FSUIPC4: the all-time number one on simMarket, for a sim you can now buy for $4.99

Microsoft Flight Simulator X: Steam Edition is on sale on Steam at the time of writing for $4.99. FSUIPC4, the add-on that lets you assign your throttle quadrant to it properly, costs €24.99 — $29.00 at simMarket's own conversion. You can buy the simulator nearly six times over for the price of the utility that makes it behave.

And FSUIPC4 is still the number one product on simMarket's all-time best sellers list. Not this week's list, where it does not appear at all. The all-time list, checked on 19 August 2026, where it sits above AES Credits, above FSRealistic Pro, above FSUIPC7 — its own successor for a simulator with a hundred times the player count. Nothing else in the shop has sold like it, and nothing else is likely to.

The last build was 4.977, released on 24 November 2021. Peter Dowson, who wrote the first FSUIPC in 1997 for FS98, announced his retirement on 17 December 2018 at the age of 75 and handed the code to his son John, who has run it since 2019. John Dowson is still answering FSUIPC4 support threads — the most recent ones on the board are dated 14 May 2026. So the honest question is not whether this product is abandoned. It plainly is not. The question is whether the thing it plugs into still justifies €24.99 in 2026, and whether the free build most people already have installed does everything they actually need.

Those two questions pull in opposite directions, and the record does not resolve them cleanly. Nineteen store reviews, every one of them five stars, spanning 2016 to May 2026. A decade of AVSIM threads in which the same argument recurs verbatim: one camp says they would uninstall FSX before they gave up FSUIPC, the other says SimConnect made it redundant years ago and Joy2Key does the button mapping for nothing. Both camps are describing real setups. Which one you are in decides everything.

This review reads 26 public sources: the simMarket listing and its full rating breakdown, the developer's own site and changelog dates, the current FSUIPC support forum including threads from December 2025 and May 2026, the closed simFlight forum, five AVSIM threads spanning 2010 to 2020, three video tutorials, two written explainers, and the Steam store and player data for FSX itself.

What you are actually buying

FSUIPC4 is a 32-bit DLL that loads into Flight Simulator X and exposes the simulator's internal state through a stable set of numbered "offsets" that other programs can read and write. That interface is the foundation of most of the FSX add-on ecosystem — hardware drivers, external instrument panels, ACARS clients, networked cockpit software, GoFlight and Saitek modules, Project Magenta.

The critical thing, and the developer says it himself on the product page, is that you do not have to pay for that part. The unregistered download is free and does the interface job in full. Buying a key unlocks the user-facing layer: axis assignment and calibration, per-aircraft profiles, button and key programming, weather and wind smoothing, autosave, and the assorted FSX fixes.

	Price	Simulators	Status
FSUIPC4 (unregistered)	Free	FSX, FSX:SE, P3D 1.4–3.4	v4.977, 24 Nov 2021
FSUIPC4 (registered key)	€24.99 / $29.00 / £21.39	FSX, FSX:SE, P3D 1.4–3.4	Same build, features unlocked
WideFS7	€24.00	Networked client PCs	v7.160, 7 Oct 2021
FSUIPC6	€29.99	Prepar3D v4, v5, v6	v6.2.2, 14 Feb 2025 — separate product
FSUIPC7	€29.99	MSFS 2020, MSFS 2024	v7.5.7, 6 May 2026 — separate product

Get the version right, because the money is not transferable. If you fly Prepar3D v4 or later you want FSUIPC6. If you fly MSFS 2020 or 2024 you want FSUIPC7 — the simMarket listing for FSUIPC4 says so in bold at the top of the page, which is more than most stores would bother with. inFlight News has reviewed FSUIPC7 separately.

One wrinkle worth flagging. simMarket sells FSUIPC4 with "Prepar3D 1.4, Prepar3d V2, Prepar3d V3.x" in the requirements box. The developer's own downloads page is blunter: those versions "will not be supported in the future. Prepar3D users should move on to Version 4." If you are on P3D v3 and thinking of buying FSUIPC4 today, you are buying into a dead branch and you should read that sentence first.

Is the older, cheaper thing the sensible buy?

Here it is inverted. The older, cheaper product is FSUIPC4 — and for most FSX users the sensible buy is the free build of the same product. That is not a criticism. It is the developer's own recommendation, printed on his own store page since 2006: "For most such programs you do not need to actually purchase FSUIPC4."

