For the last five years, the U.S. Air Force has been trying to replace its manned U-2/TR-1 reconnaissance aircraft with RQ-4 Global Hawk UAVs. This has not worked out well. While there have been some problems with Global Hawk reliability and dependability, the big issue has been in the superiority of the sensors carried by the U-2. So why not just install the U-2 sensors in the Global Hawk? The problem here is weight and space. The U-2 is a larger and heavier aircraft, and even with a pilot, has more carrying capacity. Air force suppliers keep promising that they have the problem solved, but after several generations of Global Hawk sensor redesigns and improvements, it will still be a few years before the Global Hawk will be competitive, and the U-2 will be out of a job.
Then there's the UAV software, which has still not matched the capabilities of pilots. The humans still have an edge over robotic systems, especially when it comes to emergencies. But another advantage that the U-2 has is that it has been around for half a century. Its quirks and foibles are well known. The Global Hawk is not only new, but is also the first of a new kind of robotic aircraft.
Global Hawk has crossed the Pacific, from North America to Australia, using onboard computers to run everything. While impressive, Global Hawk still has a tendency to get into trouble unexpectedly, and not know how to recover. More work needs to be done on the software and, to a lesser extent, the hardware used by Global Hawk. Since no one can (or at least will) swear when Global hawk reliability will be up to acceptable standards, plans are being made to keep the U-2s around for a while longer. Just in case.
This popularity is running the U-2s ragged. Two years ago, for example, 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. All U-2s have been upgraded to the Block 20 standard, so they can be kept in service until the end of this decade. Or at least until the robotic 13 ton RQ-4 Global Hawk is completely debugged, and available in sufficient quantity to replace it. The U-2 has been in service since 1955, and only 86 were built. Less than 900 pilots have qualified to fly the U-2 in that time.
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 30,000 feet. This puts more strain on the pilot's body. That, and the fact that they breathe pure oxygen while up there, means they tend to be completely exhausted after returning from a long mission.
This wasn't supposed to happen. Five years ago, the U.S. Air Force wanted to retire its 33 U-2 reconnaissance aircraft, 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), and because 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 kind of recon work the two aircraft can do, that both pilots and robots will coexist for a while. The current plan is to retire the 32 active U-2s by 2015, unless Congress, or reality, intervenes again.