Pratt & Whitney F135 engine undergoes accelerated mission testing

A Pratt & Whitney F135 engine undergoes accelerated mission testing in Sea Level Test Cell 3 at Arnold Air Force Base, Tenn., Nov. 15, 2021. The F135 is the engine used to power the F-35 Lightning II. (U.S. Air Force photos by Jill Pickett)

WASHINGTON — An executive from Pratt & Whitney today pushed back at a report from the Government Accountability Office warning of a depleted stockpile of F135 engines that risked production for the F-35 Joint Strike Fighter, with the company executive describing the report as out of date on the issue.

“Never once did Lockheed Martin’s production line reach and pull for a ready-for-install engine and not have one,” Jen Latka, vice president of the F135 program, said in a briefing with reporters, organized just hours after the GAO report [PDF] was released on Tuesday. “So it did not impact Lockheed’s production line the way it was kind of stated in the report.”

According to GAO, the program relies on a stockpile of engines “to mitigate the late deliveries in prior years,” which could either be plugged into fighters undergoing assembly or replace engines in the field. But after an F-35B crashed in December 2022, and the program pressed pause on new engine deliveries, the surplus of powerplants, known as an “engine buffer,” was cleared out.

Latka explained that a mitigation effort for the vibration issue that allowed F135 deliveries to resume and return grounded jets to flight enabled Pratt to get “all the engines back to the warehouse” that had been held up, where the powerplants can then be called up to move through Lockheed’s “engine shop” in preparation for installation. In the process, she said, the buffer recovered. 

“It’s also kind of unfortunate that [GAO] struck their point in time talking about this issue in February because by March 1, we were back to normal,” she said. “All of the engines that we had been building in January and February, we delivered. So it’s not an issue. We’re back to having the same buffer.” 

GAO found that in 2022, Lockheed delivered only half of aircraft on time and that Pratt delivered 97 percent of engines late. According to GAO, late aircraft were largely related to supply chain and manufacturing issues exacerbated by the pandemic.

Similar problems have affected recent engine deliveries, though powerplant tardiness has been a “long-standing issue,” with Pratt consistently delivering engines behind schedule since 2017, auditors found. Pratt submitted a “corrective action plan” to address the delayed deliveries, which was accepted in September 2022, “but late deliveries persist,” GAO wrote. (Latka did not discuss that issue during her comments to media.)

Engine And PTMS Modernization

GAO’s lengthy report in part focused on the Pentagon’s plan to modernize the fighter’s engine and power and thermal management system (PTMS), which is needed because the PTMS requires more bleed air from the F135 engine than originally expected to cool off the aircraft’s subsystems. Overworking the powerplant forces it to run hotter and risks the need for more costly maintenance, which GAO estimates will add $38 billion to the program’s lifecycle costs.

But both the F-35 Joint Program Office (JPO) and Pratt believe that cost can be mostly avoided if the engine is modernized, which officials are pursuing through an engine core upgrade (ECU)

“The JPO is already very confident we can minimize the $38 billion impact simply with ECU,” JPO spokesman Russ Goemaere said in a statement. “The ECU will restore engine life, and the PTMS solution will ensure that the air vehicle can support future capability growth. We are treating the ECU and PTMS as a single modernization effort because they must be designed together to work with the greatest efficiency.”

Latka today said the ECU “eliminates all the additional maintenance that would be required due to being out of [specifications] on this power and thermal management issue.” She also explained that a more pressing need for a PTMS upgrade would materialize after Block 4.

“The engine core upgrade can fully enable Block 4. When we go beyond Block 4, regardless of engine, there has got to be an upgraded PTMS solution,” she said. 

Even though the JPO is grouping together the engine and PTMS upgrades as a single modernization effort, the Pentagon’s stance on the latter is still unclear, GAO found. Latka emphasized it’s critical for the two systems to be developed in tandem to shape how the ECU is designed. 

“If we know what that [PTMS] solution is, we can optimize the engine design around it. So we want to be designing the engine core upgrade in parallel with whatever decision is made or whatever changes are made to the PTMS solution,” she added. “When you do those two things, you have a tremendous amount of power and cooling margin for the future beyond Block 4.”

The fighter’s Block 4 upgrade suite, whose cost growth GAO found is unaccounted for, will require even more cooling for new capabilities that will be fielded through 2035. Though those needs are generally understood, GAO said, the military services that fly the jet still need to set specific requirements. Until then, Block 4 needs are “notional,” according to the watchdog. 

Additionally, GAO said “it is unclear how far into the future any PTMS and engine upgrades will be able to support the F-35,” which is planned to keep flying into the 2070s.

Though officials have publicly said they’re going with the upgrade to the current engine, the GAO also assessed another option, that of a completely new adaptive engine, and said it offers the “greatest performance improvement” among engine modernization choices but echoed officials’ previous concerns that it is not compatible with the F-35B, a lack of commonality that risks driving up costs. 

RELATED: GE’S lobbying message to Congress on F-35 engine: ‘Take this to the next logical milestone’

The report further found other aspects of the aircraft like its fuel thermal management and electrical power systems may also need upgrades, which are linked to how the program separately approaches the engine and PTMS. Those choices were identified in a graphic compiled by GAO that is included below.

GAO F-35 chart

Until requirements for the engine and PTMS are formalized, GAO warned, the military services “risk selecting modernization options without understanding the future cost, schedule, and technical effects” of both.