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In this answer, Ralph wrote:

Spare fluid is common enough to have on-board to replenish the resevoirs.

After reading that, I recalled an incident I was told about nearly twenty years ago about a hydraulic failure on an Aeroflot flight, where Lemonade (a Russian soft drink of sorts) was used in a pinch. I've always wondered about the authenticity of this story.

Is it true? Is it plausible? Have there been other times where some non-ideal fluid was used in place of hydraulic fluid?

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Related: Russian Plane Comes In On A Wing And Lemonade – mins Feb 22 at 22:07
@mins Thanks for that! I didn't even think to do a search. I wonder if it was real or just a colorful addition to the story... – Steve Feb 23 at 0:55
Apart from the Lemonade example, what do you mean by non-hydraulic fluid? Compressible fluid, or fluid other than standard hydraulic fluids such as 5606 or Skydrol? – Jonathan Walters Feb 23 at 2:19
Please note that the lemonade was used only for very short time and on very low altitude - from discovery that landing gear won't extend, they took only as much time to circle as it was necessary to pour the lemonade in, and then landed immediately. So most of the extreme conditions that regular fluid has to endure (freezing temp, low pressure) didn't apply here. – Agent_L Feb 23 at 8:52
@JonathanWalters I mean anything ... I honestly don't know much about hydraulics. – Steve Feb 23 at 12:55
up vote 13 down vote accepted

Is it possible? Sure. Theoretically. Once.

I can't speak to your Aeroflot scenario without more details, but broadly speaking any liquid will work as "hydraulic fluid" within certain limits: At the most basic level all that is required is that it be incompressible and flow at an acceptable rate (i.e. It is a liquid with a viscosity that falls within a certain range). Engine oil is frequently used as a hydraulic fluid for certain applications (constant-speed propellers on small aircraft), and special-purpose hydraulic oil for others (the brakes on small aircraft).

There are some challenges to using any old liquid as "hydraulic fluid" though, which are why I say this is only theoretically possible (and probably only once). The two big concerns are the fluid's operating temperature range - its freezing and boiling points - and its corrosion potential.

Let's consider MIL-PRF-5606H (a typical hydraulic oil) and ordinary lemonade as examples:

Operating Temperature Range

In terms of temperature 5606 hydraulic oil has a much wider operating range than lemonade: It's "pour point" is below -60°C and its boiling point over 175°C, while lemonade's operating temperature range is probably close to water: It will freeze somewhere close to 0°C and boil somewhere close to 100°C (the exact numbers will depend on the chemical composition of the lemonade).
If we use both of these fluids in a brake system there is a chance the lemonade will boil when the brakes are applied (creating a gas pocket that's compressible, and reducing brake effectiveness), or that it will freeze in flight and the brakes will fail to apply because the ice is blocking the lines.

Corrosion Potential

As far as corrosion potential 5606 hydraulic oil is a mineral oil with some red dye in it: Since it doesn't contain dissolved water or electrolytes it's generally non-corrosive. Lemonade on the other hand is nothing but water and dissolved electrolytes (mainly citric acid to make it taste like lemon) plus sugar and a few other things. If we ran hydraulic actuators on this stuff we should expect them to corrode (rust) and eventually fail.

So it's possible that under the right circumstances an airplane could complete a flight with lemonade, water, etc. as "hydraulic fluid", but having done so the maintenance department will have to purge the "creative" fluid from the system, and probably follow some steps to remove any corrosive residue from the system (possibly up to and including replacing major system components). Long term operation in such a condition would probably damage components to the point where many of the system's moving parts (actuators, valves, etc.) would need to be replaced.

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In short, yes!

The thing is, a fluid is fluid. Any fluid that is similar to water or oil can be used. But, you would need to be on the ground and have enough of that fluid. Also, using such a fluid will introduce potential damage and corrosion to the system, so a flush should be done as soon as possible.

In addition, there might be hydraulic systems that experience higher temperatures, like brakes - in that case, regular water would be a bad idea or anything water based.

Lemonade is a a carbonated drink, and using carbonated drink as a hydraulic fluid is a bad as it would emit a lot of gas, and has significantly lower boiling point. So I would speculate that if they did use it, they probably let it vent out all the CO2 first.

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Lemonade is not typically carbonated... at least not in the US. – Ron Beyer Feb 22 at 22:12
Lemonade is a brand of Carbonated drink in Russia, so I assume that since it's Aeroflot, it's carbonated lemonade ;) – Alexus Feb 22 at 22:18
Some (many?) aircraft do have hydraulic fluid reservoirs for their hydraulically actuated landing gear systems that can be checked and replenished in the air. – Jonathan Walters Feb 23 at 2:15

In LR-JET type airplanes Learjet uses an emergency air bottle in the event hydraulic fluid is lost. The emergency air is pressurized between 1800 and 3000. When the bottle is opened to extend the gear or apply pressure to the brakes, the air will move shuttle valves causing air pressure to extend the gear or stop the airplane.

I know it is not the same as replenishing it with some soft drink but it does answer the question of whether or not something like this is possible.

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I know a few military airplanes that have a similar system of emergency gear extension. But I guess it would be much harder to keep, say, control surfaces operational in this manner. (On the other hand, on non-FBW aircraft this may not be necessary). – Zeus Feb 23 at 5:38
1800 and 3000 what ? en.wikipedia.org/wiki/Pressure#Units – kebs Feb 23 at 15:06
1800 o'clock and 3000 ft AGL ;) – Piskvor Feb 23 at 15:41

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