According to materials engineer Carl Ed Snyder Jr., purification of military aerospace hydraulic fluids can reduce the need to procure new fluid, improve component life and boost hydraulic system performance. It also has potential to shrink the second biggest waste stream in the Air Force – used hydraulic fluids.

Snyder, who is with the Air Force Research Laboratory at Wright-Patterson Air Force Base in Dayton, Ohio, spoke in May at the Society of Tribologists & Lubricant Engineers annual meeting in Cleveland. He described ongoing efforts to achieve an Air Force-wide mandate for aerospace hydraulic fluid purification. Snyders office, the Air Forces Materials and Manufacturing Directorates Nonstructural Materials Branch, is currently co-managing fluid purification field tests at two Air National Guard bases, along with the Air Force Aeronautical System Centers office of Environmental Pollution Prevention.

Hydraulically actuated mechanisms are responsible for a large number of aircraft functions, including flight controls, landing gear operation, rudder flap control and accessory door actuation. On average, the Air Force spends about $9 million per year on new aviation hydraulic fluid, plus another $2.1 million on disposal of waste hydraulic fluid. Now if we can reduce that by even just 50 percent, its a substantial yearly cost savings, Snyder pointed out.

We expect with cleaner hydraulic fluids to have less erosive wear, too – particulates having been shown to cause erosion in components, pumps, servo valves and actuators, he said. Eliminating particulates should cut down on servo valve jamming (so-called uncommanded flight control inputs) and also decrease wear in hydraulic pumps.

The purification systems the Air Force is testing also help to lessen water content in the fluid. This prevents servo valve jamming which can be caused by ice forming in the fluid at cold temperatures, as in winter or high-altitude flight. Removal of free water in the hydraulic fluid can also minimize corrosion of components.

Air was another contaminant the researchers started paying more attention to, after seeing the improvements the purification process made when it removed entrained and dissolved air. If not removed, such air can result in spongy-feeling flight controls, pump cavitation and overheated fluid.

The elimination of free air results in a more responsive hydraulic system, which can greatly improve the handling of the aircraft. The Army implemented it in CH-47 [helicopters], and after a couple of cycles, the pilots said these CH-47s handle more like sports cars than semis, Snyder said. The free air in there was causing them to be very sluggish.

And with F-16 fighter planes, crews can accomplish preflight operational checks in half the time, thanks to purified fluid. Pre-flight checks basically involve full deflection of rudders and full deflection of flaps, he said. With a more responsive system, you can respond quicker and find out sooner if its OK to take off.

Purification can trim the Air Forces new fluid procurement needs significantly, Snyder told STLE attendees. We pay to dispose of used fluids, he said. Some can be used as a fuel supplement, but most of it can be considered hazardous waste. In most of the country, you have to find a place that can dispose of it. If we can reduce such fees, we can save a lot of money.

Its important to understand what hydraulic fluid purification is and isnt, Snyder emphasized in his presentation, which was co-authored with his colleague, engineer Lois Gschwender. One thing we have to keep clarifying is it is not reclamation. It is merely removing impurities from the fluid. These tend to be particulates, solvents, moisture, anything volatile, and excess air. All of these can be detrimental to the system performance, if these contaminants are present.

He said that purification does not remove fuel, additives or other materials from hydraulic fluids. If its not particulate, if its not volatile, it will not be removed. It is not reprocessing, and does not involve any reformulations. Basically its an easy, simple process, as long as you apply it correctly.

Snyder said purifiers work by mechanical means, rather than a chemical process. They create a large surface area, either by spinning disc, or by making an aerosol mist in a vacuum chamber, he explained. Thats to remove volatiles, water, solvents, things like that. After that it goes through a very high capacity, fine filter to remove particulates, and we circulate it at an average of 3 to 5 gallons a minute.

Three commercial portable fluid purifiers have been approved for Air Force use, models made by Hydraulics International, Malabar International and Pall Corp.

Using these systems, the Air Force has found it can purify about 55 gallons of highly contaminated fluid, cleaning it to like-new quality, within four hours. Its very effective and very easy to run – typically you turn the switch on, walk away, come back. Theres no need to look at or stay with it.

