Downloaded from https://www.everyspec.com ADS-79D-HDBK The reduction gearbox assembly, mounted to the output shaft, houses the reduction gear train required for use in driving APU accessories and customer-furnished equipment (i.e., APU starter, electrical generator, pneumatic pump, hydraulic pump). Additionally, the gearbox serves as an oil sump for the APU self-contained lubrication system. Controls and accessories include those for APU operation (i.e., Electrical Sequencing Unit (ESU), fuel system, lubrication system, and ignition system).
More advanced APUs include a fully- automatic electronic control system that properly sequences control of fuel and ignition during starting and operation. On ESU equipped APUs, APU speed is regulated by the ESU that directs delivery of the correct amount of fuel regardless of ambient conditions and load requirements. Overspeed protection is provided by electronic overspeed shutdown logic that is automatically actuated within safe limits at a predetermined speed. G.5 INTRODUCTION Turboshaft engine monitoring systems have evolved significantly over the past four decades.
Simple engine monitoring systems (EMSs) that reported a few engine parameters such as oil pressure and exhaust gas temperature to the pilot have now become sophisticated engine health monitoring systems (EHMS) that record engine usage, diagnose faults, and predict the time to component failure (see Table G-I). These systems may reduce maintenance costs and improve safety when part of an engine CBM program. In the ―on condition‖ maintenance approach, engine service is typically performed based on indications from the monitoring system rather than at a predetermined time between overhaul (TBO) interval.
As long as the system reports that the engine is in good health, the engine can remain in operation until it reaches its performance limits or component life limits. 26 FIGURE G-3. Auxiliary power unit (reproduced from TM 55-2835-205-23 27 ) 26 SAE AIR 1873 ―Guide to Limited Engine Monitoring Systems for Aircraft Gas Turbine Engines‖, pg. 4. 27 TM 55-2835-205-23 Technical Manual, Aviation Unit and Intermediate Maintenance, Gas Turbine Engine (Auxiliary Power Unit – APU) Model T-62T-2B, Headquarters, Dept. of the Army, March 14, 1983. 116