WASHINGTON — The Air Force is set to begin turning the first of three KC-135R tankers into WC-135 Constant Phoenix “nuclear sniffers” in fall 2019, a conversion the service’s top general said Tuesday is necessary given the legacy aircraft’s advanced age.

The Air Force’s two WC-135s planes are among the most rare and specialized aircraft in the service’s inventory, outfitted with highly classified equipment that allows its crew to monitor the atmosphere for signs of nuclear explosions. But the planes date from the mid 1960s and are reaching the end of their lifespans.

“The current airplanes are old. They’re wearing out,” Air Force Chief of Staff Gen. Dave Goldfein told the Senate Armed Services Committee on Tuesday.

“Our mission capable rates, and more importantly our aircraft availability rates to go do this mission, are much lower than not only the secretary of defense but the combatant commander’s requirements for that mission.”

The Air Force’s solution? Take three KC-135R tankers and upgrade them, transforming them into WC-135s — an effort that increases the total inventory and thus allows “for better availability to execute time sensitive missions,” Air Force spokeswoman Ann Stefanek said in response to emailed questions from Defense News.

Then, retire the old Constant Phoenix aircraft, which are assigned to 45th Reconnaissance Squadron at Offutt Air Force Base, Nebraska.

The service’s plan, which was spelled out for the first time in the fiscal year 2019 budget, “allows us to give more time to be able to continue to accomplish this mission,” Goldfein said.

It also can be viewed as part of a larger Defense Department focus on the modernization and expansion of its nuclear capabilities, driven in part by a new Nuclear Posture Review and growing concern about Russian, Chinese, Iranian and North Korean weapons.

The Air Force is requesting $208 million in FY19 for the Constant Phoenix upgrade effort, with an additional $8 million planned in FY20. Its analysis showed “that it was more cost-effective to convert KC-135R aircraft into WC-135Rs than to modify existing WC-135W aircraft,” budget documents said.

Although the service rarely speaks about the Constant Phoenix or the mission it conducts, the aircraft has garnered more attention over the past year as tensions with North Korea ramped up. It deployed to Kadena Air Base in Okinawa, Japan, in September, where it was sent to sniff out radioactive clouds after North Korea’s sixth nuclear test that month.

The first KC-135R is projected to start the modification process in September 2019, Stefanek said. L3 Technologies, which conducts depot maintenance and modifications for all variants of RC-135 special mission aircraft, will convert the tankers at its facility in Greenville, Texas.

During the upgrade period, the Air Force will purchase modification kits — basically, the unique atmospheric collection suite and mission sensors carried by the WC-135 — which L3 will integrate on the tankers. It will take about 18 to 24 months for each aircraft to undergo the modification process.

“Additional inductions will be addressed in future budgets,” Stefanek said.

Meanwhile, “due to the age of the engines and cockpits, the plan is to divest the current WC-135W and WC-135C and send them to the Aerospace Maintenance and Regeneration Group, more commonly called the boneyard, at Davis-Monthan Air Force Base, Arizona,” she said.