The engine starting system controls the sequence of the engine start cycle. It supplies electrical energy to the engine ignition system, fuel to the engine fuel system, and high-pressure air to the engine cranking system.

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The cranking system turns the engine for engine starts or for motoring. Bleed air that is supplied to the air turbine starter (ATS) gives the necessary torque to turn the N2 engine shaft.

The system can use air pressure from the auxiliary power unit (APU), one engine, or from a ground air supply to start an engine. Bleed air from the APU or an engine that operates can be used in flight or on the ground to start one other engine. For crossbleed starts on the ground, the engine must be set to supply 54% fan speed. The engine for crossbleed starts in flight must be set to supply air between 65% and 97.6% fan speed

The pilot operates the START switch to start the related engine. The start switch is a three-position switch (CRANK/OFF/START). In a usual start sequence, the switch must be set and held in the START position for one second. The CRANK position is set to turn the engine with no ignition to do air motoring and a fire test. Motoring is done for engine maintenance.

The START switch operates the electronic control unit (ECU) from the full authority digital engine control (FADEC) system and the controller from the integrated air system (IAS).

When the switch is set to CRANK or START, the IAS controller automatically closes the flow control valves and opens the crossbleed valve. All of the bleed air is supplied for the engine start. The ECU sends the input to energize the solenoid of the starter control valve. Bleed air from the starter control valve goes to the air turbine starter (ATS) which increases the speed of the N2 engine shaft. At this point, if the switch is set to START then the ignition can occur . If the switch is set to CRANK, then no engine start occurs. The IAS controller opens the flow control valves again when the bleed-air pressure to the ATS is 10 psig or more.

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The starting system can make usual ground starts for altitudes from sea level to 15,000 ft (4,572 m). In flight starts can be made to a maximum of 30,000 ft (9,144 m). The lower recommended altitude for starts is 5,000 ft (1,524 m) above sea level.

The engine starting system has these functions:

• Increase the speed of the engine core (N2 shaft) and connected airframe accessories to the engine light-off speed.

• Help to increase the speed of engine and connected airframe accessories from the light-off (14.2% N2) to starter cutout speed (49.8% N2).
  
  
• Control the exhaust gas temperature (EGT) when the engine is started on the ground.

Air Turbine Starter

A V-coupling holds the air turbine starter (ATS) to the mounting flange on the gearbox There is an output shaft with splines between the ATS and the related gear wheel in the accessory gearbox. The output shaft is made to be the weak link in the transmission of power between the starter and the engine. Thus, if there is an overload during the operation of the starter, the output shaft breaks to prevent damage to other components.

The ATS includes an overrun clutch, which disengages the ATS from the output shaft when the transmission of power changes (the accessory drive gearbox turns the ATS). At 49.8% N2, the clutch disengages and the ATS stops.

When the engine stops, the output shaft slowly decreases the speed to engage the ATS for the subsequent start cycle.

The ATS has a reduction gearbox. The gearbox has a self-contained oil supply with the related fill plugs and drain plug.

The ATS receives an airflow through the inlet housing from the starter control valve. The air goes into the ATS and expands across the turbine.

The ATS has a turbine wheel that changes the airflow energy to torque. The torque turns the drive shaft of the accessory gearbox. The accessory gearbox is connected to the high-pressure rotor of the engine through the tower drive shaft. The high-pressure rotor of the engine turns with the accessory drive gearbox.

Air Turbine Starter Duct

The air turbine starter duct is on the forward part of the ATS. It has a collector ring and a exhaust duct. The turbine starter exhausts its air into the collector ring. The collector ring directs the air into the duct which sends it out through the bottom of the engine nacelle.

Starter Control Valve

The starter control valve is solenoid operated and controls the flow of air from the ATS duct to the ATS. V-coupling holds the starter valve on the related flange on the forward end of the ATS. Starter valve has a butterfly valve that is operated by servo pressure to the open position and by spring pressure to the closed position. It has one electrical connection and two air connections (one for the air in and one for the air out).

The valve controls a constant downstream pressure of approximately 34 psig (234.42 kPa) with a pressure regulator. The electronic control unit (ECU) controls an electrical input to the starter solenoid valve which energizes the solenoid. When the inlet of the valve has sufficient air pressure to give the necessary servo pressure, the valve opens to supply air to the ATS. When the solenoid de-energizes, the servo pressure no longer holds the butterfly valve open, spring pressure then moves it to the closed position. The valve has a position switch to give a signal to the ECU when the butterfly valve is in the closed position.

The starter control valve has a manual override device so that the valve can be operated manually without the solenoid.

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To start the engine on the ground, make sure the aircraft has electrical and air power. The air power can be from a ground air supply, the auxiliary power unit (APU) or from an engine cross-bleed.

The engine control panel and the throttle quadrant are the control panels in the flight compartment which are used for starts. There are three switches to start the engine: RUN, START and IGNITION switch. The three switches give five different procedures to start the engine. The START switch has three positions (start/crank/off). To start the engine, set the related RUN switch to on. This turns on the fuel DC boost-pump and operates the ECU start cycle. Set START switch to the start position and hold it for one second to operate the usual ground or air start.

The start sequence is automatically started by the ECU which controls ignition, engine feed-shutoff valve and starter cutout signal. After the ignition of fuel in the combustion case, the ATS disengages from the gearbox. The ECU sends a signal to the generator control unit (GCU) of the aircraft after the ATS has disengaged.

The control starter valve opens fully at approximately 3.6% N2 and bleed air goes to the ATS. At the engine light-off (approximately 14.2% N2), the starting system gives the necessary torque to motor the engine.

After the ignition, the torque caused by the engine is not sufficient to turn the engine. At the minimum self sustaining speed (approximately 35.8% N2) the engine has the necessary torque to turn. At 45% supply of air stops. If the speed increases as usually, at 49.8% N2 the ATS disengages from the gearbox. Then, the engine increases the speed until the ground idle speed.

The L (R) START ABORTED caution message will show when the related ground engine start has been aborted.

The L (R) STARTER FAIL caution message will show when the related engine starter has failed.

The L (R) STARTER FAIL ON caution message will show when the related engine starter has failed ON.

The L (R) ENG IGN FAULT advisory message will show when the related engine ignition system has a loss of redundancy.

The following EICAS messages are related to the engine starting system: 

EICAS MESSAGE(S)	LEVEL (COLOR)
L START ABORTED	CAUTION (amber)
R START ABORTED	CAUTION (amber)
L STARTER FAIL	CAUTION (amber)
R STARTER FAIL	CAUTION (amber)
L STARTER FAIL ON	CAUTION (amber)
R STARTER FAIL ON	CAUTION (amber)
L ENG IGN FAULT	ADVISORY (cyan)
R ENG IGN FAULT	ADVISORY (cyan)

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The engine starting system interfaces with the following components:

• Engine Control Panel
• Throttle Quadrant
• DC Boost-Pump Cartridge
• Engine-Feed Shutoff Valve
• Integrated Air System (IAS) Controller
• Flow Control Valve
• Cross-Bleed Valve
• Electronic Control Unit

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