What are the differences between the proportional flow control FCU and pressure control FCU?
The pressure control system is a typical system as fitted to a turbo-propeller engine where the rate of engine acceleration is restricted by a propeller speed controller

The proportional flow control is a typical system where H.P. compressor shaft r.p.m. is governed by a hydro-mechanical governor which uses hydraulic pressure proportional to engine speed as its controlling parameter

Therefore the pressure control system is one step ahead as it looks at the tendance of speed, of course the pressure control system alone is not sufficient because even if the rate of speed increase is not excessive, the final speed could become excessive, that’s why also a speed limiter is included in the system

Today’s FADECs directly protect the engines for excessive speed and for excessive speed rate

Please refer to to the following Website

http://aeromodelbasic.blogspot.com/2012/01/fuel-control-system.html

Also please refer to the following website

http://aeromodelbasic.blogspot.com/2012/02/proportional-flow-control-system.html