Thirty-five members of the U.S. Congress say they are concerned about an Air Force proposal to use its Advanced Engine Technology Program (AETP) propulsion system to replace the F-35 fighter’s current powerplant.

In January, Air Force Secretary Frank Kendall launched a market survey to find potential suppliers for an F-35 Adaptive Engine Replacement Program, in advance of a possible program launch in fiscal 2024. Since then, F-35 program officials have released information about the limitations of the F135 engine.

In a July 22 letter, the lawmakers led by Rep. John Larson (D-Conn.) appealed to Pentagon acquisition chief Bill LaPlante, arguing a new effort to build an engine to replace Pratt & Whitney’s F135 would be costly—up to $6 billion—and undermine commonality with partner nations.

The lawmakers revisited then-President Barack Obama’s opposition to the development of another F-35 engine, quoting then-Air Force Chief of Staff Gen. Norton Schwartz as saying, “The alternate engine is not for anybody else but the Air Force. The Navy isn’t going to operate an alternate engine aboard ships. The European partners are not going to operate two engines. You’re talking about focusing this on your Air Force, which is problematic in my view.”

The letter asks LaPlante to lay out a timeline for modernizing the F-35 engine, and to specify what roles the Joint Staff, the Office of the Secretary of Defense, the other U.S. military services and international partners on the program would have in creating requirements. It also asks whether the Pentagon would require an independent cost assessment of the program, whether the Pentagon would assess the impact on the industrial base and if the Pentagon might also replace the F-35 air vehicle.

“The F135 has outperformed its original specifications, including bleed air draw and time on wing, enabling it to support three major airframe and payload upgrades without engine modernization,” the letter says. “At the same time, the average engine cost has been reduced by more than 50% to date. Meanwhile, the U.S. services have spent over $7 billion upgrading the airframe and payloads. The current engine can support the upcoming Block 4 air vehicle and payload upgrades. However, there will be an impact to engine life and increased sustainment costs as a result.” 

Congress is likely to weigh in on the Air Force proposal to adapt the AETP engine for use on the F-35. The Senate is poised to consider a provision that would ask the comptroller general to review the business case for alternatives to F-35 engine upgrades. And the House has passed a fiscal 2023 Defense Authorization Act that directs the head of the Joint Program Office, Air Force Lt. Gen. Michael Schmidt, to brief congressional defense committees by Oct. 4 on the results of an ongoing cost-benefit analysis about replacing the F-35’s power and thermal management system, though the bill is not likely to become law before year-end.