(3)  
For motoring the engine, the duty cycle is 2 minutes on and 5 minutes off.

F.  Engine Starting Using Auxiliary Power Unit
(1)  
Before an engine start is attempted, the pack valves and the wing thermal anti-ice valves must be closed, the applicable engine bleed valve must be open and the bleed air isolation valve switch must be in AUTO (Fig. 4).

(2)  
With the APU operating at governed speed, the APU bleed valve switch is positioned to ON. This opens the APU bleed valve and supplies compressed air to the starter valves. When the applicable engine start switch is positioned to GRD start, electrical power is supplied to the starter valve solenoid to open the valve, and through the normally closed contacts of the starter cutout switch, to energize the start switch holding solenoid. On some airplanes pressure downstream of the start valve actuates the start valve pressure switch and energizes the START VALVE OPEN light on the center instrument panel, indicating that the valve has opened (Fig. 5).

(3)  
When compressed air is supplied to the starter, the starter begins rotating the N2 compressor of the engine. The rotation of the N2 compressor causes rotation of the Nl compressor and the turbines. When the engine has accelerated to between 15 and 17% N2 rpm (15% minimum), fuel and ignition are supplied by advancing the start lever to the IDLE detent. Engine light up occurs within 20 seconds after the start lever movement. The starter continues to assist the engine until starter cutout speed is attained. At cutout speed (35 to 40% N2 rpm), the centrifugally operated cutout switch opens interrupts electrical power to the start switch holding solenoid. The start switch returns to OFF position, interrupting power to the starter valve, and the valve closes. The closing of the start valve is evidenced by sudden increase in pneumatic duct pressure which can be observed by monitoring the dual pressure indicator on the forward overhead panel and a closing sound shall be audible from the starter valve solenoid (Fig. 4). On some airplanes closing of the start valve also extinguishes the START VALVE OPEN light (Fig. 1). For engine starting and shutdown procedures, (Ref Chapter 71, Operating Procedure).

NOTE:  To ensure adequate supply of compressed air, both air conditioning packs must be off.
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CAUTION:  IF START VALVE LIGHT REMAINS ILLUMINATED, SHUT ENGINE DOWN IMMEDIATELY. CONTINUED ENGINE OPERATION WITH START VALVE LIGHT ILLUMINATED MAY RESULT IN DAMAGE DUE TO STARTER DISINTEGRATION.
DO NOT EXCEED THE RECOMMENDED STARTER DUTY CYCLE. FOR STARTER DUTY CYCLE REFER TO PARAGRAPH 4.E.
SHOULD THE ENGINE FAIL TO START AND ACCELERATE NORMALLY, DO NOT ATTEMPT ANOTHER STARTING OPERATION UNTIL THE ENGINE HAS COMPLETELY STOPPED ALL ROTATION. (ALLOW APPROXIMATELY 30 SECONDS AFTER THE TACHOMETER READS ZERO).
IF ENGINE START WAS ABORTED DUE TO HUNG START, CHECK POSITION OF TURBOFAN VALVE (REF 21-52-0) AND AIR CLEANER PURGE VALVE (REF 21-12-0). VALVES MUST BE IN CLOSED POSITION BEFORE ATTEMPTING ANOTHER START.
5B6 
Oct 20/84  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  80-11-0 Page 11 

Pneumatic Starting System Flow Diagram  508 
80-11-0  Figure 4  Nov 01/74 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5B6  Pneumatic Starting System Circuit 
Jan 20/83  Figure 5  80-11-0 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

(4)  A manual override button on starter valve permits opening the valve if it fails to open electrically. When the override button is actuated, the valve solenoid is overridden, and the valve opens pneumatically. The valve manual override button must be released when the N2 tachometer indicator reads 35 to 40% rpm. The valve then closes, and the starting cycle is terminated.
CAUTION:  IF THE MANUAL OVERRIDE BUTTON IS NOT RELEASED WHEN THE N2 TACHOMETER INDICATES 42% RPM, THE STARTER WILL OVERSPEED AND CAN BE SEVERLY DAMAGED. THE INTERPHONE SHALL BE USED FOR COMMUNICATIONS WITH THE CONTROL CABIN.
NOTE: For access information to the manual override button, refer to par. 3.
G.  Engine Starting Using External Air Source
(1)  This type of engine starting is dependent upon having external low pressure air source connected to the ground pneumatic connection. The procedure for starting is the same as using the APU except that bleed air valve for the applicable engine must be opened manually (Ref Chapter 36) if ac power is not available.
H.  Engine Starting Using Engine Cross-Bleed Air
　
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