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Skydrol VS. HyJet?

Wed Dec 04, 2013 7:53 pm

I have asked a similar question before, but not quite the same.

I was reading on the Skydrol cans that it should "ONLY BE USED WHEN AIRCRAFT OR HYDRAULIC SYSTEM IS FITTED FOR SKYDROL." So, what happens if one airline uses Skydrol on their aircraft, and that aircraft is sold to an airline that uses HyJet on their aircraft? Do they have to refit the Hydraulic system?, or do they just flush the system, and add their choice fluid?
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RE: Skydrol VS. HyJet?

Wed Dec 04, 2013 8:39 pm

I think they are completely interchangeable. When DL merged with NW each had separate fluids for the first few months. Then we switched all the DL planes to HyJet like they used at NW. I don't recall anything but topping off the systems for the change. I think the warning is for non ester based hydraulic systems, ie 5606. The o rings and seals in those systems will not react well with Skydrol or Hyjet.
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RE: Skydrol VS. HyJet?

Thu Dec 05, 2013 5:00 am

dalmd88, which aircraft use non-ester based Hydraulic systems? I know that AirTran uses (or at least in 2006 used) Skydrol. IIRC, WN uses HyJet, and if DL uses HyJet, does that mean they will soon replace the Skydrol with HyJet in the 717s. Also, FL used ONLY Exxon (Later BP) Turbo Oil 2197 in the BR715s. I don't know if DL will continue with that, or will they use Aeroshell Turbine Oil 560?
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RE: Skydrol VS. HyJet?

Thu Dec 05, 2013 10:10 am

Quoting Grisee08 (Reply 2):
dalmd88, which aircraft use non-ester based Hydraulic systems?

Most GA aircraft use MIL-H-5606, the old red hydraulic oil. The PC-12 uses 5606 for the landing gear, the only system powered by hydraulics, and use MIL-H-83282 for the brakes.
139, 306, 319, 320, 321, 332, 34A, AN2, AT4, AT5, AT7, 733, 735, 73G, 738, 739, 146, AR1, BH2, CN1, CR2, DH1, DH3, DH4,
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RE: Skydrol VS. HyJet?

Thu Dec 05, 2013 3:39 pm

You definitely cannot mix hydrocarbon based hydraulic fluid (IE 5606, 83282, 87257) with Skydrol, and the reason is the seals. The ester-based fluids like Skydrol require ethylene propylene rubber, for example, so they don't break down and disintegrate. Hydrocarbon based oils need buna-N (and suitable sub) and other natural rubber type o-rings to operate correctly without breakdown of the material. I have a TO that states that 5606, 83282 and 87257 hydraulic fluids are totally miscible and can be mixed together in small quantities without adverse effect to the operating system, whereas ester-based fluids cannot be mixed with non ester-based fluids under any circumstances. If you want, I can check the same TO to see what it says about mixing ester-based fluids with other ester-based fluids to see if that would be acceptable.
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RE: Skydrol VS. HyJet?

Thu Dec 05, 2013 10:54 pm

Might sound obvious that it's a bad idea to mix phosphate esters and red oils, but I worked on an engine testbed project for a prominent french bizjet manufacturer where they wanted to do exactly that. Needless to say I warned them strongly against it. Never heard what the outcome was...

As for Hyjet and skydrol, they're the same thing, just different brand names for the same fluid spec, AS1241.
All the phosphate ester types are intermiscible unless stated otherwise for the specific application.

If you're really interested in aircraft hydraulic systems, AIR5005 and AIR1899 are great specs to read if you can get a hold of them, they have schematics and specs on just about everything flying, civil and military.

[Edited 2013-12-05 15:08:20]
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