Frequently Asked Questions

Frequently Asked Questions

Q: What specifications (military or industrial) do Skydrol hydraulic fluids meet?
A: There are no military specifications for Skydrol hydraulic fluids. The list below contains most of the existing industry specifications and approvals:
  • S.A.E. - AS1241
  • Boeing Seattle - BMS3-11
  • Boeing Long Beach - DMS2014
  • Lockheed - LAC C-34-1224
  • Airbus Industrie - NSA307110
  • British Aerospace - BAC M.333.B
  • Bombardier - BAMS 564-003
  • Fokker
  • Embraer
  • Westwind
  • Cessna
  • Gulfstream

Q: Are Skydrol hydraulic fluids fire-proof?

No. There are conditions under which Skydrol will burn. Skydrol fluids are "fire resistant." Traditionally the term "fire resistant" has been used to describe phosphate esters, as opposed to mineral oil based hydraulic fluids, because they are very difficult to ignite at room temperature. In standardized testing, a fine mist spray of Skydrol cannot be ignited with an oxyacetylene torch. The same fine mist spray of a mineral oil hydraulic fluid, such as MIL-H-5606 or MIL-PRF-83282, produces a large fireball when touched by the torch.

The basic parameters of fire resistance include: 1) resistance to ignition, and 2) resistance to propagation of the flame from the source of ignition. The test methods followed by the industry are described in AMS 3150 and various ASTM test methods. For more detail, please ask for our Publication No. 9118 on fire resistance.

Q: Under what conditions will Skydrol burn?
A: Skydrol fluids must be heated to high temperatures before they will sustain combustion. The fire point of Skydrol LD-4, for example, is 360°F. This is the temperature the fluid must be heated to in order for it to be ignited in the ASTM D92 cup apparatus. The autoignition temperature of LD-4 is 880°F. This is the temperature at which the vapors will ignite spontaneously.

Q: What kinds of tests are used to evaluate fire resistance?
A: There are a total of 10 tests that can be used to evaluate the fire resistance of Skydrol hydraulic fluids. There is not one single test that can adequately describe fire resistance. Flash point and fire point, for example, do not simulate real world conditions that may occur in the event of a high pressure leak from a hydraulic supply line spraying against a hot engine manifold or brake. The following tests are industry standards for evaluating fire resistance:
  • High temperature/high pressure spray test
  • Hot manifold drip test
  • Low pressure spray test
  • Wick flammability test
  • Hot compartment spray test
  • Hot manifold spray test
  • Hot brake flammability test
  • Spontaneous ignition temperature
  • Autoignition temperature
  • Flash and fire temperature points

Q: Does Solutia still make Skydrol 7000 or Skydrol 500A fluids?

No, these are earlier versions that are obsolete. Each generation of fluid has been designated by a Type number. Only Type IV and Type V fluids are made and sold today. The table below lists the dates when the older fluids were discontinued:

Fluid Type Brand Name Date Discontinued
---------- Skydrol 7000 1974
Type I Skydrol 500A 1969
Type II Skydrol 500B 1981
Type III Skydrol LD 1981
Type IV Skydrol LD-4, 500B-4 Current
Type V Skydrol 5, Skydrol PE-5 Current

Skydrol 500B-4 (Type IV Class 2) is the standard density fluid. It is most useful in ground based equipment where weight saving is not necessary. Mists of 500B-4 are much less irritating than those of low density fluids, making it more pleasant to work with.

Skydrol LD-4 (Type IV Class 1) is a low density fluid based on tributyl phosphate. Its low density makes it a popular choice for saving weight on aircraft, and thereby reducing fuel consumption. Skydrol LD-4 has been in use worldwide since 1978 with an outstanding record of performance.

Skydrol 5 is the lowest density fluid on the market today. It was specifically formulated to allow operation at higher temperatures than Type IV fluids. Other benefits include: lower toxicity than tributyl phosphate based fluids, less aggressive toward aircraft paints, fuel savings due to low density.

