Pratt stacks. Know this shape? Sure you do. It’s the distinctive exhaust stack of Pratt & Whitney Canada’s PT6A series of turboprop engines. Created in 1961, and with total sales of some 37,000 engines, it’s one of the most popular in the world. PT6s use a reverse-flow design—meaning that intake air enters from the rear of the engine, then flows forward, passing through gas-generator and power turbine sections, where the latter can turn at speeds of 30,000 rpm. From there, a reduction gearbox turns the propeller at more modest speeds—say, 1,900 rpm. In keeping with the “backwards” flow pattern, engine exhaust exits near the front of the engine—and out those stacks. This causes another unique feature of PT6s—their propensity to put heavy soot deposits on nacelles and fuselages. Stock stacks from the factory are often replaced with chromed beauties like the one shown above, made by American Aviation. Frakes Aviation makes a similar replacement stack. Both companies promise less soot, more speed, and even a bit of extra jet thrust. But let’s face it. It’s all about seeing your admiring reflection in this bit of modern art. That, and all the polishing it takes to keep them bright.