Skydrol aerospace hydraulic fluid is fire resistant and retains a low viscosity at the very low temperatures at altitudes of 10,000 metres and higher. Both are big things for aviation and aerospace. It is used in aircraft hydraulic systems in place of hydraulic mineral oils. For example, the braking system and hydraulic actuators in the undercarriage will use Skydrol fluid.
“Skydrol” is the trade name for a phosphate ester hydraulic fluid used on aircraft. Douglas developed it in the 1940’s, Monsanto in the USA manufactured it for years and more recently Eastman took the product into their portfolio. If you see some, it is a thin purple oil. The quart cans are quite handy. We buy it from Silmid or LAS Aerospace.
Over the years, there have been various versions of the product. B500 has been around for years, but LD4, Skydrol 5 and Exxon’s Hy Jet 4 are more recent. The different versions are used on specific aircraft systems and you would refill with the correct grade. That said, in principle the grades can be mixed (so our research would have you believe), so a top up pump would not need all its seals changed to move from one grade to another.
Related: Skydrol Hand Pumps
What questions do users ask about skydrol?
Much as it has some big pluses, it isn’t easy to work with. Read our guide on the good, the bad and the ugly of working with Skydrol. What have we learnt about working with Skydrol and making Skydrol hydraulic systems over 30 years?
What materials and components are compatible with Skydrol?
This is what we have learnt and where we look for answers. Web links are at the end of the article.
What Seals are compatible with Skydrol? Elastomers are the big issue. Don’t get it wrong.
Nitrile, Buna-N, silicone, fluorosilicone and Viton fluorocarbon are totally unsuitable. They will swell up massively. PTFE is inert and suitable bearing in mind that it is suitable only for static seals. Peroxide cured EPDM is the material to use. EPDM is very widely available as sulphur cured for hot water and steam. Is it suitable for Skydrol? Strictly speaking, no. We had an issue years ago with the plasticiser leeching out of a sulphur cured seal and it halving in size. That is almost worse than a seal swelling up. Buy in the proper seals. We have always been told that EPDM bonded washers have the correct peroxide cure, and that 9 series SAE o-rings are likewise. As always, we are happy to be corrected. Using industrial quick disconnects fitted with EPDM seals for Skydrol? It is widely done and seems to work, although we are not sure whether they are peroxide cured. Aeroquip QD couplings from someone like R F Saywell in the UK are the best option, at a price!
Watch out for metric vs imperial seal sizes, as you can’t necessarily swap over the seals on European kit such as filters. Imperial sizes are a much better bet. One top tip. If you have fitted the correct EPDM seals to an assembly, hard stamp it “E” to identify the configuration. You need to avoid doubt on what has been changed.
Which is the best hose material for Skydrol?
Start with the correct Aeroquip or Parker hose material and completely avoid Nitrile and fluorocarbon (Du Pont “Viton”.) Alternately, PTFE with a stainless overbraid is a nice solution up to a certain pressure. Aeroquip and Titeflex are big suppliers on these for aerospace but do offer industrial ranges as well. For Aeroquip aerospace, Saywells in the UK are your go-to. Just like the quick disconnects, there is a gap on price between the “aerospace” parts and “industrial”. PTFE hoses with a stainless overbraid were traditionally limited on pressure. Aeroquip make high pressure Kevlar reinforced PTFE used in Aerospace. Titeflex also offer ptfe up to 400 bar. EPDM steel reinforced hoses are offered by Aeroquip up to 300 bar, although check whether they offer plated steel or stainless fittings. At higher pressures up to 700 bar, on the face of it a polyamide liner and sleeve should provide a safe hose material although we have yet to find a manufacturer who provides this combination on smaller bore hoses of -4 and -6 sizes. Let us know if you source them.
Which metals are compatible with skydrol and is skydrol corrosive?
Anodised alloy and stainless steel are no problem. Steel pumps like Dynex Rivett rotary piston pumps are widely used as well, so steel is ok. For hand pumps, we recommend our stainless Micropac pumps for skydrol duty, but can offer lower cost anodised alloy / stainless wetted parts as an alternative at lower cost. That said, we would always use stainless for a reservoir and stainless pipework. We have been advised in the past to always use a stainless reservoir on LD4. We are happy to receive feedback on this.
