AEROCONSULT LTD
AEROCONSULT LTD
737-300/400/500 (CLASSIC) TYPE
TRAINING
ATA 74- ENGINE IGNITION
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ENGINE IGNITION -- INTRODUCTION
Purpose
The ignition provides a high current electrical spark to ignite the fuel/air mixture in the
engine combustion chamber during starting. Takeoff, landing and operation in adverse
weather conditions requires continuous ignition.
Location
The ignition system controls are in the flight compartment on the P5 forward overhead
panel and the control stand.
General Description
The two ignition exciters are on the left engine fan case. An igniter plug is on each side
of the engine.
The controls on the P5 panel are:
- Two 4 position rotary engine start switches
- One 3 position toggle ignition selection switch.
The engine start switch has these selections:
- GRD, normal engine starting
- OFF, ignition not connected
- CONT, continuous ignition
- FLT, engine starting in flight.
The ignition selection switch allows selection of left or right igniter, or both igniters.
The engine start levers operate ignition switches.
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ENGINE IGNITION -- IGNITION SWITCHES - GENERAL DESCRIPTION
Purpose
The ignition switches control power to the ignition system for engine starting and
continuous ignition operation.
Location
The four ignition switches (two for each engine) are in the control stand below the
engine controls. Access to the switches is through a left and right access panel
in the control stand.
General Description
The ignition switches are microswitches which close to allow power to the ignition
system.
The switches are mechanically actuated by the operation of the start levers. The cams
attached to a start control drum actuate the switches. As the start lever is moved, the
linkage rotates the drum allowing cams to actuate the switches.
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ENGINE IGNITION -- IGNITION EXCITERS - GENERAL DESCRIPTION
Purpose
The two exciters supply starting and continuous duty ignition on demand. Each exciter
can operate independently. Alternate use of exciters is recommended.
Location
The two ignition exciters are near the top of the right fan case.
General Description
The ignition exciter is a capacitor discharge type exciter. A housing encloses the exciter
and components which are secured mechanically or with silicone adhesive to the
housing. The housing is hermetically sealed for proper operation under varying
environmental conditions.
The power for the exciter operation is 115v ac from standby and transfer buses. The
input ac power is rectified and stored in the capacitors. When the capacitors get to 2850
volts each, a combined voltage of 5700 volts is discharged. The discharge process
repeats at a rate that depends on the charge time of the capacitors.
WARNING:
EXCITERS MUST BE DEENERGIZED BEFORE WORKING ON IGNITION SYSTEM.
VOLTAGE OUTPUT CAN BE DANGEROUS AND ELECTRICAL CONTACTS MUST
NOT BE TOUCHED. THE EXCITERS MAY CONTAIN AN ELECTRICAL CHARGE,
EVEN WHEN NOT ENERGIZED
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ENGINE IGNITION -- HIGH TENSION DISTRIBUTION SYSTEM - GENERAL
DESCRIPTION
Purpose
The high tension distribution system conducts the high voltage/high current electrical
energy from the ignition exciters to the igniters.
Location
The ignition leads go from the ignition exciters through a horizontal fan strut to the two
igniter plugs.
Physical Description
The ignition leads are made of insulated wire in a sealed flexible conduit. Each lead has
a cooling shroud at the connection to the igniter. The shroud is in two sections held
together by a clamp. The cooling air is from the booster compressor in the fan strut.
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ENGINE IGNITION -- IGNITER SYSTEM - FUNCTIONAL DESCRIPTION
Functional Description
The electrical power for start system is from 28v dc battery bus, 115v ac transfer bus,
and 115v ac standby bus. The start and ignition selector switches control the system. A
solenoid holds the start switch in the GND position. The ignition switch permits selection
of one or two igniters for engine start.
The fuel and ignition come on when you move the start lever out of the CUTOFF detent
when the N2 speed gets to 20% rpm and N1 turns. Engine light-up occurs shortly after
the start lever movement. At cutout speed the holding coil releases the start switch, and
it goes back to the OFF position.
To keep continued combustion during takeoff, inclement weather, and landing, one or
the other igniter plug can be used. You select continuous ignition when you put the start
switch to the CONT position.
To use ignition for an in-flight start of an engine, put the start switch in the FLT position.
Then put engine start lever to the ADV position. When the start switch is in the FLT
position, there is a bypass of the ignition selector switch, and the in-flight start uses
the two ignitors.
B737-300/400/500 (CL) TYPE TRAINING 8
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