(2)
When upstream pressure is not available at the actuator diaphragm, a compression spring exerts a closing force on the actuator diaphragm to move or hold the butterfly in the closed position. The open side of the diaphragm is vented to ambient through the open passage to the 3-way solenoid positioned valve to prevent any force during the shutoff operation.
(3)
On airplanes with start valve open light on center instrument panel, the shutoff valve includes an additional port downstream of the butterfly valve for connecting a START VALVE OPEN light pressure switch (Fig. 1 and 3).
C. The solenoid switcher can be operated by the manual override button when the solenoid malfunctions (Fig. 3, Detail A). The valve also incorporates a second override feature that requires opening the engine left cowl panel CSD servicing access door and turning the valve butterfly position indicator with an allen wrench (Fig. 3, Detail B). A hole forward of the CSD servicing access door in the cowl panel provides access to the valve manual override button.
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4. Operation
A. Electrical power for the starting system is obtained from the 28-volt dc battery bus (Fig. 5). The system is controlled by two start switches, one for each engine. The start switches are four position rotary switches, located on the forward overhead panel (Fig. 1). The four positions are: GRD start, OFF, LOW IGN, and FLT start. The switches are momentary in ground start position. Once a switch is positioned to ground start, a holding solenoid in the switch is energized and holds the momentary contacts closed. When the starter cutout switch opens at cutout speed, the holding solenoid is de-energized, and the start switch returns to off position. In all other positions the switch will remain as positioned until another selection is made (Fig. 5).
B. Other controls used in conjunction with the starting system are the air conditioning control switches, wing thermal anti-ice control switches and the start levers. Refer to Chapter 21, Air Conditioning; Chapter 28, Fuel; Chapter 30, Ice and Rain Protection; Chapter 36, Pneumatics; Chapter 49, Airborne Auxiliary Power Unit; and Chapter 74, Ignition for integration of these systems with Chapter 80, Starting.
C. Engine starting can be performed in the following ways:
(1)
Using auxiliary power unit (APU).
(2)
Using ground air source.
(3)
Using bleed-air from the other engine.
D. Starter valve inlet pressure limits:
NOTE: The maximum pressure limits shown in the table are just guides to normally expected values of these pneumatic starting sources.
AIR SOURCE FOR PNEUMATIC STARTING CONTROL CABIN GAGE READINGS PRIOR TO VALVE OPENING
MINIMUM PSIG MAXIMUM PSIG
Onboard APU 30.0 53.0
Ground Cart (Mobile Type) 30.0 55.0
Other Ground Supply (Permanent Type) 30.0 48.0
Cross-bleed from Other Engine 30.0 48.0
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E. The pneumatic starter duty cycles are as follows:
(1)
For normal starting, the duty cycle is 30 seconds (maximum) on and 60 seconds off.
(2)
For slow starting engine, the duty cycle can be extended to a maximum of 60 seconds on at speeds up to starter cutout speed (35 to 40% N2 rpm), and 60 seconds off. The extended duty cycle can be repeated once and then a 5-minute cooling period must be observed between extended duty cycle starts.
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