Is Aircraft Hydraulic Fluid Flammable? (A Comprehensive Overview)

Is Aircraft Hydraulic Fluid Flammable?

Most aircraft hydraulic fluids in use today are flammable. Aircraft hydraulic use pressurized hydraulic fluids. A malfunction in these systems alongside an ignition source can cause a fire.

Hydraulic fluids can be the cause of an aircraft crashing. In 2014 in Karachi Pakistan an airbus A330-200  had its hydraulic hose fractured during flight. This led to a fire in the passengers cabin. Fortunately, the complete passengers evacuation was completed in time.

In this article the main aircraft fluids will be listed along with their general properties and fire hazards. Some precautions and recommendations on how to deal with fires caused by hydraulic fluid in aircraft will be presented.

Most Common Aircraft Fluids and their Fire Hazards

Aircraft hydraulic fluid are oils that can withstand high temperatures and high pressures as well as not freezing under very low temperatures. As such their flammability should not be accessed as a regular fluid, given that hydraulics fluids are regularly under drastic conditions.

In order to properly evaluate the fire hazards of aircraft fluids extensive tests must be carried out for each specific hydraulic fluid.

In general, aircraft hydraulic fluids are capable of catching on fire and further sustaining this fire if an ignition source is subjected to the fluids. In fact a number of aircraft accidents and incidents have been found to be related to hydraulic fluids.

As a clarification, being capable of catching on fire does not mean the same as being flammable. Hydraulic fluids are made with the intent to be fire resistant. However, given the harsh conditions airplane hydraulic systems may go through, hydraulic fluids can act as fuel in a combustion reaction.

There are Many Aircraft Hydraulic Fluids Formulas

One can divide hydraulic fluids based on their source material. Mineral oil based fluids are composed mainly (up to 85%) of petroleum distillate. They may also also contain smaller amounts of di-butyl methyl phenol ([(CH3)3C]2C6H2(CH3)OH).

Hydraulic fluids can also be hydrocarbon based. These types of hydraulic fluids are made to be more fire resistant than mineral oils. The last type of hydraulic fluid is the phosphate ester based.

In the table below some examples of aircraft hydraulic fluids that were considered representatives within each of the three main sources of oils (minerals, hydrocarbons and phosphate esters) are listed.

MineralHydrocarbon or polyalphaolefinsPhosphate ester
MIL-PRF-5606, MIL-PRF-6083MIL-H-83282, MIl-H-87257BMS3-11, HyJet V, MIL-H-8446, NSA307110, Skydrol

In the following sections the major properties and applications of some of the most commonly used hydraulic fluids in the past and in today’s airplanes will be listed.

MIL-PRF-5606

  • Mineral oil
  • Main components: hydrotreated light naphthenic distillate from petroleum (up to 99% concentration) and solvent-refined light paraffinic (10 to 30% concentration).
  • Typical flash point: 100 – 110 ºC.
  • Typical autoignition temperature: above 319 ºC.
  • Suitable extinguisher: Foam, water spray or fog, dry chemical powder, carbon
  • dioxide.

MIL-H-83282

  • Hydrocarbon based hydraulic fluid.
  • Less flammable than typical mineral oil hydraulic fluid.
  • Flash point: 205 ºC or higher.
  • Autoignition temperature: 345 ºC or higher.
  • Used for air compressor gear boxes, brakes, flap-control mechanisms, shock absorbers and other hydraulic systems using synthetic sealing material.

MIL-H-87257 

  • Hydrocarbon based hydraulic fluid.
  • Flash point: 160 ºC or higher.
  • Similar uses to MIL-H-83282.
  • More often used in newer aircraft.

Skydrol PE-5

  • Phosphate ester based hydraulic fluid.
  • One of the four main Skydrol aircraft hydraulic fluids.
  • Phosphate based oil.
  • Appearance: Clear oily liquid (it may contain green or purple dye).
  • Flash point: 160 ºC or higher.
  • Autoignition temperature: 400 ºC or higher.
  • Contains a number of additives that help inhibit corrosion and erosion damage to servo valves.
  • Approved for use by Airbus, Boeing and BAE Systems

Skydrol PE-5 composition as published by the developing company:

Chemical name and formulaContent in % (w/w)
Tributylphosphate, (CH3(CH2)3O)3PO56 – 68
Triisobutylphosphate, ((CH3)2CHCH2)3PO8 – 10
Tri(isopropylphenyl)phosphate, ((CH3CHCH3)C6H4O)3PO5 – >10
Triphenylphosphate, (C6H5O)3PO1.3 – 1.9
2-Ethylhexyl 7-oxabicyclo[4.1.0]heptan-3-ate5.5 – 6.5
2,6-Di-tert-butyl-4-methylphenol, [(CH3)3C]2C6H2(CH3)OH0.1 – 1

HyJet V

  • Phosphate ester based hydraulic fluid.
  • Flash point: 174 ºC or higher.
  • Autoignition temperature: 427 ºC or higher.
  • Compatible with elastomers.
  • Used by Airbus and Boeing aircraft.

In the table below the main constituents of HyJet V are presented:

Chemical name and formulaContent in % (w/w)
Tributylphosphate, (CH3(CH2)3O)3PO70 – >80
Tri(isopropylphenyl)phosphate, ((CH3CHCH3)C6H4O)3PO10 – >20
2,6-Di-tert-butyl-p-cresol, C15H24O0.1 – <1
Bis(2-hydroxymethyl) tallow amine0.025 – 0.1

How to Avoid Fires Caused by Aircraft Hydraulic Fluid?

