BOEING 737 MAINTENANCE MANUAL

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PNEUMATIC STARTING SYSTEM - DESCRIPTION AND OPERATION
1. General
A. The pneumatic starting system provides means for rotating the engine to the rpm range where
starting can be accomplished when fuel and ignition are supplied.
B. The system consists of two pneumatic starters two starter valves, and the associated
pneumatic plumbing. It is controlled by two start switches, located on the forward overhead
panel. For component location, (see Fig. 1).
C. With air pressure in the pneumatic manifold, actuating the engine start switch will supply
electrical power to open the starter valve. Low pressure air acts on the turbine blades of the
pneumatic starter causing it to rotate. Rotation of the starter is transmitted to the N2
compressor through the accessory drive gear system. When the engine has accelerated to
starting speed, application of fuel and ignition, by advancing the start lever, result s in the
engine starting. For engine start procedure, see Chapter 71, Power Plant. At starter cutout
speed, electrical power is interrupted mechanically, the starter valve closes, and the starting
cycle is ended.
D. The pneumatic starting system can use low pressure air from three separate sources. Normally,
the engines are started with bleed air from the auxiliary power unit (APU). Secondly, the low
pressure air can be obtained from a ground source through the ground pneumatic connection
(See Fig. 1). The third source of low pressure air is the cross-bleed air from an operating
engine. The cross-bleed starting is not desirable because the operating engine must operate at
approximately 80% power setting to develop adequate air pressure for starting the other
engine.

2. Pneumatic Starter
A. The pneumatic starter (Fig. 2) is a lightweight turbine-type air motor which converts the kinetic
energy of compressed air into starting torque sufficient to accelerate the engine to starting
speed. Low pressure air and electrical power are required for starter operation. The starter will
continue to assist the engine until electrical power is removed.
B. The starter consists of a scroll assembly, turbine wheel, reduction gear assembly, engaging
mechanism and an output shaft. The starter is fitted with a start valve to control the inlet airflow.
When the valve is open, it admits air to the inlet connection on the starter scroll assembly; the
air then passes through the starter vanes of the scroll assembly and is directed radially inward
through the turbine wheel imparting high-speed rotation. Exhaust air from the turbine wheel
then passes through an air outlet screen.
C. The reduction gear train translates the high speed, low torque of the turbine wheel into low
speed, high torque. This output is transmitted through a pawl and ratchet engagement
mechanism to the output shaft. From the starter output shaft the cranking torque is transmitted
to the N2 compressor by way of the accessory drive gears. A clutch mechanism provides
engagement of the reduction gear train with the output shaft for engine starting; when the
speed of the output shaft exceeds the speed of the internal gear hub, the clutch mechanism
overruns, thus providing automatic disengagement. The pneumatic starter is mounted on the
accessory drive gear case beneath the engine.

3. Starter Valve (Fig. 3)
A. The starter valve controls airflow to the pneumatic starter during engine starting. The valve is
pneumatically operated and solenoid-controlled. It consists essentially of two assemblies; a
valve body assembly with lever-operated butterfly valve, and a piston type pneumatic actuator
assembly. The valve is located in the pneumatic duct to the starter. It is attached to the duct
and starter inlet flange by clamps (Fig. 3).
B. Valve P/N 898172-1-1 (AiResearch)
(1) Valve inlet pressure is directed tttttttttthrough a pressure port, filter, and orifice, into a balltype
solenoid-actuated switcher and through the pneumatic switcher to chamber B. With
solenoid de-energized, chamber A is vented and inlet pressure in chamber B keeps the
butterfly closed. With solenoid energized, the ball closes the vent, and inlet pressure is
ported to chamber A and to the actuator of the pneumatic switcher. The pneumatic
switcher closes off inlet pressure to chamber B and connects that chamber to sensing
selector.
(2) As chamber B vents through the bleed orifice, pressure differential across the actuator
piston moves the butterfly toward open position. At inlet pressures lower than set valve
(36 +5 psig), sensing selector remains closed and keeps the butterfly in fully open
position. When inlet pressures equal or exceed set valve, bleedoff reference regulator
opens to maintain a constant reference pressure in chamber A, and the sensing selector
opens to direct downstream pressure to chamber B. This modulates the butterfly position
to maintain the desired downstream pressure.
(3) When solenoid is de-energized, pneumatic switcher and chamber A are quickly vented
and upstream pressure is ported to chamber B or closing side of the actuator, causing
rapid closing of the butterfly. As downstream pressure drops, sensing selector closes,
chamber B pressure rises, assisting the torsion spring in closing the valve completely.
(4) An additional pneumatic bleed located on the upstream side of the butterfly vane directs
hot air onto the solenoid switcher and pneumatic switcher housings to prevent icing and to
reduce moisture during cold weather operation.

3. Starter Valve (Fig. 3)
A. The starter valve controls airflow to the pneumatic starter during engine starting. The valve is
pneumatically operated and solenoid-controlled. It consists essentially of two assemblies; a
valve body assembly with lever-operated butterfly valve, and a piston type pneumatic actuator
assembly. The valve is located in the pneumatic duct to the starter. It is attached to the duct
and starter inlet flange by clamps (Fig. 3).
B. Valve P/N 2730426 (Parker Hannifin)
(1) Valve opening is initiated by energizing the solenoid. The energized solenoid moves the 3-
way poppet to close the vent to ambient and allow control air to pass through an orifice
and into the opening side chamber of the actuator diaphragm. The control air pressure
exerts a force on the actuator diaphragm and overcomes the spring closure force thereby
opening the butterfly. A slow opening rate is achieved by restricting the flow of high
pressure air by the upstream orifice.
(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.

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.