Skydrol PE-5, our newest product, has full approval from Airbus and Boeing for use in all of their aircraft models. Skydrol PE-5 was developed to meet and exceed the more demanding Type V fluid requirements. It features the longest fluid life of any commercially available fluid, low density and low viscosity at low temperature; an unbeatable combination of the best features for optimum fluid performance.

Q: Do fluid classes correspond to elastomer classes?
A: Yes, but that is changing. The recommended seal material for use with Skydrol hydraulic fluids is ethylene propylene rubber, (EPR, or EPDM). The NAS 1613 specification that lists the requirements for these seals has in the past shown two classes of EPR rubber that correspond with the classes of the fluids they are compatible with. The latest revision of the specification, however, is eliminating the two classes of rubber, and is instead requiring only one class, compatible with both fluids. During the years of transition to this new revision, there will still be seals sold that are made specifically for one class or another. Customers need to be careful in specifying seals to be sure the seal has been tested and is suitable for the fluid being used.

Q: Can the various Skydrol fluids be mixed?
A: All phosphate ester Type IV and Type V fluids are mixable and soluble in each other in all proportions. They are not, however, equally approved for use on every aircraft. Please refer to your component maintenance manual or manufacturer fluid specification document to determine which fluids are approved for your aircraft. Solutia Technical Service can advise you on where to find approval information.

Q: What types of paint are compatible with Skydrol fluids?
A: Epoxy primers are compatible, as well as two-part polyurethane top coats.

Q: What are the in-service limits of Skydrol hydraulic fluids?

Airlines should follow the specified in-service fluid limits as called out by the various airframe manufacturers. Our technical service department will make recommendations to help airlines maintain their fluid within these limits.

Q: What tests are performed and why?
Analytical Test Why it is Performed
Moisture Hydrolysis, Corrosion
Total Acid Number Thermal Stress
Particle Count Cleanliness
Gas Chromatography Composition, Contamination, Remaining life of additive package

Q: Why is the control of acidity important?
A: Acidity is the one test that defines fluid life. The additive package contains an agent to control acidity, which usually keeps the level under 0.5 mg KOH/g. When acidity reaches the range of 1.0 to 1.5 mg KOH/g, it means the additive that controls it is severely depleted. The maximum permissible acidity level is 1.5 mg KOH/g. Some manufacturers set lower limits. Consult Solutia Technical Service for further details.

Q: Why does Solutia recommend changing hydraulic fluid when the acid number reaches 1.0, while most airframe manufacturers recommend a 1.5 maximum value?
A: In many cases, when a fluid reaches 1.0 acid number, 90% of the acid control additive has been depleted. The rate of increase in acidity at this point becomes quite high, resembling an exponential curve. When aircraft are sampled only once per year, it is possible to have an aircraft exceed the limit by a wide margin if the stopping point is set at 1.5. If an aircraft should exceed 1.5 it requires multiple drain-and-refills of the reservoir to bring acidity under control. If, however, the action point is set at 1.0, one reservoir drain should be sufficient. Note: Boeing recommends taking action at 0.5 on the 757 center system. Other airframe manufacturers many have different limits. Please consult your service manual, or Solutia Technical Service for more details.

Q: Why does Solutia recommend changing hydraulic fluid when moisture reaches 0.5%, while the airframe manufacturers set higher limits?

Most used fluids analyzed by Solutia show moisture content of less than 0.5%. When a fluid does contain 0.5% water, this is an indication of a source of moisture contamination that may continue to build up in the fluid. Solutia suggests that the operator determine the source of the moisture and eliminate it. The ground cart fluid should be submitted for analysis. Fluid storage and maintenance procedures should also be examined.

While some airframe manufacturers may allow up to 0.8% water in used fluid, any level above 0.5 will severely curtail the service life of the fluid. It will lead to a build-up of acidity much earlier than normal.

Q: What happens when Skydrol fluids change color?
A: Skydrol fluids are given a purple dye to make it easy to distinguish them from other fluids. But the dye does not usually last as long as the fluid does. The color may change from purple to gray or yellow or green, yet the fluid can still meet all used fluid specifications. Color is not a reliable indicator of fluid quality, and we recommend that the system be sampled for chemical analysis to determine if it is suitable for use. A dark amber color, however, is a cause for concern, and can indicate a severely stressed fluid. Call Solutia's Technical Service for assistance if you are unsure about a fluid's color. Our technical bulletin has a color chart to use as a guide.