What the community consistently praises

Autosave is the feature people name first, and they name it with stories. A simMarket reviewer in May 2017 described three attempts to fly Dubai–Toronto, the third ending in a CTD over the UK, and FSUIPC restoring the PMDG 777 with FMC data, speeds, altitudes and fuel intact. A March 2022 reviewer called it the answer to a crash on approach after eight hours. It recurs on AVSIM in the same terms. FSX is a 32-bit application that runs out of virtual address space on long flights; autosave is the difference between an inconvenience and a wasted evening, and nothing in the base simulator does it.

Per-aircraft controller profiles are the second. A simMarket reviewer using the A2A C172 described his vernier throttles loading correctly, with buttons and keys, purely because FSUIPC swaps profiles when the aircraft changes. Another, coming from X-Plane 10 in December 2016, spent two months failing to get Saitek throttle quadrants behaving on an A320 and roughly four hours doing it in P3D with FSUIPC. The May 2026 reviewer put it most precisely: you can match a physical yoke to the virtual cockpit yoke, "which is impossible otherwise."

Wind and weather smoothing. This is a fix for a specific FSX defect — abrupt wind-layer shifts that jolt the aeroplane — and on AVSIM's PMDG forum the consensus in a long free-versus-paid thread was that no weather add-on, ASX or REX included, actually solved it. One poster said he bought the licence for wind smoothing alone. This is worth understanding as a period feature: it is a reason to buy in 2011 that means nothing in MSFS, and everything if FSX is your simulator.

It is tiny. A 2014 AVSIM post measured FSUIPC4's VAS footprint at 7MB. On a simulator where every megabyte of address space is contested, that matters, and no source disputes it.

And the support has outlived the man who promised it. The fsuipc.com pledge reads: support "provided free-of-charge (i.e. no annual subscription necessary) for the lifetime of the product (or until I retire!)". Pete Dowson did retire. Support continued anyway. John Dowson was answering FSUIPC4 questions on 14 May 2026 — nearly five years after the last code change, on a product he did not write, for a simulator that stopped being developed before he took over. That deserves saying plainly.

What keeps coming up as a problem

Installation is where new buyers get stuck. Still open. FSUIPC4 hooks FSX through SimConnect, and it needs the right SimConnect. The classic failure on FSX:Steam Edition is "Probe Manifest fails to match base SimConnect needed to load FSUIPC4" — an AVSIM thread from 21 March 2020 shows the fix is installing SimConnect from all three folders in the FSX:SE SDK, which nothing tells you. On 1 December 2025 a user on the current support forum could not get the Add-ons menu to appear at all; John Dowson's diagnosis was a missing FSX Service Pack 1 and SP2. Windows 11 has produced its own reports of SimConnect failing to register in the side-by-side store. None of this is FSUIPC's fault. All of it is the buyer's problem.

The interface. Still open, and will not change. FSUIPC4's configuration is a Win32 tabbed dialog — About, Logging, Miscellaneous, Winds, Clouds Etc., Hot Keys, Buttons + Switches, Key Presses, Axis Assignment, Joystick Calibration, AutoSave/GPSout — backed by manuals running to a hundred-odd pages. The calibration tab alone is an eleven-page menu. On AVSIM in January 2015 a user asked, reasonably, whether there was a "for dummies" guide. The community answer was a YouTube video. That video, frooglesim's HOW TO USE FSUIPC FOR FSX, has 236,000 views. A quarter of a million people needed a stranger to explain the settings screen; that is a documentation problem, however good the documentation technically is.

Registration fails on a name mismatch. Still open. An AVSIM moderator's advice from July 2020: your name has to be in exactly the same format as the purchase order, initial versus full first name included, or the key is rejected. The installer also asks for a WideFS key you may not own — leave it blank, which the manual did not say.

Thirty-two buttons. Still open, by design. On 14 May 2026 a user with a BBI-64 button box found FSUIPC4 seeing only buttons 1–32. John Dowson's reply: "Native support for > 32 buttons is only available in FSUIPC6 and FSUIPC7." The workaround is a Lua script mapping physical presses to virtual buttons. If you are building a cockpit in 2026 with modern controller boards, that ceiling is a real constraint and it is never going to be lifted.

Twenty years of troubleshooting archive were deleted. Still open, and it is the worst of it. On 5 November 2025 John Dowson announced the new fsuipc.com site and warned that "the old support forums hosted by SimMarket will be closing at the end of the year (2025)". They did. forum.simflight.com now returns a bare "This forum is no longer available" page pointing at the shop. On 23 January 2026 a thread on the official MSFS forums recorded what that cost: the new forum did not carry the old posts, and years of discussion had disappeared. For a product last updated in 2021, whose remaining users are almost all troubleshooting rather than exploring, the searchable answer to their exact problem was frequently a 2013 simFlight thread. Those are gone. Dowson has said he is copying FAQ entries across, and some have appeared, but the FSUIPC4 board on the new site currently runs to two pages of topics.

