The mission-capable rates for the Air Force's entire fleet of nearly 5,500 aircraft range from 46.98 percent for the B-1B bomber to 100 percent for the C-21C — the executive style jet used to transport senior Air Force brass.

According to officials in Air Combat Command, Air Force Special Operations Command and Air Mobility Command, mission-capable rates are affected by a number of factors including: the mission of the aircraft, its age, and the sophistication — or simplicity — of its design.

But for the aircraft listed here, the major impact on their "up" time is the crushing tempo of their continued use supporting the fighting in the Middle East.

An aircraft, says Lt. Col. Robert Ehasz, chief of the F-22 weapon system team, "is a mechanical tool. It's like your car: Over time it degrades slightly. Now, if you took your car in and got a complete overhaul, when you roll it out of the shop it feels brand new, and that's exactly what we do with these airplanes. We send them into a depot and the professionals go from nose to tail and take off every panel and look at every wire and every nut and every bolt and re-torque everything and make sure that that airplane is basically brand new coming back out that door."

According to Capt. Nick Plante, spokesman for AMC, "The high operations tempo is a constant concern for commanders in the field. Keeping this in mind, and realizing that the pressure to fix aircraft quickly could potentially drive dangerous shortcuts, the message from the top, all the way to the airmen turning the wrenches remains: 'Safe, by-the-book maintenance.' We have been able, to sustain the mission-capable rates for both the KC-135 and C-130H weapon systems through a concerted effort on the part of all support systems (maintenance, supply and the System Program Office) to provide qualified maintainers with the parts needed at the time they need them to fix the aircraft, with the program office assisting with nonstandard repairs, such as corrosion and stress fatigue repairs, as they arise."

The average age of the fleet is 27 years — with some as old as 53 — but the oldest planes are not those with the lowest availability rate. For example, the venerable B-52 bomber has a 72 percent up rate, while the CV-22 Osprey, averaging under 4 years old, is "up" only 55 percent of the time, due to "some near-term challenges in aircraft availability and maintenance man-hours per flying hour. These types of challenges are not unlike what any other new weapon system faces, and we're taking them to task to maintain our operational readiness and effectiveness," according to an AFSOC spokesman.

The B-52's availability, on the other hand, says Lt. Col. Todd Andre, division chief for aircraft maintenance at Air Force Global Strike Command, "comes from the original engineering and simplistic design of the weapon system. Most of the subsystems are based on 1970s technology. This provides for easier troubleshooting and more efficient/effective repairs."