The International Cessna^® 170 Association

If you guys were as old as me,maybe you could remember when a lot of the unpowered hydraulic brake systems such as were in the twin Beech AT-7 and AT-11, and other smaller airplanes, used a vegetable based, "Green" as opposed to petroleum "Red", hydraulic fluid. I believe it was because natural rubber seals and other parts were used in the brake systems. I can remember that if it was exposed to the air it left a gummy residue that could only be cleaned up with alcohol type solvents. Petroleum solvents only aggravated the gumminess.

I sort of remember Skydrol also being used in the cabin pressurization system on the DC-6 since it did not have the flamability of 5606.

Skydrol is advantageous in very cold temperatures as it does not freeze like H-5606 does. It's drawback is that it requires specialized materials as it is very corrosive. Spill some in the belly and it's a real clean-up issue. Mil-H-5606 is easier to design for but will burn, since it's petroleum oil. (It doesn't "explode" however, unless like any other petroleum oil, it is atomized or vaporized.)

I'll check to see where we get our 5606, we ( Continental ) get our 5606 in plastic quart bottles to put pump sprayers on them, even the latest and greatest jet aeroliner still needs their struts wiped down every day with 5606. Oh and shock struts (main and nose landing gear) still use them too. As far as skydrol goes, I can speak for all aircraft techs that deal with it, the world would be a better place without it, its nasty smell and severe irritant will literally leave a bad taste in your mouth. And you will be hard pressed not to find a Boeing product with a puddle or two of skydrol underneath after it parks for a overnight.

Just checked at work, we have 1 quart bottles from royal lubricants. We also have the castrol aero 40, its a mil-prf5606 which is straw color per Boeing spec, Douglas is red.
Happy New Year.

Hawkers have two 5606 systems ...one is an ambient pressure in the cockpit (just like our 170's) which operates the high-pressure (boosted) valves for the wheels. The low press system leaves the factory with the straw colored stuff and the wheels and main hydraulics use the red stuff. It is supposed to help diagnosis any leaks at the main brake valves so the technician can see which system is leaking. But eventually both systems end up with the red stuff in them and it makes no difference.

When I installed a Teleflex SeaStar hydraulic steering system in my boat I had to order 4 quarts of genuine SeaStar steering hydraulic fluid at $32/quart. When the stuff arrived, I noticed on the little plastic qt. bottles it was labelled as meeting Mil-5606. I could have bought a gallon of that stuff for $16 at the airport, and already had 2 gallons in the hangar!

A couple of years later, when a screw had chafed a hole in the system and dumped all the fluid while "at sea"..... I repaired the line while afloat and filled the system with what was available.......ordinary, El-Cheapo brand Type A, Suffix A auto tranny fluid. It's been there now for 12 years and I've never replaced it. Works just fine. (But I'd not try it in the airplane.)

My mechanic tells me that automatic transmission fluid is the same as 5606, but with an anti-foaming agent added. (I don't use it in airplanes.)

I strongly suspect that MMO is the same with perfume added.