During the development of the fire-resistant hydraulic fluids for DoD (Department of Defense) aircraft, candidate fluids were formulated and then characterized initially by various laboratory bench tests. Endurance pump tests were then conducted on the candidate fluids to validate their performance. Pump tests under accelerated conditions were conducted on three candidate fire-resistant hydraulic fluids and several candidate nonflammable hydraulic fluids, using pressure-compensated axial flow piston pumps and an instrumented, small-volume, closed-loop hydraulic test stand. Case-drain temperature and signal-noise on the pump outlet pressure and the case-drain flow provided better failure indicators than the case-drain flow. The fluids containing viscosity-index-improvers underwent large viscosity losses. No viscosity loss was observed for the fluids that did not contain the viscosity-index-improvers. Lower viscosity fluids performed as well as the higher viscosity fluids in these pump tests. However, the ball-bearing material and the heat treatment had a bigger impact on pump life. Use of an M-50 bearing in place of the AISI-52100 bearing doubled the pump life.
Based upon the laboratory screening tests and the validation pump tests, the best performing fire-resistant hydraulic fluid was recommended for use in the DoD aircraft. Performance of the recommended fluid was further demonstrated in successful aircraft flight tests. Only limited hydraulic system simulator and aircraft subsystem testing has been conducted on the best-performing nonflammable hydraulic fluid candidate to date, but the performance was acceptable.