Q: What happens when liquid contaminants get into aircraft hydraulic systems?
A: Liquid contamination is typically introduced by accident. An example of this is cleaning solvent contamination. The following types of contamination cause several distinct problems:
  • Chlorinated Solvents - Can lead to erosion.
  • Dissolved Water - Reduces fluid life through hydrolysis.
  • Free Water - Can destroy pumps due to lack of lubricity and can freeze causing blockages.
  • Petroleum Based or Synthetic Lubricants- Can adversely affect seals, reduce fire resistance, and cause viscosity index improvers to gel.

Q: What happens when solid contaminants get into aircraft hydraulic systems?
  • Physical damage to critical areas in the hydraulic system, as solids can move at very high velocity.
  • Deposits in lines or on filters in the hydraulic system, which can cause plugging.
  • Chemical reactions in fluids, accelerating fluid decomposition.

Q: What happens when fuel is contaminated by hydraulic fluid?
A: In laboratory tests, concentrations of up to 5% Skydrol in jet fuel were cooled to -30oF. No gel formation or any other effect was observed. 5% is the maximum concentration possible in the worst case of a leak in a fuel tank heat exchanger.

Q: What actions should be taken to correct hydraulic system contamination?

Liquid Contamination - We recommend flushing the system as soon as possible with clean Skydrol hydraulic fluid and repeating enough times to be sure the contaminating fluid has been completely removed. It may be necessary to thoroughly clean each component to remove the contamination. Procedures outlined by the aircraft manufacturer and component manufacturer should be followed.

Solid Particle Contamination - We recommend the examination of the aircraft hydraulic fluid filters and servicing as soon as possible. The flushing of the hydraulic system with clean Skydrol hydraulic fluid and filter changes might be necessary to remove the contaminants in extreme cases. Procedures outlined by the aircraft manufacturer and component manufacturer should be followed.

Q: Why take samples of aircraft hydraulic fluid?

The performance of the entire aircraft hydraulic system can be affected by the condition of the hydraulic fluid. If the hydraulic fluid additives are damaged or depleted, the hydraulic fluid may no longer perform adequately in the system. Only after sampling the hydraulic fluid for the purpose of fluid analysis can one determine the condition of the additives. This allows action to be taken before damage to the hydraulic system can occur.

Solutia's fluid sampling program offers comprehensive analysis and evaluation of the fluid with recommendations for corrective action.

Q: When should hydraulic fluid samples be taken?
  • Routine Sampling - Each system should be sampled at least once a year or every 3000 flight hours, or whenever the airframe manufacturer suggests.
  • Unscheduled Maintenance - When malfunctions may have a fluid related cause, samples should be taken.
  • Suspicion of Contamination - If contamination is suspected, fluids should be drained and replaced, with samples taken before and after the maintenance procedure.

Q: What is the procedure for taking a hydraulic fluid sample?
A: Use the airframe manufacturer's maintenance manual procedure for taking samples of aircraft hydraulic fluids. Some general guidelines for sampling are to operate the system to assure uniform mixing of the fluid. Use a clean sample bottle, available from Solutia. Drain a sufficient quantity of fluid to purge the line before collecting the sample, and collect 12 ounces of fluid for a complete analysis. The sample must be identified with the following information: aircraft number, aircraft model, system identification, aircraft hours, source of sample, and sample date.

Q: How long does it take to receive laboratory results on a fluid sample?
A: Our normal analysis time is 10 days or less from the date we receive a sample. If the situation is urgent, we can offer 24 hour turn around time to our customers.

Q: Is there a field test kit available to analyze hydraulic fluid samples?
A: Yes, but it is used only for a crude estimate of acid number, and not a precise numerical value. The Skydrol Test Kit is available from TBM Corporation, 314-721-5590, and other Skydrol distributors.

