Some of the U.S. Air Force's B-2 stealth bomber pilots fly their multi-billion-dollar warplanes just once a month.

That's one of the startling details in Jamie Hunter's feature in the latest issue of Combat Aircraft magazine.

The Air Force possesses 20 B-2s, which Northrop Grumman developed and built in the 1980s and 1990s at a cost of no less than $2 billion apiece—more if you count ongoing upgrades. There used to be 21 of the radar-evading, batwing bombers, but one crashed in Guam in 2008, fortunately sparing its two-man crew.

The B-2s boast special shaping and coatings to help them avoid detection. They have dropped bombs during U.S. operations in Kosovo, Afghanistan, Iraq and Libya—sometimes flying non-stop from their main base in Missouri.

But the B-2s' pilots—there have been only around 600—don't get much practice. Their planes are complex, unreliable and too expensive to risk in everyday training.

Just nine or so B-2s are flight-ready at any given time. The rest are in maintenance or getting upgraded. A single hour of flying in a B-2 sets back the Air Force as much as $170,000—that's 10 times what an A-10 attack jet costs.

"We have to be kind of careful with what we do," B-2 pilot Capt. Dan St. Clair told Hunter. "Obviously we have to preserve [the B-2s]—an incredibly expensive bill was paid for them."

St. Clair went on to say that B-2 pilots get to fly "between one and three" sorties per month. A single flight could last as many as 20 hours—or as few as three.

Flying hours translate into pilot skill, which translates into effectiveness in combat. "While the exact number of hours required to achieve world-class status may be subject to some dispute, there is no question that training improves performance," Air Force Lt. Col. Dan Ward wrote in his new book F.I.R.E: How Fast, Inexpensive, Restrained and Elegant Methods Ignite Innovation.

Ward highlighted the F-20, a cheap, simple fighter that Northrop developed in the 1970s specifically to make it easier for pilots to spend more time in the air.

The complex, expensive B-2 has the opposite effect. In 2013, Maj. Luke Jayne surpassed 1,000 hours flying the B-2. It took him roughly a decade—that is to say, he flew around 100 hours a year … or fewer than 10 hours per month.

The Air Force celebrated Jayne's achievement with a press release. It mentioned that only 35 B-2 pilots had logged 1,000 flying hours in the stealth bomber. None had achieved 2,000 hours.

By contrast, a B-52 pilot flies around 300 hours a year. The 1960s-vintage B-52 isn't stealthy, but it is cheap and reliable. The Air Force's 76 B-52s are ready 75 percent of the time and cost $70,000 per hour to fly. Many B-52 pilots have racked up 5,000 or even 6,000 hours flying their old planes.

On reaching 5,000 hours in the B-52 in 2006, Lt. Col. Larry Littrell complained that shifting deployment patterns were "making it harder" for B-52 fliers to get 300 hours a year in the cockpit.

He surely would have hated being a B-2 pilot.

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