The most advanced fighter America builds has a decidedly un-glamorous problem: it runs too hot. Every new sensor, jammer and processor bolted into the F-35 Lightning II throws off heat the jet was never designed to shed — and the fix has become a two-company arms race worth billions.

On 24 August 2026, one of the contenders moved ahead. Collins Aerospace, an RTX business, announced it had completed altitude testing of its Enhanced Power and Cooling System (EPACS), the next-generation heart of the F-35’s thermal management. The incumbent, Honeywell, is fighting to keep the job with a cheaper, phased upgrade of the system already in the jet.

It is a quiet contest with loud consequences: whoever wins decides how much future the F-35 has left in it.

Why the F-35 runs hot

The F-35 is a single-engine jet, and that one Pratt & Whitney F135 does a great deal more than push the aircraft through the sky. Through the PTMS, it also supplies the electrical power and the cooling for the jet’s dense electronics. As the mission systems have grown, the cooling demand has outstripped what the original PTMS was sized for.

The workaround has been to draw more from the engine — and that has a cost. Running the thermal system beyond its design point pulls extra bleed air and power from the F135, which US government auditors have linked to accelerated engine wear and shorter engine life. In other words, the cooling shortfall is quietly eating the jet’s engines.

Fixing that is the whole point of a new PTMS. Collins says its altitude campaign proved EPACS can hold up under real flight conditions.

EPACS versus Honeywell

The two bids could hardly be more different. Collins is offering a clean-sheet replacement that demonstrated roughly 80 kilowatts of cooling in laboratory testing in 2024 — more than double what the Honeywell PTMS in the jet delivers today, and enough headroom for whatever the F-35 grows into.

Honeywell, which has built the F-35’s thermal system from the start, argues for evolution rather than replacement: a phased upgrade beginning with a software change it says would lift capacity to around 40 kilowatts within two to three years, avoiding the cost and risk of ripping out proven hardware.

The RTX angle

There is a corporate subplot. Collins and Pratt & Whitney are both RTX businesses — and Pratt & Whitney is already delivering the F135 Engine Core Upgrade that will give the F-35 more power and cooling capacity in the first place. Win the PTMS competition too, and RTX supplies almost the entire power-and-thermal chain of the jet. Honeywell is the outsider defending the one big system it still owns.

The customer, ultimately, is the Pentagon and the dozens of nations flying the F-35. For them the calculation is blunt: pay more now for a system with room to grow, or bank on a cheaper upgrade and hope the jet’s appetite for cooling doesn’t outrun it again. Collins just made its case a little harder to ignore.

Sources: RTX / Collins Aerospace press release (24 Aug 2026); FlightGlobal; Aerospace Testing International; PR Newswire.