Snyder said the preferred method for aircraft hydraulic fluid purification involves attaching a portable purifier not directly to a plane but to a hydraulic test stand, commonly called a mule. The mule holds a reservoir of fresh fluid that technicians use to refill aircraft systems during maintenance.

What were trying to do is purify the fluid in the test stand that services the aircraft, and that doesnt tie up the aircraft, he said. This process doesnt happen instantaneously. Its an iterative process, which runs about 3 to 5 gallons a minute through the purifier. It removes all the contaminants – but slowly. It doesnt do it in one pass.

While its possible to hook a fluid purifier directly to an aircraft, doing so can adversely affect the availability of the aircraft. Cleaning fluid in the test stand also doesnt tie up the maintenance crews time, Snyder added. Weve got a process that doesnt require the maintenance personnel to monitor it at all – they hook it up, turn it on and walk away. It will continue as long as you have it turned on.

One Malabar unit and two from Pall Corp. have been used in the field for about two years, gaining accolades from hydraulics technicians. Theyve all said purifiers make their job easier. They dont have to do all the flushing currently required if the hydraulic fluid is highly contaminated. It has been a very, very positive thing.

Transition to the field proved a slow, laborious process though. In fact, efforts can be traced back for years, as the Air Force ran a series of purification tests on various aircraft at a number of bases going back as far as 1988. Not all of these met with success. For example, one study with an older-type fluid did not pan out because the purifier tended to remove antifoaming agents in the hydraulic fluid, along with impurities.

Snyders group first got involved by working with the militarys MIL-PRF-5606 fluid, a naphthenic mineral oil based hydraulic fluid. Their goal was to ensure the purification process would have no adverse effect on fluid performance.

Their efforts included a long-term, 1,500-hour pump test under very severe conditions, including maximum fluid temperatures. Tests were performed on both the naphthenic based Mil-PRF-5606 and on a synthetic based, fire-resistant type, Mil-PRF-83282. There was no adverse effect from purification, Snyder remarked. Essentially, the fluid was just as good at the end of 1,500 hours as it was when we put it in.

Still, there have been other obstacles to overcome, including rigid Air Force rules forbidding any alterations to approved maintenance equipment. In order to be able to hook up a purifier to a test stand, we had to modify the technical orders for that test stand, to allow fittings to be attached, Snyder said.

He added that each aircraft is governed by a program office which has authority over its type of aircraft. Just because F-15s have adopted [purification] doesnt mean other aircraft will adopt it. In the Air Force, we had to get every system program office to agree to allow purified hydraulic fluids to be used in their aircraft before we could begin implementation.

Its possible the 20-year effort to get purification accepted throughout the service may be nearing its goal. The Air Force Headquarters is considering mandating hydraulic fluid purification across the Air Force, Snyder said. One purifier costs maybe $20,000, but one purifier can purify a lot of test stands. A big base might need two or three purifiers.

Field testing continues with the purifiers at an Ohio Air National Guard base in Springfield, Ohio, and a Florida Air National Guard base in Jacksonville, Fla. We are still testing, because whenever you bring about change, you have to have mounds of data. He said the advantage of using Air National Guard bases as testing grounds is that ANG airplanes tend to remain at their home base, while active Air Force aircraft tend to keep moving around to other bases. So you can see the effect of purification on performance on those aircraft. If you have new ones coming in and others going out all the time, you cant see the impact on performance.

In addition to the Air National Guard test programs, Snyder said, the Air Force itself has a number of purifiers in the field, with orders in for at least 50 purification units.

However, to get an Air Force-wide mandate will require showing a tremendous advantage over current methods. We have about four or five factors were saying it will help, and weve seen some evidence of this in limited tests weve done. Its difficult to project that across the whole fleet.

Meanwhile, Snyder said, users of the purification systems remain the best advocates. Everyone that has used it has seen the advantages of purification and felt it made it easier for them to do their job, and was really worthwhile.