Stuck servovalves were observed during pre-flight checks in some aircraft using MIL-PRF-83282 hydraulic fluid. Analysis of hydraulic fluid samples and stuck valves as described in the paper provided a link between valve failures and barium dinonylnaphthalene sulfonate (BSN) content of the hydraulic fluid. The BSN contamination was traced to residual MIL-PRF-6083 preservative fluid that had not been drained from components before installation on the aircraft. The hypothesis was validated by laboratory tribological experiments using a reciprocating tribometer and grazing angle microscope Fourier transform infrared spectroscopy. Corrective actions included reemphasizing procedures to drain MIL-PRF-6083 from components and establishing a 10 ppm maximum limit for barium in new MIL-PRF-83282.
Since the corrective actions were taken, no valves have locked up, further validation that the preservative fluid, MIL-PRF-6083, as a contaminant in the operational aircraft, is the root cause of the valve lockups. The recommended corrective actions were: 1) removing and cleaning the hydraulic valves; 2) assuring that the barium content of the aircraft hydraulic fluid does not exceed 15 ppm.
Other factors besides the presence of BSN which contributed to the valve-sticking problem included: the elevated hydraulic fluid temperature, the valve design and materials, and the duty cycle. The simplest and most cost-effective solution to the problem was limiting the amount of BSN in the aircraft hydraulic system. Other aircraft hydraulic systems problems perceived to be caused by BSN contamination are discussed.