Every F-35 carries a small, fast-spinning turbomachine that most people never hear about. It starts the engine, cools the radar, pressurises the cockpit and, if the main generators fail, keeps the flight controls alive. When one of those machines comes out of a European jet for repair, it has to prove itself on a test stand before it flies again.

From now on, that test can happen in the Netherlands. On 8 October 2026 Honeywell Aerospace announced that, together with the Dutch firm Aeronamic, it has started a Modular Test Cell at the Royal Netherlands Air and Space Force’s Woensdrecht Air Support Command, built to test turbomachines from the F-35’s Power and Thermal Management System, or PTMS.

It is not a glamorous piece of kit. A steel building with a stack on the roof rarely is. But the F-35 PTMS test cell closes a gap in Europe’s repair chain, and on a jet whose readiness has been dogged by spare parts and depot capacity, gaps are what count.

The box that keeps the F-35 breathing

Older fighters spread these jobs across several boxes: an auxiliary power unit here, an environmental control system there, an emergency power unit somewhere else. The F-35 Lightning II rolls them into one integrated package. Honeywell describes the PTMS as combining all the functions of an auxiliary power unit, emergency power unit, environmental control system and thermal management system.

The list of what depends on it is long. According to Honeywell, the PTMS supplies liquid and air cooling for the radar, electronic warfare system and avionics, cools and pressurises the cockpit, provides oxygen to the pilot, seals and defogs the canopy, inerts the fuel tanks, and supplies emergency power to the primary flight controls if the engine or generators fail.

In other words, a jet with a sick PTMS is not flying. That is why the speed at which a removed turbomachine can be repaired, tested and returned matters so much to fleet availability.

Why a test cell, and why Woensdrecht

A repaired turbomachine cannot simply be bolted back into an aircraft. It has to pass an acceptance test first. Back in 2024 Aeronamic said that this acceptance test was still outsourced, and that it was working to bring a PTMS test cell in-house, adapting a US test cell to European regulations together with the Dutch air force and Honeywell.

With the F-35 PTMS test cell running at Woensdrecht, that gap is closed. Honeywell says operators will benefit from accelerated turnaround times, reduced logistics requirements and increased aircraft availability, because units no longer need to leave the region to be tested.

The design is not new. In October 2023 Honeywell delivered a Modular Test Cell to the US government depot at Fleet Readiness Center East in Cherry Point, North Carolina, so that PTMS turbomachines repaired there could be acceptance-tested on site. Lockheed Martin’s F-35 depot project manager, Porfirio Velasquez, said at the time that the concept could be duplicated in sustainment facilities in the United States, Europe and Asia. Woensdrecht is the European copy.

A Dutch link four years in the making

The Woensdrecht cell is the latest step in a partnership that began at the Farnborough Airshow in July 2022, when Honeywell and Aeronamic signed a licensed service centre agreement to set up a Netherlands-based depot for PTMS sustainment covering the European market. Aeronamic, headquartered in Almelo, says it also builds the F-35’s Forward Module, the starter-generator for the integrated power package.

In April 2024 Aeronamic Services at Woensdrecht announced it had passed its audit and was qualified to perform F-35 maintenance. The test cell, which the company had targeted for the end of 2025, completes the loop: repair and test in the same place.

The cooling problem behind the headlines

The F-35 PTMS test cell arrives as the system itself is under scrutiny. In May 2023 the US Government Accountability Office reported that the jet’s cooling demand had outgrown its design.

The same report put the extra engine maintenance bill at 38 billion dollars. Honeywell’s answer is incremental: in September 2026 it launched a 40-kilowatt upgrade of the existing system, up from about 32 kilowatts, expected to enter service around 2029 to 2030. Collins Aerospace is pitching a clean-sheet rival, as we covered in Refroidissement du F-35 : les défis posés par l’EPACS de Collins (Honeywell).

Either way, the turbomachines already flying in hundreds of European F-35s will need repairing for decades. Honeywell has argued that its derivative upgrade keeps existing repair facilities, tooling and expertise relevant, and a new test cell in the Netherlands is exactly that kind of infrastructure.

Honeywell’s own explainer on how its systems power and cool the F-35 Lightning II. Video: Honeywell Aerospace

Stealth, sensor fusion and Block 4 get the headlines. Availability is won in places like Woensdrecht, one tested turbomachine at a time.

Sources: Honeywell Aerospace press release, Honeywell Aerospace and Aeronamic start test cell in the Netherlands to support regional F35 sustainment, Woensdrecht, 8 October 2026; aeroTELEGRAPH, 8 October 2026; Honeywell press releases of 10 October 2023, 14 September 2026 and 22 September 2026; Honeywell Aerospace, Why PTMS is foundational to F-35 mission success, 19 April 2026; Aeronamic news, 19 July 2022 and 8 April 2024; Shephard Media, July 2022; US GAO report GAO-23-106047, May 2023; Defense News, 1 June 2023; The Defense Post, 24 September 2026; Wikipedia, Lockheed Martin F-35 Lightning II.