MIL-PRF 5606: A Common and Important Hydraulic Fluid for Military Aviation

Continuing our series of articles on military-grade aviation lubricants, adhesives, and solvents, we are examining MIL-PRF-5606, a common hydraulic fluid used in various U.S. military aircraft. From helicopters to heavy jets, almost all aircraft in the military fleet use hydraulic systems. Hydraulics power flight control surfaces, landing gears, brakes, weapon bay doors, and more, so using high-quality and the correct hydraulic fluid is extremely important.

MIL-PRF-5606 is widely used, and here is our rundown on what it is and what we offer. 

What is Hydraulic Fluid MIL-PRF 5606

MIL-PRF 5606 is a petroleum-based hydraulic fluid designed for use in aircraft, missile, and ordnance hydraulic systems. It is used in most legacy military designs, such as the KC-135, C-130, C-5, and B-52, among others, as well as in the A-10 and F-15 fighter and attack aircraft. 

The specification describes the characteristics and provides the requirements for the fluid, including excellent low-temperature fluidity and exceptional anti-wear properties. Maintaining the correct level of fluid is crucial, as is keeping the fluid clean in the aircraft. 

The fluid is identified by military symbol OHA and NATO Code No. H-515, and is widely used in military and civilian aircraft hydraulic systems, although civilian aircraft are not governed by the same MIL PRF standards, even if they are identical performance standards. 

It is a high viscosity index fluid that enhances performance in general-purpose hydraulic applications. Some of the most common uses and applications for MIL PRF 5606 on approved aircraft are:

  • Landing gear struts
  • Flaps and slat systems
  • Brake systems
  • Spoilers
  • Vertical and horizontal stabilizers
  • Speed brakes

History and Development of MIL-PRF 5606

The development of MIL-PRF 5606 dates back to the 1950s, when the US military needed a reliable hydraulic fluid for its aircraft and missile systems. Hydraulic systems were commonly used in World War II, although many more of the aircraft systems were mechanical. However, as the jet age dawned, aircraft became much heavier and faster, subjecting control surfaces, weapons bays, and weapons systems, as well as landing gears, to much higher loads, not to mention brake systems, which were tasked with stopping jets that were not only much heavier but also much faster. 

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The modern revisions included updates to the fluid’s characteristics, such as its shear stability and foam inhibitors. The fluid has undergone several changes over the years, with the latest revision being MIL-PRF-5606J. Every new revision to MIL-PRF-5606 is usable for any previous revision; i.e., an aircraft rated for MIL-PRF-5606A can use any revision thereafter, but an aircraft designed for the J revision can only use that and any revisions thereafter. Admittedly, this is not a significant issue with a legacy fluid like MIL-PRF 5606, but it does matter for aircraft in new production. 

Performance Characteristics of MIL-PRF-5606 Hydraulic Fluid

MIL-PRF-5606 and its commercial derivatives (we carry Royco 756 MIL-PRF-5606H Mineral Oil Based Aircraft Hydraulic Fluid in a one-quart can) are the required hydraulic fluid for a number of the older aircraft across the military fleets. Here are some of the key performance characteristics:

  • MIL-PRF 5606 hydraulic fluid has excellent low-temperature fluidity, making it suitable for use in cold climates. It is effective in temperatures from -54°C to +135°C (-65°F to +275°F)
  • Royco 756 has exceptional anti-wear properties, which help to extend the life of hydraulic system components. These components are subjected to extreme forces; flaps extended in a 200 knot slipstream; landing gear struts compressing for thousands of cycles on a 300,000lb aircraft (or considerably heavier depending on the airframe). The fluid is the make-or-break point to the longevity of these components. 
  • It also has good oxidation and corrosion inhibition, which helps to prevent damage to system components. Again, fluid either allows components to degrade or keeps them in great working order for decades. 

Comparison to Other Hydraulic Fluids

MIL-PRF 5606 hydraulic fluid is a mineral oil-based fluid, and compared to other hydraulic fluids, such as MIL-PRF-83282, it exhibits lower fire resistance and thermal stability. MIL-PRF-83282 is a synthetic fluid that performs better 

However, retrofitting aircraft to different types of fluid is highly prohibitive. Since commercial carriers use aircraft for much shorter timeframes, the planes that would have used mineral-oil-based fluids have been retired. However, military aircraft tend to accumulate hours much more slowly, so those airframes remain in use for decades. There are no significant enough safety concerns with modern fluids that warrant revising the weapons systems, so MIL-PRF-5606 will continue to be used until these aircraft are phased out. 

The fluid is widely available and is supported by a range of manufacturers, including Royco and AeroShell. Again, we offer Royco 756 Hydraulic Fluid by the quart for your convenience.

Operating Temperature Range and Maintenance

MIL-PRF 5606 hydraulic fluid has an operating temperature range of -54°C to +135°C, which allows aircraft that use it to operate in almost all climates globally. Like all other fluids on the aircraft, hydraulic systems require regular maintenance, including fluid analysis and filter changes, to track performance and reliability. The military uses these metrics to determine future revisions. 

The fluid is available in a range of packaging options, including resealable containers, which makes it convenient to use and store, from a single quart can to 55-gallon drums of fluid. Aircraft without pressurized hydraulic systems can easily be serviced using one-quart cans. However, aircraft that use a pressurized hydraulic system are better serviced from large drums, as the hydraulic fluid must be serviced from a cart. 

You will want to ensure that your operation can handle that much fluid, though, since it has a shelf life of either 1,095 days or 1,460 days, depending on the type. The fluid meets the requirements of MIL-PRF-5606H, which includes a shelf life of 1095 days, and 756A has a shelf life of 1,460 days. 

Safety Considerations and Handling

MIL-PRF 5606 hydraulic fluid is a petroleum-based fluid, which makes it flammable and definitely hazardous. There have been several incidents involving hydraulic fluids catching fire, including at least two instances on B-1B Lancers, where the brakes caught fire. This is one of the biggest concerns with hot brakes, especially when tires blow out (which is why wheels are equipped with thermal relief or blowout plugs). When high-pressure tires blow out, the sidewalls become incredibly dangerous. They will slice through hydraulic lines with ease, and a hydraulic system under pressure becomes a fire hose of flammable petroleum. 

Maintenance teams should handle hydraulic fluid with care and keep it away from open flames and sparks. It must also be stored in a well-ventilated area and disposed of per regulations. Hydraulic fluid is toxic and must be handled with gloves and protective clothing, such as a rubber apron, and goggles that seal around the eyes. Technicians should consult the Royco 756 Hydraulic Fluid Safety Data Sheet (SDS) for complete personal protective equipment (PPE) requirements. 

Greenwood Aerospace Provides a Full Catalog of Aircraft Lubricants

Greenwood Aerospace is your full-service supplier of aerospace lubricants, solvents, and adhesives. We carry Royco 756, along with dozens of other lubricants for your fleet requirements. Our inventory is continually expanding, so please check in with our team of procurement specialists to obtain pricing details for your order.

You can contact us via email (contact@governmentprocurement.com), phone (580) 762-2580, or you can even start a quote right here!