Overtaxing the Pratt & Whitney (P&W) F135 engine on the Lockheed Martin F-35 stealth fighter is costing the Pentagon billions of dollars in unexpected maintenance costs.

Auditors from the US Government Accountability Office (GAO) say a failure to design in adequate cooling capability for the jet’s advanced sensors and radar will add an extra $38 billion to the total life-cycle cost of the USA’s F-35 programme, including current expenses and future projections.

F-35 takeoff

Source: Lockheed Martin

Sensor advances, plus flawed projections by procurement officials, has left F-35’s cooling system underpowered 

That problem is expected to worsen as Lockheed rolls out the much-anticipated Block 4 upgrade to the F-35, which will further increase the aircraft’s electronic sensing capabilities, including data transmission at 5G speeds.

“The addition of Block 4 will require more cooling capacity,” the GAO says in a 30 May report.

The F-35’s Honeywell-designed power and thermal management system (PTMS) uses bleed air taken from the F135 engine to cool onboard sensors. 

“They’re two different systems, but they work in a symbiotic way,” says Jennifer Latka, vice-president of F135 engines at P&W, speaking following the report’s release.

Sensor improvements to the F-35, along with bureaucratic mishandling of the jet’s symbiotic cooling system, have left the fifth-generation fighter pulling electrical power and air cooling beyond its design specifications.

The GAO says Lockheed discovered the cooling issue in 2008, and in 2013 requested a design revision to address the problem. However, the Pentagon denied that request, citing cost and schedule impacts.

“Programme officials decided to continue with the F135 engine’s original design with the understanding that there would be increased wear and tear, more maintenance, and reduced life on the engine because it would need to provide more air pressure to the PTMS than its design intended,” the GAO notes.

Instead, additional bleed air is diverted from the F135 engine into the PTMS to provide the necessary cooling. This causes excess wear on the engine’s turbine, resulting in a shorter lifespan and more frequent overhauls. 

Government auditors note P&W designed the F135 engine to meet the air pressure specifications the company was given at the time. Executives at the Raytheon Technologies subsidiary say the recent GAO report supports their calls to upgrade the F-35’s powerplant.

“We do feel that the GAO report actually validates the government’s decision to pursue the engine core upgrade on the F135,” says Latka.

After months of jockeying between P&W and rival engine maker GE Aerospace, the US Air Force in March threw its support behind P&W’s engine core upgrade (ECU) proposal for the F135. GE had been pushing for the service to adopt an entirely new engine for the aircraft.

f35 final assembly

Source: Lockheed Martin

Lockheed Martin conducts final assembly of the F-35 at its plant in Fort Worth, Texas

The ECU package “restores the engine life completely”, according to Latka, while also providing additional electrical power output and cooling capability the F-35 needs. “We are trying to keep up with the growth that we know is coming.”

She notes the ECU improvements to the existing engine will provide the F-35 with enough power and cooling capability to meet current needs, as well as the future Block 4 requirements. The GAO report notes an engine upgrade will potentially offset at least some of the extra $38 billion maintenance costs the current configuration is projected to generate of the aircraft’s service life.

However, both P&W and the GAO say the Pentagon will need to upgrade not just the F135 engine, but also the existing Honeywell PTMS if it plans to continue adding capability to the F-35 beyond the Block 4 plans.

“When we go beyond Block 4, there has got to be an upgraded PTMS,” says Latka.

Without such improvements, the GAO notes, the F-35’s engine will again experience reduced wear-time as additional radar and communications capabilities are added to jet.

“The engine will need to provide even more air pressure to PTMS to support future capabilities, which will further reduce engine life,” the GAO report notes.

To date, the Pentagon has not determined how it plans to modernise the F-35 PTMS, meaning a solution is still years away. The joint procurement office that oversees F-35 acquisitions for the USA and foreign buyers says it has not yet determined the power and cooling requirements beyond 2035.

In the nearer term, P&W will be ready to start delivering core upgrades for the F135 starting in 2028, the company told FlightGlobal in March. Those improvements should be sufficient to address the power and cooling needs of the current sensor improvement effort, according to Latka: “The engine core upgrade fully enables Block 4,” she says.

The cost of the Block 4 improvements has ballooned to over $16 billion – a 55% increase from the $10.6 billion estimate provided by the Pentagon in 2018.

Meanwhile, Lockheed says it is currently flight testing the Technical Refresh-3 (TR-3) update to the F-35 that will enable functioning of the Block 4 package. Chief executive James Taiclet described the process as being “in the very late innings” during an April call with investors.

TR-3 is scheduled for inclusion on production aircraft in July, the GAO report reveals, as part of the first Lot 15 F-35s.

While that will set the stage for delivery of Block 4, those improvements will not be rolled out at least until fiscal year 2029, according to Pentagon and GAO documents – three years behind the original schedule.

Auditors at the GAO remain critical of the Block 4 development process, calling the Pentagon’s cost reporting “inadequate”.

“Congress lacks critical information for overseeing the broader Block 4 effort and holding the programme and contractor accountable,” the watchdog says.

Story updated to reflect 14 July, 2023 to reflect that the GAO estimate of $38 billion represents total lifecycle costs projected into the future, not costs already endured.