Which metal finishes are compatible with Skydrol? Beware.
As noted, anodised alloy is no problem. Aeroquip make their aerospace quick disconnects in anodised aluminium alloy. Hard chrome is also fine. We don’t know about electroless nickel, but we would not use bright zinc plating on steel for wetted parts. We had always been told that Skydrol strips plating. Maybe it does. Maybe not. We just use stainless or anodised alloy. One of the web materials data guides says “don’t incorporate zinc into the wetted parts.”
Which plastics are compatible with Skydrol? Which are the best paints for Skydrol?
Skydrol plastic and paint compatibility is a very difficult area. Many “mainstream” materials are simply rotted out very quickly. A “standard” polyurethane paint will be stripped in minutes. Epoxy will probably be softened and be no real use. And no, epoxy powder coat that is “sure to be OK” won’t from our experience. If you need paint, buy a proper paint system from Trimite or similar. Just standardise on one “house” colour and make people aware that other colours may well cost 300GBP once you have taken everything into account.
We have been great fans of laser engraved labels on black anodised alloy for some years. Plastic labels are not a good choice.
Pressure gauge faces, knobs, buttons and grips can be a challenge. Never use a pressure gauge with a plastic face as Skydrol will make it illegible. Suppliers like Badotherm who do screwed bezel 63mm gauges can supply a glass face which can be popped in place of plastic. Do clarify that it is a safety glass face that is offered to you. PVC Grips are a big worry, but at least cheap to change. There is no fix other than making a metal handle or using a compatible plastic knob or grip. Finally, electrical knobs or buttons are potentially at risk. It is more likely that they will be operated by somebody with Skydrol on their fingers and the surface becomes sticky. Just be careful when using this kit.
We would always pipe up a system in Swagelok or A-LOK stainless double ferrule fittings. If you do use bought in fittings, make sure there are not any awkward European o-rings in them that need upgrading. Awkward, only of you can’t get the sizes in peroxide cured EPDM, that is. We stick with Swagelok.
Level indicators seem to be a disaster. Your mainstream plastic indicators just will not work on Skydrol. You are into glass types, although sourcing o-rings can be yet another issue.
Some plastics are just softened in the long term. For example, homopolymer like Delrin is softened, so we are told but we have never seen it. Will your filter element that is fabricated using epoxy be suitable for Skydrol? Ask the supplier to be sure.
Finally, low pressure tubing. PVC seems to soften and swell very quickly. OK if you must for dispensing then throw it away. Nylon seems durable. If you check with material guides, “polyamide” or nylon is rated as a good polymer. Although we have tended to use stainless 316 filler caps for skydrol, we note that people like Elesa do mould their filler breathers in “polyamide”, so on the face of it, these should be suitable. The sealing washers and foam within them would need looking at, though. We have used PTFE tubing for sight level gauges on units rather than nylon, probably in the belief that this material is very inert.
How dangerous is Skydrol? Personal safety with Skydrol and that of other people.
We would say skydrol is hazardous enough to be very careful indeed.
Eye protection and Skydrol resistant rubber gloves are the order of the day. Your normal nitrile or latex gloves will disintegrate within minutes. You don’t want it in contact with skin for long. It is quite stubborn to scrub off, from our experience. Finally, get your eye wash to hand and make sure people have read the safety data sheet and know what to do if they did get some in their eye. It is horrible.
As noted, print out the material safety data sheet. You will need to risk assess whatever you are doing, as there is a genuine risk to yourself and other people if something goes wrong. Wise people would probably sort out a session on “working with skydrol” and get this competence on the Training Matrix. All this said, the fluid is not flammable, so that is a plus.
How do I handle Skydrol? Health and using Skydrol everyday in the shop.
After 30 years of using the fluid, what are our big gripes and worries?
- Skydrol will burn your skin and that of your colleagues. We are not medical folk, so can’t tell you what it is actually doing, but it does not feel too good.
- Clean those tools. We find that traces left on tools are as tricky as direct hazards like splashing on to the skin or into eyes. We suggest cleaning off all tools with solvent after you have been working on a Skydrol system. Any trace of the oil can so easily get on to fingers then into eyes.
