Downloaded from https://www.everyspec.com ADS-79D-HDBK assess the level of filter blockage and prognosticate the need for filter replacement / additional mechanical system maintenance. For guidance on lubrication condition, debris, and filter monitoring refer to Appendix M. G.6.4 Oil Level Monitoring. Turboshaft EMS/EMHS systems should automatically monitor oil levels and prognosticate the need for oil servicing when limits on consumption rates are exceeded. The monitoring system should be capable of providing accurate readings during normal vehicle operation so as not to provide maneuver driven or inaccurate oil level/consumption indications. G.6.5 Oil Pressure Monitoring.
Turboshaft EMS/EMHS systems should monitor oil pressure and indicate / prognosticate exceedance trends of maximum/minimum limits during both steady state and transient operations. G.6.6 Fuel Contamination Monitoring. Turboshaft engines have complex and sensitive fuel control systems and a series of high pressure fuel lines and nozzles that are sensitive to fuel contamination. EMS and EMHS should incorporate means to identify the faults caused by fuel contamination (typically shown by erratic fuel pressure, erratic fuel flow rate, or measured turbine temperatures).
Direct tests for fuel contamination through in-line sensors similar to oil sensors are potentially viable, but such sensors require validation to standards such as ASTM D2276. Fuel contamination can arise from the presence of water, particulates, or microbiological organisms. Current practice is to subject fuel to extensive filtration and storage processes that eliminate the presence of contaminants. As the technology for sensors advances, the use of sensors to detect fuel contamination at the aircraft may be effective as a last line of defense.
FMECA and maintenance history analysis should be used to assess the cost and benefit for using fuel contamination sensors as part of a CBM program for engines. G.6.7 Fuel Filter Monitoring. The condition of the fuel filter should be monitored on turboshaft engines to assess level of filter blockage, fuel flow, and fuel actuator capability to prognosticate the need for filter replacement / fuel system maintenance. G.6.8 Fuel Pressure Monitoring. Fuel Pressure should be monitored on turboshaft engines to indicate / prognosticate trends towards exceedance of maximum/minimum limits during both steady state and transient operations. G.6.9 Auxiliary Power Unit Monitoring.
Diagnostic algorithms for APUs have different requirements compared to bigger turbine engines. Especially in older APUs, the instrumentation selected is typically based on the control system requirements and not for health monitoring purposes 33 . Thus, the health monitoring and diagnostic algorithms have only limited data with which to work. Typically monitored parameters include speed, exhaust gas temperature, oil temperature, and discrete aircraft commands such as APU start/stop and main engine start (MES). Performance trending can be used to assess the health of an APU under consistent and repeatable conditions, such as MES.
The monitored parameters can be compared to baseline values generated by performance models of the MES condition. The results of the comparison are then trended over time to show the degradation in performance. Sudden changes in the slope of the performance trends are indicative of faults or failures. Efforts are currently underway to develop diagnostic and prognostic 33 Gorinevsky, D., K. Dittmar, M. Dinkar, N. Emmanuel, ―Model –Based Diagnostics for an Aircraft Auxiliary Power Unit,‖ Presented at IEEE Conference on Control Applications, Glasgow, Scotland, Sept. 18-20, 2002, pg. 2. 121