  • Leaks must be avoided or quickly detected. If the hydraulic fluid leaks from the pressurized system the fluid can form a flammable suspension in the air. When a liquid such as hydraulic fluid has very small particles floating in the air an ignition source can easily cause a fire.
  • Under emergent situations aircraft brakes can be heated to very high temperatures (up to higher than 500 ºC).

The pilot’s awareness of the temperatures of the brakes by attention to the temperature indicators helps avoid overheating of the brakes.

  • In the case of crash fire, the temperatures of the fire can be higher than 1000 ºC, which is way higher than the aircraft hydraulic fluid.
  • Thorough maintenance practices and flight crew diligence during pre-flight supervision contribute to minimize the risk of fire caused by hydraulic fluid leakage.
  • If possible to choose between hydrocarbon and phosphate ester based hydraulic fluid, keep in mind that phosphate ester based fluids possess a higher resistance to fire.

Nonetheless, phosphate ester hydraulic fluids can still catch on fire, either by contact with an ignition source or by itself if heated to its autoignition temperature or higher temperatures (this temperature stays around 400 – 470 ºC).

How Does an Aircraft Hydraulic System Work?

The hydraulic system is made up of three primary mechanical components plus the hydraulic fluid itself.

  • The motor (it can be a hydraulic actuator, hydraulic cylinder or a hydraulic motor), which is responsible for generating the required pressure. The motor can be referred to as the ‘pressure generator’.
  • The pipes.
  • The valves. 

Pretty much every aircraft has a hydraulic system of some sort. In light weighted common aviation aircraft, the hydraulic parts may be composed of just the wheel brakes, for instance.

In heavier aircraft, the hydraulic system may act the motive power for a series of other systems within the aircraft, such as the emergency electrical generation, flap and slat retraction/extension, the flight control actuation, the landing gear retraction/extension, 

and also in the airstairs and cargo doors.

Conclusion

A general view regarding aircraft hydraulic fluid was presented. From the information that was exposed, it can be recollected that hydraulic fluids are of utmost importance for long distance locomotion. 

And, hydraulic fluids are capable of causing fires, albeit such events are considered rare to occur, specially when all the recommended procedures and inspections are executed properly.

Frequently Asked Questions (FAQ): Is Aircraft Hydraulic Fluid Flammable? (A Comprehensive Overview)

Why is the intermixing of the aircraft hydraulic fluids not allowed?

Aircraft hydraulic fluids have very specific compositions (mineral oils, hydrocarbon oils and phosphate esters, each category with its own variations) and properties (such as viscosity, melting point, purity, pressure stability, thermal stability, fire resistance).

Intermixing two or more different hydraulic fluids will most likely drastically change the properties of the fluid, which are calculated for each specific application.

What’s the flashpoint of hydraulic fluid?

It really depends on the exact composition of the fluid. In many cases the exact flash point is not known, but the minimum flash point is. 

Phosphate ester based hydraulic fluids are the ones with the highest flash points (meaning the less likely to catch on fire). The flash point of phosphate ester based fluids is usually 160 ºC or higher. 

On the other end, mineral oil based hydraulic fluids have flash points around 100 ºC, making them considerably more dangerous. 

What color is aircraft hydraulic fluid?

The aircraft hydraulic fluid color is not as controlled as the aircraft fuel. The color of the hydraulic fluid does not necessarily indicate its purpose and type, unlike the fuel which usually has a specific color for each grade. Aircraft hydraulic fluid can be clear, green, blue, purple, blac, etc.

Can you mix engine oil with hydraulic fluid?

Engine oil and hydraulic fluid are considerably different. Mixing them will most surely make it unusable and even dangerous to use. If proper lubrication does not take place overheating will occur which could cause a fire.

Can hydraulic oil explode?

Aircraft fluids are not likely to explode or catch on fire under normal circumstances. They can catch on fire or cause an explosion under severe conditions of temperature and/stress of the hydraulic system. The flash point and autoignition temperature of hydraulic fluid are respectively 100 – >170ºC and >350 ºC for modern aircraft hydraulic fluids.

Is hydraulic fluid hazardous?

Inhalation of hydraulic fluid can lead to disorientation and dizziness, nerve damage, etc. Ingestion of hydraulic fluids can lead to severe health problems and even death.

If in contact with eyes or skins it can cause irritation or irreparable damage to the eye.

Does hydraulic fluid evaporate?

Some of the substances that make hydraulic fluids can evaporate over time. But normally that evaporation is extremely slow. It is important to be aware of the maximum usage time of the oil. Most hydraulic fluids can only be used for up to 5 years. After that the hydraulic fluid will no longer function properly.

Why is hydraulic fluid so flammable?

Despite there being many types of aircraft hydraulic fluids, all of them are made of mostly organic substances (that is they have carbons in their molecules). This means that all of them contain substances that are capable of being oxidized. So they are capable to go through a combustion reaction.

The developers and producers of hydraulic fluid make the oils to have the highest possible fire resistance keeping the required characteristics.

References

Safety measures and general information: https://skybrary.aero/articles/hydraulic-fluid-fire-source

https://skybrary.aero/accidents-and-incidents/a332-karachi-pakistan-2014
https://skybrary.aero/articles/auto-ignition-temperature
https://www.fire.tc.faa.gov/pdf/tn90-19.pdf

Specification of some hydraulic fluids:

https://www.shell.com/business-customers/aviation/aeroshell/aeroshell-hydraulic-fluid.html#specs
https://www.alcorpetrolab.com

Skydrol PE-5 information:

https://ws.eastman.com/ProductCatalogApps/PageControllers/MSDSShow_PC.aspx

HyJet V information:

https://www.exxonmobil.com/en/aviation/products-and-services/products/hyjet-v

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