What is Skydrol oil if this is called up on the specification or user manual? 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?

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.

What is special about it?

It is the main oil used in Aerospace Service Hydraulics. The hydraulic fluid is fire resistant and retains a low viscosity at the very low temperatures up at 10,000 metres. 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.

What do users need to know?

Much as it has some big pluses, it isn’t easy to work with.

Materials and the engineering of the job- what about Skydrol compatibility? 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!

Finally on elastomers, don’t go out on a limb and think you can necessarily easily swap out the nitrile seals on a European filter. Peroxide cured metric o-rings can be a nightmare. Imperial sizes are a much better bet. We have not heard of Sealjet offering a peroxide cured EPDM material, but DMR Seals can offer a good Skydrol safe peroxide cured EPDM with their machined seal process. Again, feel free to correct us. If you have fitted the correct EPDM seals to an assembly, hard stamp it “E” or similar to identify configuration. Not knowing what you have changed or risking a leak is really unacceptable, quality wise.

On hoses, buy in the correct Aeroquip or Parker hose material. Nitrile is a total no no. Alternatively, PTFE with a stainless overbraid is a nice solution up to a certain pressure. Aeroquip and Titeflex are big on these for aerospace, offering industrial ranges as well. Funnily enough, some of the motorsport people are good sources of more specialist hoses. We like AN Motorsport for Titeflex PTFE/stainless overbraid hose. Saywells are your UK people on Aeroquip aerospace hoses.  Just like the quick disconnects, there is a gap on price between the aerospace parts and “industrial”, although on hoses it fortunately is not so large. If you are specifying parts for GSE, you really do need to be very careful with exactly what you are buying.


Micropac Skydrol drum pump for long service life and controlled dispensing.

Which metals? 

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 units 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.

Finishes on metals. Beware.

As noted, anodised alloy is no problem. Aeroquip make their aerospace quick disconnects in 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.

Plastics and paints. A lot of mainstream hydraulic bits are useless.

This is a very difficult area. Some 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. You are stuck buying in a proper paint system from Trimite or similar. Just standardise on one colour and make people aware that other colours may well cost 300GBP minimum charge by the time you have bought in this and that.

We have been great fans of laser engraved labels on black anodised alloy for some years. Plastic labels are not a good call.

Pressure gauge faces, knobs, buttons and grips can be bad news. 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. PVC Grips are a big worry, but at least cheap to change. There is no fix other than making a plastic handle rip or using a harder plastic knob. 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 nasty European o-rings in them that need upgrading. 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, but in the longer term, we are not sure. We have used PTFE tubing for sight level gauges on units rather than nylon.

Personal safety with Skydrol and that of other people.

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, we have found. 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.


Micropac Skydrol dispensing and pressure testing pump on 20 litre two wheel trolley with hose stowage. Use at home in the shop, inside the hangar or on the apron.

Health and using Skydrol everyday in the shop. Product handling musts.

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. Go easy 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.
  • 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.

Equally worrying is any escape of a volume of air with Skydrol vapour in it. Inhaling this stuff cannot be good. It is horrible. Pressure testing a reservoir or hose assembly that has even a trace of Skydrol in it is something to be very, very careful about.

In passing, 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. Again, Skydrol laden air in the shop is very bad news.

Skydrol needs to be clean Skydrol. None of us want to get on a flight relying on dirty hydraulics. 

You will need to ensure fluid is clean and free from contamination. The quart cans are useful because you only open what you need. Many users use filtration either on the outlet of the pump or in the line from the pump. You can also rely on stainless systems not corroding. 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 are dispensing Skydrol from a drum, filling or charging systems like brakes 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

What a lot of hassle! Anyway that is what aerospace uses. 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 two wheel trolleys. They are at home in the shop, hangar or on the apron.

Look what we do on www.sarum-hydraulics.co.uk .

What is Skydrol and what is it used for?
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What is Skydrol and what is it used for?
Read our guide on the good, the bad and the ugly of working with Skydrol & what have we learnt about working with Skydrol for over 30 years
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Sarum Hydraulics
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