An AFRL-developed fire-resistant hydraulic fluid recently completed a B-52 flight test, and based on successful test results, systems engineers from Oklahoma City Air Logistics Center (OC-ALC) will adopt the fluid (MILPRF- 87527) for use in over 90% of the aircraft's hydraulic systems. OC-ALC engineers will conduct further tests to determine whether they can also convert the hydraulic systems controlling the B-52's landing gear and wingtip protection struts to the fire-resistant fluid. AFRL expects the improved fluid's higher flash point and reduced flammability to increase the B-52 aircraft's survivability and overall operational safety. Further, the fluid's associated thermal stability measurements and fluid film thickness data indicate it performs well over extended periods of time in hightemperature environments and in temperatures as low as -65°F.
Air Combat Command's B-52 is a longrange, heavy bomber that performs a variety of missions (see figure). The bomber can fly at high subsonic speeds at altitudes up to 50,000 ft and can carry nuclear or conventional ordnance. In a military conflict, the B-52 can perform air interdiction, offensive-counter air, and maritime operations. During Operation DESERT STORM, B-52s delivered 40% of all weapons dropped by coalition forces.
Hydraulically actuated mechanisms are responsible for a large number of aircraft functions, including highly sophisticated flight control, landing gear operation, and accessory door actuation. Systems designers also employ hydraulic fluids to lubricate aircraft systems, cool heat-generating components, and otherwise function in highpressure hydraulic systems near a variety of ignition sources. In the past, the B-52 used a flammable, petroleum-based hydraulic fluid (MIL-PRF-5606) because it could operate at -65°F, a capability required for some deployments.
AFRL nonstructural materials experts have dedicated significant research and development effort to preventing hydraulic fluid hazards. Though fireresistant fluids will burn, they are significantly more difficult to ignite than non-fire-resistant fluids and/or they exhibit a lower propensity to propagate a fire after ignition. Thus, earlier researchers eventually developed two hydrocarbon-based fire-resistant hydraulic fluids to meet B-52 requirements, and these synthetic fluids were also compatible with the systems of other aircraft using the flammable, MILPRF- 5606 fluid. Ultimately, a materials development program instituted in the 1960s and extending through the 70s led to development of MIL-PRF-83282, a hydraulic fluid compatible with, and an appropriate drain-and-fill replacement for, MIL-PRF-5606. Another advantage of the newer, MIL-PRF-83282 fluid was that it did not require any retrofit of hydraulic system materials or components.
Air Force (AF) officials subsequently converted all aircraft—except those required to be airborne on short notice—to MIL-PRF-83282. They did not convert aircraft meeting the shortnotice criteria because the MIL-PRF- 83282 fluid exhibits a higher viscosity at -65°F than does MIL-PRF-5606 at the same temperature; therefore, aircraft operating in these extreme temperatures require longer warm-up times prior to takeoff. Since military planners considered the longer time-to-takeoff unacceptable, these short-notice aircraft continued to use the more flammable, MIL-PRF-5606 fluid.
Operational commands consequently developed a requirement for a compatible, drain- and-fill hydraulic fluid with improved fire resistance and lowertemperature viscosity to replace MILPRF- 5606. AFRL scientists responded by employing a modified synthetic hydrocarbon, polyalphaolefin, to develop MIL-PRF- 87527, a fluid that extensive testing and evaluation have since proven is an appropriate replacement for MIL-PRF-5606.
AFRL scientists and engineers have continually sought to develop and promote improved fluids and lubricants for field application because of their safety, cost, and operational benefits to both the warfighter and the aircraft maintainer. With the conversion of the B-52 to a fire-resistant hydraulic fluid, only a few AF aircraft continue to use the flammable, MIL-PRF-5606 fluid. AFRL experts hope that these aircraft will convert to one of the safer, superiorperforming, fire-resistant hydraulic fluids in the near future.
Mr. Carl E. Snyder, Jr., Ms. Lois J. Gschwender, Dr. Shashi K. Sharma, and Mr. Tim Anderl (Anteon Corporation), of the Air Force Research Laboratory 's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document ML-04-31.