Replacements are on the way for two decrepit 55th Wing jets that give the Air Force critical information about nuclear detonations in North Korea and elsewhere in the world.

The giant defense contractor L3Harris and the Air Force’s “Big Safari” acquisitions group are scheduled to start in May converting the first of three former Air National Guard fuel tankers into WC-135R radiation-detection aircraft. Work will start on the other two next year.

When they’re finished, all three aircraft will be assigned to the 55th Wing at Offutt Air Force Base. The first is scheduled for delivery in 2022, said Maj. Malinda Singleton, an Air Force spokeswoman, with the other two following in 2023.

The conversions will cost $218 million. Congress appropriated the money in the 2019 and 2020 budget years. Nebraska Sen. Deb Fischer and Reps. Don Bacon and Jeff Fortenberry worked to ensure funding survived the Washington budget wars.

“The world needs this capability to sense nuclear tests,” Bacon told The World-Herald.

During the conversions, the long refueling booms will be removed from the former KC-135R tankers, Singleton said. The avionics and flight systems will be upgraded to match the RC-135 Rivet Joint and Cobra Ball aircraft already in the 55th Wing fleet at Offutt.

They will be equipped with sensing pods on each side of the fuselage, over the wing. Filters inside the pods capture tiny particles from the nuclear blast, giving analysts critical information about the composition of the bomb or other radiation source.

“There are a number of intelligence items you can only get from atmospheric particles,” said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists. “If you can get the bomb components, you can get a wealth of data.”

The planes will also be equipped with compressors that channel air samples into tanks for further study. Each conversion is expected to take almost 24 months.

The aircraft are part of a mission called Constant Phoenix, which traces its roots back to the late 1940s.

In 1949, fallout from the first Soviet nuclear test was detected by an Air Force WB-29 weather plane, according to “The Boeing KC-135 Stratotanker: More than a Tanker,” a 2017 history by Robert Hopkins III, a former 55th Wing recon pilot.

During the Cold War, a series of aircraft types flew the missions frequently over the Pacific and Arctic Oceans. In the mid-1960s, the Air Force converted 10 former C-135B transport planes for Constant Phoenix, which came to be called the “nuke sniffing” mission. The prefix “W” was added to their name because they also collected weather data.

Hopkins said the newly named WC-135s could fly much longer and much higher than previous types.

“It doesn’t have to fly into hostile territory,” Hopkins said. “They can tell you everything you need to know about the bomb, and the level of sophistication.”

For decades, the Offutt-based Strategic Air Command used them to monitor compliance with the 1963 Nuclear Test Ban Treaty. One was used to monitor fallout from the 1986 nuclear disaster at Chernobyl, though it later had to be scrapped in part because of residual radiation.

After the fall of the Soviet Union, then-Air Force Chief of Staff Gen. Merrill McPeak moved quickly to retire them.

“McPeak was determined to get rid of everything SAC,” Hopkins said. “They started just dragging them off to the boneyard.”

But the rise of China as a nuclear power, and the 1998 nuclear standoff between India and Pakistan, made it clear there was still a need for “nuke sniffers.” That was reinforced by North Korea’s unsettling development of nuclear weapons.

Today, the 55th Wing’s 45th Reconnaissance Squadron operates two WC-135s, which were built in 1961 and 1962. They are among the oldest jets in the Air Force, and have not received the same upgraded engines and flight systems as the 55th Wing’s RC-135s.

The 55th Wing allows only very experienced pilots to fly the current WC-135s because of their old-fashioned flight controls and engines, a noisier, smokier, gas-guzzling motor last produced in 1985. They break down frequently.

In 2017, an engine on one of the planes caught fire on a flight from the remote Indian Ocean island of Diego Garcia and Okinawa, Japan. After a frightening descent, the plane made a safe emergency landing at a civilian airport in Banda Aceh, Indonesia. It was sidelined there for repairs for more than a week.

“The current airplanes are old. They are wearing out,” Gen. David Goldfein, the Air Force chief of staff, told the Senate Armed Services Committee in April 2018. “Our mission-capable rates and, more importantly, our aircraft availability rates to be able to do this mission are much lower than not only the secretary of defense but the combatant commanders require.”

Hopkins said the three KC-135s selected for conversion rolled off the Boeing assembly line in August and September 1964 — which means they are among the youngest of that type. Each has about 25,000 to 27,500 flight hours, Singleton said, compared with between 29,500 and 36,000 for the two current Constant Phoenix jets.

Though only two to three years newer than the aircraft they replace, all three were upgraded with new, quieter, turbofan engines in the early 1990s, and are equipped with newer avionics and flight controls. That means most any 55th Wing pilot will be able to fly them.

All have served most recently as National Guard tankers, which Hopkins said means they are in better shape than the 55th Wing workhorses. He said Guard tankers typically rack up fewer flight hours and are maintained by National Guard mechanics who care for the same aircraft for years, unlike in the active-duty Air Force.

“Offutt flies their jets hard,” he said. “Airplanes that come out of the Guard and Reserve have been pampered.”

Hopkins has criticized the Air Force for failing to fund new reconnaissance aircraft, but he does believe refitting the aged tankers is the right move now.

Flight training and parts replacement will be simplified because the cockpit, engines and airframes will be the same as all but two other 55th Wing jets.

“It’s a prudent choice,” Hopkins said.

Bacon said he is glad to see the planes coming into the pipeline.

“The world situation tells us we need these,” Bacon said. “We might as well go out and get it done.”

Offutt Air Force Base through the years