Tribotechnical Diagnostics of Aircraft Emergency Hydrogenerator NP-27 T

Aircraft emergency hydraulic pumps are safety-critical units whose intermittent operation complicates condition assessment and reduces the diagnostic value of conventional bulk physicochemical oil properties. This study evaluates the applicability of tribotechnical oil analysis for monitoring degradation of the NP-27T emergency hydraulic pump under controlled bench conditions. Four NP-27T units were tested on a dedicated hydraulic bench, and oil samples were collected at defined intervals during operation and after test completion. The diagnostic methodology combined elemental spectrometry (ICP-OES), particle counting interpreted with reference to ISO 4406, and analytical ferrography. The results showed that flow performance deterioration was accompanied by measurable changes in oil-borne wear indicators, although the sensitivity of the individual diagnostic channels varied among the tested units. In several cases, the coarse particle fraction (>15 μm) exhibited the clearest response to degradation, while Fe and Cu concentrations provided useful but not uniformly monotonic trends across all pumps. Using a pragmatic early warning criterion based on the first exceedance of 100 particles/mL in the >15 μm fraction, the coarse particle signal provided lead times of approximately 13–345 min before the flow-based rejection limit was reached in the four tested units. Ferrographic analysis identified cutting and fatigue debris, together with larger wear particles, in units approaching or reaching the flow-based rejection limit. Overall, the findings demonstrate that the combined use of elemental analysis, particle counting, and ferrography provides a practical multi-indicator framework for relating oil-diagnostic signals to functional degradation of the NP-27T hydrogenerator. Under the present bench conditions, flow proved to be a more sensitive degradation indicator than pressure. The proposed approach therefore represents a useful complementary tool for maintenance decision-making and for integration with vibration-based condition monitoring of aircraft hydraulic systems.