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 & Whitney PW4084) or the Lockheed Martin F-35 Lightning II (Pratt & 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,
Are there specific reasons for not using designated hydraulic fluids such as Skydrol for all hydraulic applications?