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A normal aircraft has multiple hydraulic circuit, identified by colors (e.g. according to the ATSB final report of the QF32 the green hydraulic line was disabled; according to the BEA final report of the AF4590 the Concorde had a yellow, a blue and a green circuit, ...). I understand the need to be equipped of several independant hydraulic system (redundancy) and the need of being able to identify those.

Thanks to the previous example, I figured out the hydraulic circuits are identify by colors. Does it mean the hydraulic fluid is colored (useful in case of leak)? Does it means the elements of each circuit is painted accordingly (usefull for maintenance)?

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  • $\begingroup$ Only some manufacturers label hydraulic systems by colours. Airbus names them Green, Yellow and Blue, but Boeing usually names them A, B and Standby. $\endgroup$ – Jan Hudec Sep 17 '14 at 18:03
  • $\begingroup$ and at least some of the Dassault aircraft number the systems (1, 2, etc.)... $\endgroup$ – Lnafziger Dec 14 '15 at 21:55
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A common hydraulic fluid used in aircraft is called Skydrol. The fluid is artificially colored, but this does not depend on which circuit it is in. The FAQ on the manufacturer's site addresses the color.

Skydrol fluids are given a purple dye to make it easy to distinguish them from other fluids. But the dye does not usually last as long as the fluid does. The color may change from purple to gray or yellow or green, yet the fluid can still meet all used fluid specifications. Color is not a reliable indicator of fluid quality, and we recommend that the system be sampled for chemical analysis to determine if it is suitable for use. A dark amber color, however, is a cause for concern, and can indicate a severely stressed fluid. Call Solutia's Technical Service for assistance if you are unsure about a fluid's color. Our technical bulletin has a color chart to use as a guide.

Initially the color helps to distinguish from other fluids, and having multiple colors would make it more complicated. It notes that the color also does not remain constant, so even if it were colored initially, it would probably end up changing anyway. It also mentions that a dark amber color can be an indicator of issues.

I can't find any good pictures, but the lines are probably not color coded. Any colored hydraulic system components are generally purple (like the fluid). The lines themselves can be labeled, but the standard for hydraulic lines is blue and yellow. The end fittings are colored to identify the tube material type.

The maintenance manuals will show the layout and label the different lines. Also, on this ASRS page, one report is from a mechanic that confused the green and yellow system fittings, which probably wouldn't have happened if they were color coded.

Jar of Skydrol showing purple color

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It is just the components that will be labeled accordingly.

Having each fluid be colored differently is a maintenance nightmare, you need to have all 3 colors available in the hangar and each system must be fully isolated (3 different pumps, buffers, actuators etc.) to avoid contamination.

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    $\begingroup$ That would actually be an excellent idea. The plane's systems are separate, maintenance can just stock the clear fluid and some jars of dye. Hydraulic fluid is naturally a clear oil, the common red colour is already an additive, and a top-up wouldn't need any dye added (just mixes in with the rest). $\endgroup$ – paul Sep 17 '14 at 10:47
  • $\begingroup$ top-up would dilute the dye, and wouldn't solve the weight issue for the fully separate systems $\endgroup$ – ratchet freak Sep 17 '14 at 10:54
  • $\begingroup$ I think this is a good idea to use colors as labels. This could have been used to visually identify either the fluid itself or the several components (pumps, actuators, pipes) $\endgroup$ – Manu H Sep 17 '14 at 11:38
  • $\begingroup$ @ratchetfreak a fluid top-up won't make red/ green/ blue dye fade out to an indistinguishable colour. If it does, then it's more than a top-up. $\endgroup$ – paul Sep 17 '14 at 12:59
  • $\begingroup$ Having 3 different fluids in maintenance may be inconvenient, but it does eliminate a common failure mode (contamination at the source). $\endgroup$ – MSalters Jul 20 '17 at 13:23
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The colour codes of hydraulic systems referred to, common for 1960's-1970's British Jets, and since then all Airbus aircraft, do not refer to the colour of the fluid!

It is just an identification system for each separate hydraulic branch, the fluids themselves have nothing to do with 'green, yellow, blue' system identifications.

It should also be noted that the American convention is not to use colour codes, on Boeings they typically refer to 'left, right and center' systems.

In terms of the fluid itself, the original question mentioned Concorde. Concorde used Chevron M2V 'Oronite' fluid, which is silicate ester and is a light straw colour.

Almost all civilian airliners used one of the main fire-resistant phosphate ester fluids, either Skydrol or Hyjet - phosphate ester is also naturally a light straw colour, but as specified it is artificially dyed purple for identification. However, phosphate ester aviation hydraulic fluid changes colour when overheated, getting darker, turning from purple to dark purple to grey, to black in extreme cases - or if it leaks and becomes oxidised it turns back to the normal straw yellow colour.

Final mention is for many business jets(Gulfstreams and Citation X excepted) and military aircraft, they use 'red oil' hydraulic fluids, either Mil-5606, 87257 or 83282. They again are probably a straw colour to begin with, but are artificially dyed red per their relevant specifications.

You can generally mix the different red oils, or you can mix the different phosphate esters, but never ever mix red oil and phosphate ester - they have different seal swells etc.

Getting back to the original question: you don't use fluid colour to identify the system, you use the system schematics and the tube/hose identification labels. The colours 'blue/green/yellow' are primarily for illustrative purposes in the pilot operating manual and system descriptions - go to smart cockpit dot com for exmaples.

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  • $\begingroup$ Your answer is quite good and may be enhanced with references and links for further readings. $\endgroup$ – Manu H Jul 22 '17 at 13:12

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