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Most aircraft hydraulic braking systems are designed for mineral oil. This is NOT compatible with automotive brake fluid. The norm is MIL-spec hydraulic fluid MIL-H-5606; any aircraft maintenance organisation will have some. Auto transmission fluid is a reasonable substitute if you do not operate in extremely cold conditions.

 

 

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Most aircraft hydraulic braking systems are designed for mineral oil. This is NOT compatible with automotive brake fluid. The norm is MIL-spec hydraulic fluid MIL-H-5606; any aircraft maintenance organisation will have some. Auto transmission fluid is a reasonable substitute if you do not operate in extremely cold conditions.

I wonder why 'regular' brake fluid isn't used in aircraft brake systems?

 

 

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Mil-H-5606 stays liquid down to around minus 50 C - which means aircraft hydraulic systems remain functional at high altitude (seems like a good idea, don't you think?). Since undercarriages tend to take a while to warm up after cruising at altitude, I suppose it seemed like a good idea for them, too. As a fringe benefit, you don't tend to get the sort of local pitting corrosion that plagues automotive brakes. I think automotive systems started using the glycol/alchohol type brake fluid (to replace castor oil) because it allowed the continued use of natural rubber brake cylinder cups - whereas aircraft went to synthetic rubber (Buna-N - Neoprene) about the time they stopped using castor oil as hydraulic fluid, in the 1940s. The practice has continued to this day - tho synthetic rubber is now cheaper than natural rubber - in fact, I expect brake cups are nowadays mostly made from nitrile rubber. However if you want the metal parts of the brake system to last indefinitely, mineral-base brake fluid is definitely a step in the right direction.

 

Some car manufacturers have seen the light - Citroen for one; they changed to LHM (liquide huile minerale) long ago - about the time they introduced the DS series, in fact. Their brake calipers just don't give trouble, in my experience. However, most car manufacturers are interested in planned obsolescence, so brake parts that corrode out are probably viewed favourably.

 

I use "regular" brake fluid as a mild paint stripper, to remove the decorative paint trim from my aircraft without damaging the polyurethane anti-corrosion finish; it's great for that purpose. Not much good for anything else, IMHO.

 

 

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From the installation instructions which came with my Matco's, you ONLY use hydraulic oil. The cylinder and caliper seals are not compatible with auto brake fluid which also is hydroscopic (absorbs atmospheric moisture). The mil spec oil I purchased came in a sealed quart can, but looks and feels much like the red auto trans fluid, eg Dexron. III.

 

 

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A bit off topic, but if stuck just use plain water.Worked for me for a few times in the bush years ago after damaging brake lines.

Phil.

That may be OK for a temporary fix but still risky. Water, at sea level, boils at 100 degrees. When it turns to gas, it won't be of any use to apply your brakes. Brake fluid has a much higher boiling point.

 

 

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A bit off topic, but if stuck just use plain water.Worked for me for a few times in the bush years ago after damaging brake lines.

Phil.

Works fine for a brief fix, and in an aircraft where the brakes usually don't get the same use as a car it would be good to get home, I remember as a first year apprentice being told by a tradesman that hot water works best,,,,,but it has to be hot ,once it cools it's no good ,,,,,,,

Matty

 

 

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Probably a light mineral oil would do for a short while but you would have to replace all rubber components after wards. Did you say use PLANE water? PLAIN water comes from the plains.... NO? Nev

 

 

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Probably a light mineral oil would do for a short while but you would have to replace all rubber components after wards. Did you say use PLANE water? PLAIN water comes from the plains.... NO? Nev

How about "Raw water"?

 

Seriously, I have done this a couple of times to get out of trouble.

 

The water doesn't seem to damage any components in the short term.

 

Phil.

 

 

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Probably less damaging from the servicing point (the water) but the water wouldn't give a lot of braking capability once it got hot. People used to use metho and castor oil mixed way back and it doesn't hurt rubber. Still wouldn't be good enough for disc brakes the temps they reach. Alcohol will flush out the water drops, afterwards, when servicing.. Silicone type works for ever but some systems that require a better lubricant don't recommend it..Nev

 

 

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Probably less damaging from the servicing point (the water) but the water wouldn't give a lot of braking capability once it got hot. People used to use metho and castor oil mixed way back and it doesn't hurt rubber. Still wouldn't be good enough for disc brakes the temps they reach. Alcohol will flush out the water drops, afterwards, when servicing.. Silicone type works for ever but some systems that require a better lubricant don't recommend it..Nev

Nowadays, you have to be more specific as to what you mean by "rubber"; there are at least a dozen rubber-like substances in use for elastomeric components, with totally different chemical basis. I assume you mean "natural rubber" (as from rubber trees); however that has mostly been replaced by nitrile rubber, except in rubber bands and speargun rubbers and shock cord. Other common forms include Neoprene, polyether, silicone, polyurethane, viton, etc. Each of these works with some fluids but not with others. I'm no expert on this, but it's a minefield unless you know what you're doing.

 

 

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Yes, that's true. I'm not using rubber in the generic sense. Neoprene is a trade name too . Once a rubber (there we go again) tyre has had oil left on it for any time I would consider it unserviceable. It generally swells and softens. Ozone damages many of these materials too. (Electrical discharges).. nev

 

 

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