Stuck servovalves in aircraft hydraulic systems

Stuck servovalves were observed during preflight checkup in some aircraft u sing MIL-PRF-83282 hydraulic fluid. Analysis of hydraulic fluid samples and stuck valves provided a link between valve failures and barium dinonylnaph thalene sulfonate content of the hydraulic fluid. The barium dinonylnaphtha lene sulfonate contamination was traced to residual MIL-PRF-6083, the prese rvative fluid for components, which had not been drained from the component s prior to installation on the aircraft. Validation of that hypothesis was proven by laboratory tribological experiments using a reciprocating tribonm eter and grazing angle microscope Fourier transform infrared spectroscopy. Corrective actions included revising the operational procedures to drain MI L-PRF-6083 from components and establishing a 10 ppm maximum limit for bari um in new MIL-PRF-83282.