ENGINE STARTING SYSTEM

TM 1-1520-238-10 Change 9 2-29 ENG START switch is placed to START, pneumatic mo- toring of the engine starter takes place and the ignition system is energized. Ignition cutout is automatic after the engine starts. Following aborted starts (Chapter 9, Emer- gency Procedures), the engine must be motored with the ignition system disabled. This is done by placing the ENG START switch to IGN OVRD. Chapter 5 contains the start- ing cycle limitations. 2.30 ENGINE STARTING SYSTEM. The engine uses an air turbine starter (fig 2-16) for engine starting. System components for starting consist of the engine starter, a start control valve, an external start con- nector, check valves, controls, and ducting. Three sources may provide air for engine starts: The shaft driv- en compressor (SDC) (normally driven by the APU for en- gine starts), No. 1 engine compressor bleed air, or an ex- ternally connected ground source. In any case, the start sequence is the same. With the ENG FUEL switch ON and MASTER IGN switch ON, placing the ENG START switch momentarily to START will initiate an automatic start sequence. This will be evidenced by the illumination of the ENG 1 or ENG 2 advisory light above the ENG START switch on the pilot PWR lever quadrant. Com- pressed air is then directed through the start control valve to the air turbine starter. As the air turbine starter begins to turn, an overrun clutch engages which causes the engine to motor. The starter turbine wheel and gear train auto- matically disengage from the engine when engine speed exceeds starter input speed. At approximately 52% N_G, air to the starter shuts off, and ignition is terminated. If the engine does not start, the PWR lever must be returned to OFF and the ENG START switch must be placed at IGN OVRD (which aborts the automatic engine start se- quence) momentarily before another start is attempted. If the engine is equipped with a DECU , fuel flow to the engine will be automatically shut off if TGT exceeds 900 �C during the start sequence. If this occurs, the PWR lever must be returned to OFF and the ENG START switch must be placed in IGN OVRD (which aborts the automatic engine start sequence). 2.30.1 Engine Start Using APU. The APU provides on-board power for system check by ground personnel. The APU is capable of driving the main transmission ac- cessories which include the ac generators, the hydraulic pumps, and the shaft driven compressor (SDC). The APU is normally left on during both engine starts, but it may be shut down after one engine has reached 100% N_p. When N_p has reached 100% and the APU shut down, that en- gine may be used to drive the SDC through the transmis- sion and accessory gear train for starting the second en- gine. A complete description of the auxiliary power unit appears in Section XII. 2.30.2 Engine Start Using External Source. An exter- nal air receptacle under the No. 1 engine nacelle provides an attachment point for an air line to start either engine from an external source. The assembly contains a check valve to prevent engine bleed air or SDC pressurized air from being vented overboard. An external air source of 40 psig and 30 pounds per minute (ppm) pressurizes the start system up to the engine start control valves which re- quires only that electrical power be applied for a normal start sequence. 2.30.3 Engine Start Using Engine Bleed Air Source. When the No. 1 engine is operating and it is necessary to start the No. 2 engine, bleed air may be used from the No. 1 engine compressor under certain circum- stances. This technique is not normally used; however, it is fully automatic, and provides an alternative starting ca- pability if the SDC shaft fails or if the SDC throttle valve or surge valve clogs (Section VI, Pressurized Air System). The starting sequence is the same as for APU starting; only the source of air to the start motor is different. When using this technique to ensure an adequate air pressure and flow to the No. 2 engine starter, collective pitch must be increased to a value that will increase the N_G of the No. 1 engine to a minimum of 95%.