Q: Do Skydrol fluids have a limited shelf life?
A: All Skydrol hydraulic fluids have a 10 year shelf life from the date of manufacture. This applies to properly stored factory sealed containers.

Q: Where and how should I store Skydrol fluids?
A: The basestocks of Skydrol hydraulic fluids are blends of phosphate esters, which are mildly hygroscopic. In order to keep moisture in the air from absorbing into the fluid, fluid containers should remain closed when not in use. Vent holes on reservoirs should fitted with dessicant breathers to absorb moisture from the air. Containers of Skydrol should be stored indoors. If drums are stored outdoors, they should be sheltered from rain. If they cannot be sheltered from rain, then they should at least be stored horizontally to prevent water from collecting around the bung. Alternatively they can be stored vertically but tipped slightly to accomplish the same result.

Q: What chemicals in Skydrol fluids cause irritation?
A: The phosphate ester base stock is the cause of irritation. Phosphate esters are good solvents, and can dry the skin. They contain mild organic acids that give a slight burning sensation on sensitive skin areas, and in the eyes they are quite painful. In all cases of exposure, however, as soon as the phosphate ester is removed, by washing, the pain subsides. Skydrol fluids are not known to cause allergic skin rashes, although repeated or prolonged exposure may dry the skin. If left unattended, this could result in complications such as dermatitis or even secondary infection from bacteria.

Q: What can I use to protect my hands?

To avoid exposure, a worker should wear gloves that are impervious to Skydrol hydraulic fluids. Solutia has tested several different glove materials and has published guidelines in the publication titled "Glove Facts." Contact Skydrol Technical Service to obtain a copy.

Solutia does not recommend the use of barrier creams in place of gloves. Barrier creams are not a reliable method of protection.

Q: What is the proper first aid treatment for eye exposure to Skydrol fluid?
A: Solutia is not aware of any case of eye damage from exposure to Skydrol fluid. When the fluid gets into the eyes, it can cause severe pain, but the pain will subside as soon as the fluid is removed. First aid is washing with tap water, or a standard eye irrigation solution. 15 minutes of washing with water will usually be enough to remove the Skydrol and cause the pain to cease. Secondary treatment, such as with sterile mineral oil, should only be given under the care of a medical professional, as it goes beyond recommended first aid. Milk is not recommended as an eye wash, because it is not sterile and can cause infections in the eye. Always remember to wear safety glasses or chemical goggles to prevent eye exposure when working around Skydrol.

Q: What happens if I breathe in Skydrol fluids?

Upper respiratory irritation, including nose and throat irritation and tracheitis and/or bronchitis, can occur from inhalation of a mist. People with asthma may have a more marked reaction.

If through some accident liquid Skydrol fluid is aspirated directly into the lungs, such as by swallowing a large amount and breathing in at the same time or by breathing in while vomiting, it is quite possible that chemical pneumonitis could occur. This occurs following deep aspiration of any foreign material into the lungs. We have never heard of this happening with the Skydrol fluids. The possibility of it happening under normal industrial conditions does not appear likely.

When mist or vapor is possible because of high pressure leaks or any leak hitting a hot surface, a respirator capable of removing organic vapors and mists should be worn.

Q: What is the best first aid treatment if Skydrol fluid is ingested?
A: Immediate first aid is not likely to be required. Consult a physician or a Poison Control Center.

Q: What are the key points for working safely with Skydrol fluids?

Use glove and goggles or safety glasses.

Eye baths should be available in case of eye exposure.

When mist or vapor is possible, use an organic vapor/mist respirator.

Consult Solutia's MSDS for further details.

Q: What is the best way to clean up Skydrol spills?
A: For tools and equipment use SkyKleen. Other suitable solvents include Stoddard solvent, acetone, and mineral spirits. On floors, absorb Skydrol fluid with paper towels or clay absorbents. Wipe with a suitable solvent. A final wash with soap and water will finish the job. Be sure the solvent selected will not damage the flooring material.

Q: How do I dispose of waste Skydrol liquids?
A: There is no approved reclamation system or any known technology to reclaim Skydrol fluids for return to aircraft hydraulic system usage. We recommend incineration in accordance with federal, state, and local guidelines.