And the marketing copy has not been touched in twenty years. The weather section of the product page still reads that the options are "not all fully useful yet due to FSX problems which will be resolved in due course. Please keep an eye on the Support Forum, where developments will be announced." Nothing will be resolved in due course. That forum is closed.

Performance and stability

The product itself is not where the instability lives. FSUIPC4 is a small, mature module with a 7MB memory footprint and no reported frame-rate cost in any source read here. What it does to stability is mostly positive: the free build has been credited on AVSIM since at least 2014 with reducing g3d.dll crashes in FSX, and the paid autosave feature turns the crashes it cannot prevent into a thirty-second reload.

The failure mode is environmental. FSX is a 2006 32-bit application, its SimConnect is delivered through a Windows side-by-side mechanism Microsoft no longer maintains, and both FSX and FSUIPC4 are now running on operating systems released fifteen years after either shipped. The reports that surface — SimConnect not registering on Windows 11, version-manifest mismatches on FSX:SE, service packs missing — are all handshakes between old software and new Windows. FSUIPC4 sits in the middle and gets blamed. Its own code has not needed a change since November 2021, which is either a sign of maturity or a sign that the platform stopped moving. In this case it is both.

Who should buy it

The FSX or FSX:SE cockpit builder with multiple devices — buy it. Yokes, quadrants, rudder pedals, GoFlight or Saitek panels, a networked second PC. This is what the product is for, it is the only thing that does it properly on this simulator, and the €24.99 is trivial next to what your hardware cost. Budget for WideFS7 at €24.00 if you are networking. Accept the 32-button ceiling before you buy.

The long-haul FSX flyer who hits out-of-memory errors — buy it, for autosave alone. Multiple independent reviewers say the same thing across a decade: it recovers the flight, the FMC state and the fuel. If a single salvaged eight-hour leg is worth €24.99 to you, it pays for itself once.

The Prepar3D v1–v3 holdout — buy it with your eyes open. It works. The developer has said in writing that it will not be supported on those versions going forward, and the store still sells you the compatibility anyway. That mismatch is on the listing, not on you, but you should know about it.

The FSX:SE flyer with one joystick who just wants their add-ons to work — do not buy it. Install the free unregistered build, which is all your add-ons need, take the g3d.dll crash reduction for nothing, and spend the €24.99 on scenery. This is the developer's own advice on his own store page, and it is right. If you later find yourself fighting throttle detents, come back.

The Prepar3D v4/v5/v6 user — do not buy this. Buy FSUIPC6 at €29.99. FSUIPC4 will not load into a 64-bit Prepar3D and your key will not carry over.

The MSFS 2020 or 2024 user — do not buy this. Buy FSUIPC7 at €29.99. The simMarket listing tells you this at the top of the page; believe it.

Where it stands today

FSUIPC4 has not changed since 24 November 2021 and it is not going to. The simulator it serves was finished by Microsoft in 2009; Dovetail's Steam re-release, which is what most remaining users run, averages under 400 concurrent players and is currently discounted to $4.99. Judged as a 2026 software purchase, a €24.99 utility for that platform looks indefensible on the face of it.

It is not indefensible, though, and the reason is that "frozen" and "abandoned" are different words. The code is finished because the target stopped moving, not because anyone walked away. The support promise made by a man who retired in 2019 is being honoured by his son in May 2026, on threads about a build from 2021, for free. That is rarer in this hobby than any feature. The store record — nineteen ratings, nineteen of them five stars, running from 2016 to three months ago — is not the record of a product coasting on nostalgia.

What has genuinely degraded is everything around it. The twenty-year simFlight archive that answered most FSUIPC4 questions was deleted at the end of 2025, and the replacement board has two pages of threads. The product page still promises fixes "in due course" and points at a forum that no longer exists. Modern button boxes hit a ceiling FSUIPC4 will never raise. A new buyer in 2026 is walking into a smaller, quieter room than the one all those five-star reviews were written in.

So: the score reflects a product that does its job superbly for a shrinking number of people who need exactly it, sold at a price set for a market that has moved on, with a free version that covers most of the room. If you are the cockpit builder, this is a 4.6 and you should have bought it already. If you are anyone else on FSX, it is a 2.5 and the free download is the answer. The headline figure is the honest average of two very different reviews, which is what happens when a twenty-year-old utility outlives its simulator and keeps selling anyway.