Match the plane to the pilot & mission

[Big Pistons Forever]

Originally Posted by AdamFrisch

The 680FLP is essentially the same airframe as the later 685 and Turbine models. However, there is a big difference in the way the 680FL/FLP achieves pressurisation compared to the later 685. In the 685 the pressurisation is taken from the bleed air from the turbos, just like on a turbine, essentially. On the 680FL/FLP they're driven by a 3000psi hydraulic cabin pump/supercharger made by New York Air Brake. It's almost impossible to find replacement parts for this pump should it break. Not only that, it doesn't use red hydraulic fluid, but a nasty and expensive compound called Skydrol. This stuff will strip the paint off your plane and is a general nightmare to deal with.

Also, the 680FLP only carries 225gal of fuel as compared to the 322gals of the 685, which makes it a relative short legged long range tourer.
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I believe that most Mr RPM conversions removed the pressurization pump and replaced it with a turbocharger bleed air pressurization source like the one in the Aero Commander 685 and all Cessna pressurized piston aircraft. You are right about the fuel though.