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what is the meaning of PRF here and can you differentiate them visually ?

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About 25 years ago, there was a major effort to revise MIL standards and specifications. The biggest change in MIL specs was to move away from a design or manufacture spec to a performance spec. The downside of the old specs were that they didn't allow for an improved product without getting the spec revised. By switching to performance spec (what it must do as opposed to how to make it), improvements can be made without spec revisions and contract revisions (that call out the specs) ultimately saving everyone time and money while delivering products that use up to date technology.

In your case, MIL-H-5606 is the 'old' spec. The H is taken from the primary application, in this case "Hydraulics". The current spec is MIL-PRF-5606. PRF is used to indicate a 'performance' specification.

'MIL-PRF' and other abbreviations are defined within DoD 4120.24-M, [2], Defense Standardization Program (DSP) Policies and Procedures, March 2000, OUSD (Acquisition, Technology and Logistics).

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MIL-H-5606 and MIL-PRF-5606 are US specifications for hydraulic fluid based on mineral oil. They are used in US military 3000 PSI hydraulic systems.

The biggest draw back is their flammability, which is why their use in new commercial aircraft designs has been eliminated. However, many piston general aviation aircraft still depend on -5606 hydraulic fluid.

MIL-H-5606 is obsolete and superseded by MIL-PRF-5606.

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    $\begingroup$ Depends on you definition of 'commercial'. Small aircraft like Cessnas and Pipers use 5606 for brakes, gear, and some autopilots. Some of those are in commercial service. $\endgroup$ – JScarry Nov 13 '17 at 22:06
  • $\begingroup$ Many business jets also use MIL-H-5606 for their hydraulic systems. $\endgroup$ – Lnafziger Mar 30 '18 at 20:19
  • $\begingroup$ Cessna, Bananza, and Navion use MIL-H-5606 for their hydraulic systems. $\endgroup$ – jwzumwalt Apr 2 '18 at 2:35
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Commonly MIL-H-5606 is replaced with the compatible MIL-H-83282 or now PRF due to its increased flammability resistance. As long as the operating temp range stays above -40. It also is not restricted to piston engine aircraft, Beechcraft King Air with a PT-6 as well as many turbine commercial helicopters use one or both of these fluids. Sikorsky, Bell and Airbus helicopters to name a few.

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PRF is from the MIL-PRF portion of the Performance Specification and is not specific to this item number. See MIL-PRF.

From AeroShell handbook on hydraulic fluids...

Although the military did not move to phosphate ester type fluids they did identify the need for a more fire resistant fluid as a direct replacement for MIL-H-5606. As a result a synthetic hydrocarbon-based fluid, MIL-H-83282 was developed. This fluid is completely compatible with MIL-H-5606 fluids and MIL-H-5606 hydraulic system materials. All physical properties of MIL-H-83282 (now MIL-PRF-83282) were equivalent to or superior to those of MIL-H-5606 (now MIL-PRF-5606) except for low temperature viscosity. In particular all fire resistant properties of MIL-PRF-83282 are superior to those of MIL-PRF-5606. More recently MIL-PRF-87257 was introduced in order to address the concerns over the low temperature viscosity of MIL-PRF-83282.

Visually, there is no distinctive difference between the old 5606 and the replacement variants of 83282.

My take is that we should all be moving to 83282. Another part number to remember...

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