Key Points and Summary – The F-35 program is under intense pressure due to severe problems with its critical Block 4 modernization.
-The new TR-3 hardware, the foundation for the upgrade, is late, meaning new jets delivered since 2023 have non-combat-ready software that has required pilots to reboot systems in flight.
-This software crisis is compounded by a long-standing hardware flaw: the jet literally overheats, reducing engine life.
-The U.S. Air Force is so frustrated with the delays and what one general called “pipedreams” that it has slashed its planned 2026 procurement of the fighter in half.
The F-35 Fighter Challenge ‘Heats Up’
From its anonymous digs in Crystal City, the Pentagon’s high-rise civilian annex, the Joint Program Office for the Lockheed Martin F-35 Lightning wrote $36 billion in contracts in 2023 alone. The program is still critical to US military power even if the Boeing F-47 hogs the headlines.
The program’s recent performance has earned a stiff rebuke from the US Air Force, by far its largest customer, which has not yet approved for combat usage a new set of critical software. That problem has added to the woes of a complex set of hardware upgrades called Block 4. And the aircraft needs an upgraded engine and new power and cooling system to fix a long-running problem: the F-35 literally overheats in flight.
At the current pace, the last of the US Air Force’s nominally required 1,763 F-35s will be delivered in 2051. It’s halfway there (full-scale development began in 2001) but the project is under pressure. In the fiscal 2026 budget, the USAF hacked its planned F-35 procurement from 48 to 24 units, the lowest number since 2015, and chief of staff General Dave Allvin stated explicitly in a July interview with Defense One that the service was delaying its buys until it could get ‘F-35s that are most relevant for the fight.’
F-35s delivered since 2023 do not have combat-ready software. Lockheed Martin said in June the fix was awaiting air force approval. But that’s just step one in the huge Block 4 effort to modernise the F-35’s mission systems. The last estimated development bill for Block 4 was $16.5 billion, 60 percent as much as the cost of developing the complete Northrop Grumman B-21 bomber. This wider mission-system modernisation effort has also hit delays, and a promised ‘re-imagining’ plan has not been revealed.
Root of the Problem
The root cause of the problem is the well-intentioned management philosophy to invite competition for replacing subsystems when new technology is available; this encourages innovation and avoids sole-source bids. But when the program office and Lockheed Martin sought a new integrated core processor in the late 2010s, L3Harris undercut incumbent Northrop Grumman, won the contract—and quickly fell behind schedule. Late-arriving hardware delayed software testing. During flight tests in 2023, pilots frequently had to reset their F-35s’ systems in flight.
The integrated core processor is the center of the Tech Refresh 3 (TR-3) hardware upgrade, which in turn is the foundation stone for Block 4. Under Block 4, more than 80 individual improvements (including the new Northrop Grumman APG-85 radar and Raytheon EO-DAS all-round infrared imaging system) were planned to drop into the F-35 production line every two years between the first TR-3 delivery in 2023 and the end of the decade. F-35s ordered in 2029 and delivered in 2031 would be at the full Block 4 standard.
Congress and the Government Accountability Office said repeatedly that Block 4 was so large an effort that it should be its own major acquisition program—that is, like one for a completely new aircraft. But the program office continued it under the principle of Continuous Capability Development and Delivery it adopted in 2019, in line with the then-universal enthusiasm for ‘agile’ system development.
But there were too many sub-programs to be run this way, particularly since many of them were interdependent. In hearings in early 2024, Lieutenant General Michael Schmidt, then the Joint Project Office director, disclosed that the Block 4 project was being ‘reimagined’, reduced to a ‘subset of capabilities that give us the most bang for the buck’ because ‘we have signed ourselves up to pipedreams’. That reimagination is incomplete—hence the USAF’s decision to cut its fiscal 2026 order. There’s no way for outside observers to tell whether, with the reduced suite of improvements, Block 4 will be delivered as previously scheduled.’
A parallel program, not part of Block 4, is aimed at alleviating thermal management problems that have dogged the program since early flight tests. One F-35 stealth design principle is to dump heat from avionics, actuators and other systems either into the engine bypass duct or into the fuel and cooling air provided by the power and thermal management system (PTMS). Otherwise, internal systems start to exceed their design temperature limits.
This heat-dispersal system falls short in some circumstances (higher speeds and lower altitudes), so more air needs to be bled from the engine to the PTMS, making the engine run hotter and reducing its life. (In 2023 alone the program office ordered 40 more engines than airframes.) And Block 4 and subsequent developments are expected to need even more cooling.
Part of the solution has been to give the F-35 a new-design engine. Until 2022 it was to be one of an efficient but technically challenging configuration called ‘adaptive cycle’. But such an engine would be hard to incorporate on the short take-off and vertical-landing version of the fighter, the F-35B, and Pratt & Whitney lobbied hard to instead improve its F135 engine, the one that the F-35 already uses, with a new high-pressure section. A contract for this, the Engine Core Upgrade, was signed in 2023, with the aim of delivering engines by 2029. But it was disclosed in June 2025 that the critical design review for the engine upgrade, scheduled for this summer, is running a year late.
The second front in the thermal war is to adopt a new PTMS design, with Lockheed Martin running a contest between RTX Collins and incumbent Honeywell. The PTMS is a complex system, combining environmental control, auxiliary and emergency power and engine starting.
The schedule for introducing these new technologies into production is uncertain, but another question is what to do with more than 1,000 pre-TR-3 F-35s, including those flown by Australia, South Korea, Japan and Singapore? Bringing even half of these up to a Block 4 standard, and adding the engine improvement and new PTMS, would be the largest aircraft upgrade program ever carried out, and the unit price tag is unknown.
And, as the USAF seems ready to switch attention to the Boeing F-47, Lockheed Martin chief executive Jim Taiclet has been talking up a future F-35 variant that could use technology from the company’s work on its unsuccessful competitor to the F-47. This technology could deliver ‘80 percent of the capability, potentially, at 50 percent of the cost per unit aircraft. Eventually, there’ll be 3,500 of those [F-35] chassis out there at various stages of technology and capability.’
That sounds less like Block 4 than an all-new avionics system—possibly based on one Northrop Grumman is developing for the B-21—and an adaptive engine that would offer more cooling and a 30 percent increase in range. Some airframes could be retrofitted, but there would be scope for a new production version with the larger wing of the current F-35C and refinements made possible by eliminating provision for short take-off and vertical landing.
It’s a robust strategy because the US Air Force and US Navy must keep buying F-35s to stop their fighter forces ageing out. The F-47 will be too expensive and arrive too late for that, and the Department of Defense doesn’t want to fund a new fighter that’s been in the works for the navy, the F/A-XX.
Even if the air force did seek a completely new low-end fighter to supplement the F-47, there’s little or no chance of any of the three possible suppliers wanting to develop one: Lockheed Martin would be happy to build an updated F-35 instead, Northrop Grumman, a partner in making F-35s, would probably have the same view, and Boeing would be focused on the F-47.
That means that a new low-end fighter would have to be the heavily improved F-35 that Lockheed Martin is talking about. How that would dovetail with Block 4 upgrades and retrofits is anyone’s guess.
One way or another, the F-35 will be around for a long time.
About the Author: Bill Sweetman
Bill Sweetman is a veteran, award-winning journalist and aerospace industry executive. He is the author of Trillion Dollar Trainwreck: How the F-35 hollowed out the US Air Force. This first appeared in ASPI’s The Strategist.
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