- Careless pouring. Take care with any pouring or dispensing. You don’t want it left on bench tops or splashed on to the shop floor where it will simply strip off floor paint in seconds. This must be an issue in the aerospace industry, as customers moaned to us both about sourcing low cost dispensing pumps and also being unable to dispense direct from those 1 gallon and 5 gallon drums. Our Micropac MI series drum and can dispensers meet this need exactly. Alternatively, use our MP series filling, charging and test 2 wheel trolley units for safe and easy working in the hangar, test shop and on the tarmac.
- Finally, a big risk is a leak or burst from high pressure Skydrol. Check all fittings a second time and sign it off on the works order before pressurising. Hence our enthusiasm for double ferrule fittings if leaks are a pain. Put a polycarbonate / lexan guard in front of the job during testing.
Avoid inhaling any escape of a volume of air with Skydrol vapour in it. Watch out when pressure testing a reservoir or hose assembly that has even a trace of Skydrol in it.
Whenever we have specified an air driven pump unit for one of our Skydrol filling and pressure test trolley units, we have always used a distance piece between the air motor and hydraulic section. This will ensure that any fluid leak from the high pressure seal will not be blown out with exhaust air from the air motor.
Can I clean Skydrol?
The answer is yes using a hydraulic filter. The previous notes on materials will apply, so a skydrol cleaning rig will be quite specialist. Be careful to use compatible filter elements. MP Filtri would be our first port of call. We have found that dispensing direct from the Eastman drum or can provides cleaner fluid than dispensing from a larger tank. Micropac MK dispensers were designed to fit the one gallon or five gallon Eastman containers. Use a Micropac MB drum pump on the larger drum. Keep in mind the five year shelf life on fluid in cans / drums and the need to check for contamination / water ingress into a hydraulic system. If you are designing a system, you will be thinking about contamination control on Skydrol and also some easy means of sampling the Skydrol to take a small amount for cleanliness testing. We normally tee in a Swagelok bleed valve or little two way ball valve.
Safe working and how Micropac pumps can provide an easy solution.
Whether you need a Skydrol drum pump, are moving volumes of fluid using a Skydrol Transfer Pump, or are simply filling, charging or pressure testing, Micropac pumps were designed to provide well engineered solutions.
- Modular pump design allows you to specify exactly what you want
- Designed for Skydrol on day one, so easy seal supply. No long lead times
- Easily serviceable
Typical Skydrol solutions using Sarum Hydraulics Micropac pumps
Is there an alternative to Skydrol?
There are other similar phosphate esters like Hi-Jet which have the same requirements. Fifty years on, there might well be an alternative fluid that could be used. However, the fluid is well entrenched so unlikely to change in a hurry. All we would say is if you have Skydrol kit, label it with one of our suggested rot-proof alloy labels. You cannot swap fluids from Skydrol to DTD585 and back again without changing all the seals. “Oh, we can flush it” does not work.
Go to somebody who has the know-how. Our Micropac filler and test units are proper kit for filling and pressure testing on Skydrol. Our pumps are portable hand pump or hand / air driven units either for carrying around or on MP series two wheel trolleys. We also make our custom dispensers, drum pumps and transfer pumps. They are at home in the shop, hangar or on the apron.
Look what we do on www.sarum-hydraulics.co.uk.
Links to “skydrol” and other phosphate ester manufacturers.
Links to people who will sell you a quart, 5 gallon or larger can.
Web resources on suitable materials for skydrol.
Elastomers. Most of the big seal manufacturers will identify that you need peroxide cured epdm. The issue is finding it in stock.
Note that DMR seals will machine you a seal in peroxide cured epdm, which we have found an invaluable service. Useful people.
Polymers suitable for skydrol.
Not much luck here on a comprehensive guide. Eastman tell us in their handbook that anodised aluminium is compatible and also that zinc should not be used in skydrol hydraulic systems. We would review plating finishes case by case on a new system.
Paints and finishes
Hoses and somebody who will sell you one.
Note that having a material in a catalogue and being able to supply a hose is not the same. The majors now seem to offer long lead time and a high MOQ if your own hose supplier doesn’t have the material in stock.
Our own Micropac skydrol pressure test, charging and dispensing pumps
Our preferred manufacturer of high pressure piston pumps for skydrol
Our preferred double ferrule fittings offering high integrity in use on high pressure skydrol