamined to see if that upgrade would work on the UAV. The problem is that the U-2 has a larger payload than the RQ-4 and the presence of a pilot makes it easier to deal with any hardware problems. That led the air force to delay U-2 retirement plans at least until 2030 and instead seek to use both the upgraded U-2 and soon the RQ-4. The air force is also retiring 24 older RQ-4s and will only have a dozen of the latest (Block 40) RQ-4s. These RQ-4s could take over some of the U-2 missions, ones that could be handled by the less capable (because of smaller payload and no pilot) UAV.
The other possible replacement is the proposed (and not yet accepted) manned TR-X. This new aircraft was designed and proposed in 2015 by the U-2 manufacturer. It is based on the current U-2 design. The air force was interested but not yet convinced that a 24 ton manned successor to the U-2 would be worth the larger investment required. The TR-X has the same 2.3 ton payload but two engines that enable it to fly higher. This enables TR-X to disregard stealth because it would be too high for any current or planned air defense weapons. TR-X could also carry electronic countermeasures for any air defense systems that eventually can reach it. TR-X is tempting but not enough to get any air force investment. At least not yet. U-2 has turned out to be one of those systems, like the C-130 transport and Sidewinder missile, both of which entered service in the 1950s, that are still in use. Both have been continually upgraded and kept up with proposed successors. This is not a common phenomenon but it is not all that rare either.
The U-2 has been in service since 1955 and only 103 were built, of which 31 remain in service, including four two-seat trainers. The current U-2s aircraft were built as TR-1s in the 1980s and later refurbished and renamed U-2s. Fewer than a thousand pilots have qualified to fly the U-2 so far.
The heavy use of the U-2 has been hard on the pilots. Missions can be as long as 12 hours and pilots operate in a cockpit pressurized to conditions found at 10,000 meters (31,000 feet). This puts more strain on the pilot's body. That, and the fact that they breathe pure oxygen while in the cockpit, means they tend to be completely exhausted after returning from a long mission. U-2s often fly missions daily over the Middle East, Afghanistan/Pakistan, and East Asia.
This wasn't supposed to happen. In 2006 the U.S. Air Force came up with a plan to retire its U-2s and replace them with UAVs like Global Hawk. But Congress refused to allow it, partly for political reasons; jobs would be lost, which is always a live political issue. There were some in Congress (and the air force) did not believe that Global Hawk was ready to completely replace the U-2. This turned out to be correct. New Global Hawks continue to appear but there is so much demand for the kinds of recon work the two aircraft can do that both pilots and robots will coexist for a while.
There have been other efforts to build a successor to the U-2. One of the most successful of these was the 30 ton, twin-engine SR-71. This aircraft could fly a lot faster than the U-2 and a bit higher. This was built by Lockheed, which also built the U-2 and proposed the TR-X. The SR-71 proved useful but was too expensive to build and operate. Only 32 were built and it was retired in 1998. The replacements were the new RQ-180 jet-powered stealth aircraft. This looks like a smaller B-2 bomber, and is also very expensive, and not operating on a large scale yet. Meanwhile the U-2 has taken over much of the SR-71 workload. In effect the U-2 replaced its 1960s successor.
Eventually the old reliable U-2 will be retired. Yet like the C-130 and Sidewinder, no one is sure when a suitable replacement will show up.