The F-117 Nighthawk has been spotted over the Nevada desert occasionally in recent years, raising questions why a “retired” plane has made its way onto a flightline.

Technically categorized as “flyable storage,” the remaining single-seat, twin-engine aircraft in the Air Force inventory are tucked away at test and training ranges in Tonopah, Nevada.

But in accordance with the National Defense Authorization Act of 2017, passed Dec. 23, the Air Force will remove four F-117s every year to fully divest them — a process known as demilitarizing aircraft, a service official told Military.com on Monday.

“Flyable storage” aircraft are not considered classified, said the official, who requested anonymity to free discuss the program. This is why aviation enthusiasts may have spotted the stealth aircraft flying in 2014 and again in 2016 and again as they were taken out for training flights.

“We had to keep all the F-117s in flyable storage until the fiscal ’17 NDAA gave us permission to dispose of them,” the official said. “Once we have it, [Congress] doesn’t let us to get rid of anything, but do it in phases, like keep it in backup inventory, primary aircraft assigned, or flyable storage.”

Congress gave authority in 2007 and 2008 to retire a total of 52 F-117s from the inventory, but wanted them maintained so they could be recalled into future service in case they were needed for a high-end war, the official said.

But in coming years, the stealth attack plane — capable of attacking high value targets without being detected by enemy radar — may permanently fade to black.

“We’re supposed to dispose of one [Nighthawk] in 2017 and approximately four every year thereafter,” the official said.

One is scheduled to be divested this year.

The transition to “demilitarize” and decrease the inventory of the aircraft is defined in the Defense Department’s 41-60.21, “Defense Materiel Disposition Manual.”

Depending on what the aircraft does or what DoD plans to do with it — such as put it in a museum, for example — the manual dictates how to dispose by eliminating the functional capabilities of the plane.

“Sometimes the boneyard does that, sometimes they don’t,” the official said, referring to Davis-Monthan Air Force Base, Arizona, where retired planes call home after they’ve been deactivated.

Furthermore, once the aircraft is declared as “excess after deactivation,” sometimes the aircraft can be sold off to other federal agencies in need, if it applies.

Four C-17 Globemaster III aircraft flew out just before midnight on Saturday to transport approximately 300 health care personnel in preparation for Hurricane Irma disaster response operations -- even as parts of Florida were still evacuating before the Category 4 storm hit.

At the request of the U.S. Department of Health and Human Services, the medical staff -- physicians, nurses, veterinary staff, paramedics, fatality management professionals, and experienced command and control staff -- flew from Washington Dulles International Airport to Florida’s Orlando International Airport, Air Mobility Command officials said in a release.

The doctors and staff are part of the National Disaster Medical System, “which provides response capabilities to augment existing healthcare systems in affected areas,” AMC spokesman Maj.Korry Leverett said in the release.

The C-17s -- from Scott Air Force Base, Illinois; Dover Air Force Base, Delaware; and March Air Reserve Base, California -- were pre-positioned by Air Mobility Command's 618th Air Operations Center at Scott. The AOC plans, tasks, and executes airlift, aerial refueling and aeromedical evacuation missions throughout the globe.

“Additionally, there are about 20,000 pounds of medical equipment and supplies pre-positioned at Scott to provide assistance when called upon,” AMC spokesman Col. Chris Karns told Military.com.

The service on Thursday deployed a 10-bed patient staging facility and 28 airmen from Travis Air Force Base, California, to St. Croix, to support patient movement from Schneider Regional Medical Center, St. Thomas, to Puerto Rico.