The U.S. Air Force readiness rate (percentage of available aircraft able to do their job) varies by type and technology. Age has less to do with it than you might think. For example, the current rates for fighters are;
F-15C, 71 percent
F-15E, 72.5 percent
F-16, 75.4 percent
F-22, 60.9 percent
A-10, 70.5 percent
The F-22 rate is lower partly because it is a new aircraft, and problems are still being uncovered. But another problem is that the many stealth features of this aircraft require attention, and provide more items that can break.
For bombers, the rates are;
B-1B, 43.8 percent
B-2, 54.9 percent
B-52H, 74.6 percent
The B-1B is cursed with additional components (especially hydraulics) that enabled it to fly fast and low. That capability is not used anymore, but the equipment is still there, and when any of it breaks, the aircraft doesn't fly. The B-2 has lots of stealth stuff, but the air force has overcome a lot of those problems. Meanwhile, the ancient, but relatively simple, B-52 has the highest readiness rate, and is the cheapest to operate.
Transports also are relatively simple in terms of tech, and their readiness rates show this;
C-130E, 76.7 percent
C-130H, 73.8 percent
C-130J, 82.3 percent
C-17A, 84.4 percent
C-5A, 52.7 percent
C-5B, 59.6 percent
CV-22, 54.3 percent
HC-130, 69.9 percent (search and rescue)
The lowest rates are either the result of age (the C-5) and lots of tech (the tilt-rotor CV-22).
Electronic warfare aircraft, despite all the complex electronics carried, are basically transports.
E-3, 71.6 percent (AWACS)
E-8, 81.1 percent (JSTARS)
EC-130H, 70.6 percent
EC-130J, 65.2 percent
Aerial tankers are transports, and quite elderly;
KC-10A, 74.8 percent
KC-135R, 81.1 percent
KC-135T, 80.4 percent
The readiness rate for these old, and heavily used, aircraft does not reflect the large number (up to 20 percent) of aircraft that are pulled for major rebuilds. Thus the most decrepit tankers are not counted, keeping the readiness of aircraft in squadrons at a high rate.
UAVs are simple aircraft, and their high readiness rate reflects this;
MQ-1, 93 percent
MQ-9, 91.9 percent
RQ-4, 41.6 percent
One exception is the RQ-4 Global Hawk, which is a jet propelled, trans-oceanic aircraft. It is a much more complex beast, and has long had maintenance problems. The air force is not happy with this.
Trainers are simple aircraft, and their readiness rates reflect this;
T-1A, 79.7 percent
T-38A, 80.4 percent
T-38C, 76.1 percent
T-6A, 80.3 percent
The air force only has one type of strategic (long distance, high altitude) recon aircraft. More than most jets, the U-2 is a powered glider. It is old (in design), simple and very reliable.
U-2, 81.2 percent
The air force doesn't have many helicopters, and they are army models. The army designs its helicopters for easy maintenance and heavy use.
HH-60, 74.6 percent (search and rescue)
UH-1, 80.9 percent (transport)
New aircraft designs create often unique maintenance problems. For example, the F-22 aircraft is proved more difficult to maintain than planned. Because of the time required to service the stealth features of the aircraft, only 60 percent of them are available for service at any given time. Non-stealth fighters have a readiness rate of 70 percent. This is not a new problem.
As recently as four years ago, only about seven of the U.S. Air Force's 21 B-2 "Stealth" bombers were ready to go at any time. The air force was already trying to solve the problem, using a combination of robots, sprayers and quality control in an attempt to double the readiness rate. This was essential, because the B-2 was frequently getting called a "Hangar Queen" (an aircraft that spends too much time in the hangar for maintenance or repairs).
Six years ago, the U.S. Air Force introduced the use of robots to reduce the maintenance efforts required to keep their B-2 bombers flying. The B-2 uses a stealth (anti-radar) system that depends a lot on a smooth outer skin. That, in turn, requires that the usual access panels and such on the B-2 must be covered with tape and special paste to make it all smooth. The F-22 uses a similar system. After every flight, a lot of this tape and paste has to be touched up, either because of the result of flying, or because access panels had to be opened. All this takes at lot of time, being one of the main reasons the B-2 required 25 man hours of maintenance for each hour in the air. Since most B-2 missions have been 30 or more hours each, well, do the math. The readiness rate of the B-2 fleet (currently 20 aircraft) has been about 35 percent, which was less than half the rate of most other aircraft. This means, that whenever there is a crises that requires the attention of B-2s, there are not many of these bombers ready to fly.
The main base for B-2s is in Missouri, and over a thousand maintenance personnel were assigned to take care of 20 aircraft there. A team of four robots were installed, to liquid coating to B-2s, thus cutting maintenance hours in half. But there were quality control problems with the liquid coating, often forcing maintenance crews to go back to tape and paste. Eventually, the quality control problems were solved, and, readiness rate of B-2s went up to over 50 percent.
B-2s still require a special, climate controlled hangars. There are some portable B-2 hangars, that can be flown to distant bases, thus keeping the bombers in the air less, and reducing the amount of maintenance needed. B-2 quality hangars were built at Guam, in the Pacific, and Diego Garcia in the Indian ocean