Why is fuel used as a hydraulic fluid? Why is fuel used in some servo-systems as a hydraulic fluid (fueldraulic systems) of modern civil and military aircraft such as the Boeing 777-200/-300 (Pratt &amp; Whitney PW4084) or the Lockheed Martin F-35 Lightning II (Pratt &amp; Whitney F135)?
Examples for fueldraulics are the actuation of


Fuel Valves, Start Bleed Valves, Cooling Valves,
Variable (Inlet) Guide Vanes (VGV/VIGV) / Variable Stator Vanes (VSV) for the compressor, and
Thrust Vectoring Exhaust Nozzles,


see for example Eder, M.: "Engine Fuel System Design Issues", Aviation Fuels with Improved Fire Safety, A Proceedings, 1997.

Here is a sketch of a fueldraulic system actuating the Bleed Valve and the VIGVs for a compressor, taken from "Rolls-Royce plc: The Jet Engine, 5th Edition 1996, Derby, England, ISBN 0902121235":


Are there specific reasons for not using designated hydraulic fluids such as Skydrol for all hydraulic applications?
The primary reason is availability -- the high pressure fuel pump is running whenever the engine is, while the airframe hydraulic systems may not be available in case of an accessory gearbox failure, hydraulic leak, or maintenance shutoff.

Also, this keeps hydraulic lines from needing to be plumbed deep within the bowels of the engine -- this has benefits in that the hydraulic system is confined to the accessory gearbox fire zone (where the engine driven pump lives), instead of having to run in several engine fire zones.