The U.S. Air Force officially retired its 52 surviving F-117 Nighthawk stealth fighters in 2008, transferring their radar-evading attack mission to B-2 bombers, F-22s...

The U.S. Air Force officially retired its 52 surviving F-117 Nighthawk stealth fighters in 2008, transferring their radar-evading attack mission to B-2 bombers, F-22s and — eventually — F-35s.

The Air Force claimed it would preserve the F-117s for future use, but it’s possible most of the Nighthawks actually wound up in a landfill inside the Air Force’s highly-secure Tonopah Test Range in Nevada. But the flying branch has held on to at least two of the sensor-dodging F-117s, which first entered service in the early 1980s.

Amateur plane-spotters packing powerful cameras have photographed and videotaped F-117s flying over the desert test range and taxiing on a remote runway, sometimes singly and sometimes in pairs. The most recent snapshot of F-117s in flight are dated July 22 and can be found here.

Why would the Air Force want to keep a few F-117s operational, despite their age, complexity, high cost and the fact that Serbian air-defense forces figured out how to detect the planes and actually shot one down during the 1999 U.S.-led air war on Serbia?

Aviation expert Tyler Rogoway has an idea:

On the radar and infrared tracking side of argument, the F-117 is also a near-perfect and highly available low observable aircraft to test everything from ground based radars and SAM systems, both foreign and domestic, AWACS modifications, fighter radars and even infrared search and track systems.