I’ve been testing the Turbo Arrow III, and it appears to suffer from the same flaw in fuel mixture logic as other turbocharged piston planes in the sim right now. In real-life, a turbocharged engine can maintain sea-level performance all the way to the critical altitude for the turbocharger. It is not necessary to lean the mixture with altitude in a turbocharged engine. But in the Turbo Arrow III, if I leave the mixture at full-rich, fuel flow begins to drop above approximately 6,000 ft, and by 10,000 ft, the engine has lost so much power that it can hardly climb. Leaning the mixture at that point results in much higher fuel flow and more realistic engine power.
The same behavior exists in other add-on turbocharged airplanes in the sim (the Seneca V from Carenado and the Turbo Bonanza mod by Robert Young). To my knowledge, each of those add-ons is using the sim’s core engine tuning, rather than implementing their own engine model from scratch. As such, it appears to be a problem with the core logic for mixture in turbocharged engines for MSFS. Just Flight recently confirmed this in a post on their own community forum. I’ve created a bug report describing the problem: