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BOEING 737 SYSTEMS REVIEW
Page 1
POWERPLANT
1. GENERAL
¡ The aircraft is powered by two CFM International high bypass ratio turbofan engines.
¡ The engine is a dual rotor assembly consisting of a fan rotor (N1) and a compressor rotor (N2).
• The N1 rotor consists of a single stage fan and a three stage booster section connected by a
through shaft to a four stage low pressure turbine.
• The N2 rotor is a nine stage axial flow compressor connected by a through shaft to a single
stage high pressure turbine. The first four stages of the compressor are variable.
¡ Fan air and combustion gasses exit through separate nozzles at the rear of the engine.
737 Classics PMC
¡ The Main Engine Control (MEC) schedules fuel to provide the thrust called for by the Thrust Lever
setting. This fuel flow is further refined electronically by the Power Management Control (PMC)
without moving the Thrust Levers.
737 NG FADEC
¡ The Full Authority Digital Electronic Control (FADEC) schedules fuel to provide the thrust called for by
the Thrust Lever setting. The FADEC can adjust the fuel flow without moving the Forward Thrust
Levers.
Reverser
¡ A sliding sleeve, fixed vane thrust reverser system is installed which redirects bypass fan air to aid in
stopping the aircraft.
2. MAIN COMPONENTS & SUBSYSTEMS
737 CLASSICS POWER MANAGEMENT CONTROL
¡ The thrust control system consists of a hydromechanical MEC unit and a PMC unit mounted on each
engine. The PMC is an electronic system with limited authority over the MEC. The PMC uses MEC
thrust lever angle, N1 speed, inlet temperature and pressure to adjust, or trim, the MEC to obtain the
desired N1 speed. The PMC adjusts fuel flow as a function of thrust lever angle.
¡ The PMC provides a constant thrust climb feature. When thrust is set for the climb, the PMC
automatically maintains that thrust throughout the climb with no further thrust lever adjustments. If
the thrust lever is repositioned, the PMC maintains the setting corresponding to the new thrust lever
angle.
¡ The PMC includes failure detection and annunciation modules which detect PMC failures and provide
a signal to the crew. For detectable failure conditions, the PMC schedules a slow N1 drift over
approximately 30 seconds and then illuminates the PMC INOP Light, the ENG System Annunciator
and the MASTER CAUTION Lights. For a PMC failure, the PMC can be selected OFF by a P/B on
the aft overhead panel. The engine speed is then controlled by the hydromechanical MEC only. The
PMC INOP Light is suppressed below starter cutout engine speed (46% N2).
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737 CLASSICS IDLE RPM
¡ There are two engine idle speeds, low idle and high idle.
¡ The minimum engine speed for all flight phases is high idle, which varies with flight conditions. As
temperature and airspeed decrease, high idle speed also decreases. The average high idle setting is
approximately 32% N1.
¡ Engine idle speed is reduced to low idle, approximately 22% N1, four seconds after touchdown. This
delay is provided to enhance engine speed acceleration for reverse thrust.
737 NG FADEC
¡ The FADEC consist of an Electronic Engine Control (EEC) and a Hydraulic Mechanical Unit (HMU).
The EEC has two independent control channels and automatically switches channel if the operating
channel fails. With each engine start or start attempt, the EEC alternates between control channels.
The EEC receives thrust lever inputs to automatically control forward and reverse thrust and can
operate in Normal or Alternate mode.
¡ The EEC also provides N1 and N2 redline exceedance protection in both Normal and Alternate
modes but does not provide EGT redline exceedance protection.
EEC NORMAL MODE
¡ In the normal mode, the EEC uses sensed flight conditions and bleed air demand to calculate N1
values. The EEC compares commanded N1 to actual N1 and adjusts fuel flow until actual N1 equals
commanded N1.
¡ Full rated take-off thrust is available at a thrust lever position less than the forward stop. If the thrust
lever is advanced to the forward stop, the EEC limits thrust to the maximum certified thrust ratings for
current conditions.
EEC ALTERNATE MODE
¡ The EEC can operate in either of two alternate modes, soft or hard.
¡ If required signals are not available to operate in the normal mode, the EEC automatically changes to
the soft alternate mode. When this occurs, the ALTN switch illuminates and the ON indication
remains visible. In the soft alternate mode, the EEC uses the last valid flight conditions to define
engine parameters. Thrust rating shortfalls or exceedances may occur as flight conditions change.
The soft alternate mode remains active until the hard alternate mode is entered by either retarding the
thrust lever to idle or by manually selecting ALTN with the EEC switch on the aft overhead panel.
¡ When the hard alternate mode is entered, the EEC reverts to the alternate mode thrust schedule.
Hard alternate mode thrust is always equal to or greater than normal mode thrust for the same thrust
lever position. The maximum certified thrust rating can be exceeded. If the hard alternate mode is
entered by reducing the thrust lever to idle while in the soft alternate mode, the ALTN switch remains
illuminated and the ON indication remains visible. When ALTN is selected manually, the ON
indication is blanked.
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BOEING 737 SYSTEMS REVIEW
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737-800 IDLE RPM
¡ The EEC automatically selects ground minimum idle, flight minimum idle and approach idle.
¡ Ground minimum idle is selected for ground operations and flight minimum idle is selected for most
phases of flight.
¡ Approach idle is selected in flight if flaps are in landing configuration or engine anti-ice is ON for either
engine. Approach idle provides a higher % RPM than flight minimum idle improving engine
acceleration time in the event of a go-around. Approach idle is maintained until TBD-seconds after
touchdown, when engine idle speed is reduced to ground minimum idle.
¡ In flight, if a fault prevents the EEC from receiving flap or anti-ice signals, approach idle is active
below FL 150.
2. ENGINE FUEL SYSTEM
B737 CLASSICS
¡ Fuel leaves the fuel tank and enters through the Engine Fuel Shutoff Valve, located at the engine
mounting wing station. The Engine Fuel Shutoff Valve is electrically controlled by the Engine Start
Lever and the Engine Fire Warning Switch. When the engine fuel shutoff valve is closed, the FUEL
VALVE CLOSED Light located on the forward overhead panel illuminates dim.
¡ Fuel passes from the first stage of the engine driven fuel pump through a fuel/oil heat exchanger to a
filter. Provisions are made to bypass the heat exchanger or the filter in the event of failure or
blockage. Illumination of the FILTER BYPASS Light indicates an impending or actual bypass of the
fuel filter due to contamination.
¡ The second stage fuel pump provides high pressure fuel to the Main Engine Control (MEC). As the
fuel leaves the second stage fuel pump, a portion of the fuel is diverted to operate the MEC. This fuel
is filtered again and then routed through the fuel heater a second time. The fuel heater uses engine
oil to heat the fuel for anti-icing purposes.
¡ The MEC, in conjunction with the Power Management Control (PMC), uses thrust lever angle, fan
inlet pressure and temperature, N1 RPM and N2 RPM to meter the correct amount of fuel to the
combustor. Fuel flows from the MEC through the MEC fuel shutoff valve. The MEC fuel shutoff valve
is mechanically controlled by the Engine Start Lever. A fuel flow transmitter measures the rate of fuel
flow from the MEC.
737 NG
¡ Fuel leaves the fuel tank and enters through the Spar Fuel Shutoff Valve. The Spar Fuel Shutoff
Valve is electrically controlled by the Engine Start Lever and the Engine Fire Warning Switch. When
the Spar Fuel Shutoff Valve is closed, the SPAR VALVE CLOSED light located on the forward
overhead panel illuminates dim.
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¡ Fuel from the first stage fuel pump passes through two heat exchangers where the fuel is heated by
IDG oil and engine oil. Fuel then flows through a filter. Provisions are made to bypass the main
engine oil heat exchanger or the filter in the event of failure or blockage. Illumination of the FILTER
BYPASS Light indicates an impending or actual bypass of the fuel filter due to contamination.
¡ The second stage fuel pump provides high pressure fuel to the Hydraulic Mechanical Unit (HMU).
Excess fuel from the HMU is directed back to the main engine oil cooler.
¡ The EEC uses thrust lever angle, fan inlet pressure and temperature, N1 and N2 RPM to calculate
the correct amount of fuel. The EEC sends electric commands to the HMU. The HMU controls the
fuel flow. Fuel flows from the HMU through the Engine Fuel Shutoff Valve. The Engine Fuel Shutoff
Valve is controlled electrically by the engine start lever and the Engine Fire Warning Switch. When
the Engine Fuel Shutoff Valve is closed. The ENG VALVE CLOSED light located on the forward
overhead panel, illuminates dim. A fuel flow transmitter measures the rate of fuel flow from the HMU.
3. ENGINE OIL SYSTEM
¡ Oil from the engine oil tank is circulated through the engine to lubricate the engine bearings and
accessory gearbox. Oil quantity is displayed on the Oil Quantity Indicator.
¡ The oil system is pressurized by the engine driven oil pump. The oil leaves the oil pump, passes
through an oil filter, and continues to the engine bearings and gearbox. Sensors for the Oil Pressure
Indicator and the LOW OIL PRESSURE warning are located downstream of the oil filter, prior to
engine lubrication. The oil is returned to the oil tank by means of engine driven scavenge pumps.
From the scavenge pumps the oil passes through a scavenge filter. Should the filter become
contaminated, oil automatically bypasses the filter. Prior to the oil bypassing the filter, the OIL
FILTER BYPASS warning illuminates. The oil then passes through the fuel/oil heat exchanger where
it is cooled by engine fuel to maintain proper oil temperature prior to returning to the tank. Oil
temperature is displayed on the Oil Temperature Indicator and is measured B737 Classics in the
scavenge line; B737 NG in the supply line.
4. ENGINE START SYSTEM
¡ The engines may be started with air from the APU, from a ground source, or by using engine
crossbreed. The Engine Start Switch GRD position uses DC power from the battery bus to open the
starter valve and allow pressure from the pneumatic manifold to rotate the starter.
¡ When the starter valve is not closed, the amber START VALVE OPEN indication illuminates.
¡ The starter is a turbine-type air motor which rotates the N2 compressor through the accessory drive
gear system. When the Engine Start Lever is advanced to the IDLE position, fuel is supplied to the
combustor where the fuel ignites.
¡ At cutout speed (B737 Classics 46% N2 RPM; B737 NG 56% N2 RPM), power is interrupted to the
start switch holding solenoid, allowing the Engine Start Switch to return to the OFF position and the
starter valve to close.
¡ During an engine shutdown, the start switch holding-solenoid is held in the cutout position until
engine speed falls below 30% N2 RPM. The starter should not be re-engaged until engine speed has
decreased below 20% N2 RPM.
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BOEING 737 SYSTEMS REVIEW
Page 5
5. IGNITION SYSTEM
¡ Two high energy AC systems are provided. The ignitors can only be energized when the Engine
Start Lever is in the IDLE position. With the Engine Start Switch in the GRD position, the starter valve
opens and the selected igniter(s) are energized.
¡ The CONT position energizes the selected igniter(s).
¡ The FLT position energizes both igniters.
¡ The Ignition Select Switch selects either the LEFT, RIGHT or BOTH igniters for both engines. The
Ignition Select Switch is bypassed when the Engine Start Switch is in FLT.
• IGN L, powered by the AC transfer bus, provides single high energy ignition to the left igniter.
• IGN R, powered by the AC standby bus, provides single high energy ignition to the right igniter.
737 NG ABNORMAL START PROTECTION
¡ During ground starts, the EEC monitors engine parameters to detect impending hot starts, EGT start
limit exceedances, and wet starts. These protection features do not function during inflight starts.
¡ If an impending hot start is detected by a rapid rise in EGT or EGT approaching the start limit, the
white box surrounding the EGT digital readout flashes. The flashing white box resets when the start
lever is moved to CUTOFF or the engine reaches idle N2.
¡ If the EGT exceeds the starting limit, the EGT display, both box and dial, turn red. The EEC
automatically turns off the ignition and shuts off fuel to the engine. The alert terminates and the
display returns to white when EGT drops below the start limit. Following engine shutdown, the EGT
box turns red to remind the crew of the exceedance.
¡ A wet start occurs if the EGT does not rise after the start lever is moved to IDLE. If a wet start is
detected, the EEC turns off the ignition and shuts off fuel to engine 15 seconds after the start lever is
moved to IDLE.
737 NG Auto Relight
¡ An auto-relight capability is provided for flameout protection. Whenever the EEC detects an engine
flameout with the ENGINE START switches in OFF, both ignitors are activated. A flameout is
detected when an uncommanded rapid decrease in N2 occurs or N2 is between 57% and 50%.
6. THRUST REVERSER
¡ Each engine is equipped with a hydraulically operated thrust reverser, consisting of left and right
translating sleeves. Aft movement of the reverser sleeves causes blocker doors to deflect fan
discharge air forward, through fixed cascade vanes, producing reverse thrust.
¡ Hydraulic pressure for the operation of engine No. 1 and engine No. 2 thrust reversers comes from
hydraulic systems A and B, respectively. If hydraulic system A or B fails, alternate operation for the
affected thrust reverser is available through the standby hydraulic system. When the standby system
is used, the affected thrust reverser will deploy and retract at a slower rate and some thrust
asymmetry can be anticipated.
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BOEING 737 SYSTEMS REVIEW
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¡ The thrust reverser can be deployed when either radio altimeter senses less than 1 0 feet altitude, or
when the air/ground sensor is in the ground mode. Movement of the Reverse Thrust Levers is
mechanically restricted until the Forward Thrust Levers are in the idle position. When reverse thrust is
selected, the isolation valve opens and the thrust reverser control valve moves to the deploy position,
allowing hydraulic pressure to unlock and deploy the reverse sleeves. An interlock mechanism
restricts further movement of the Reverse Thrust Lever until the reverser sleeves have approached
the deployed position. The movement of the Reverse Thrust Levers into reverse thrust engages
locking pawls which prevent the Forward Thrust Levers from moving. Terminating reverse thrust
removes the locking pawls and restores forward thrust lever movement ability.
¡ When either reverser sleeve moves from the stowed and locked position, the 737 Classics
REVERSER UNLOCKED Light, located on the center instrument panel, illuminates; 737 NG amber
REV indication on the upper DU is displayed. As the thrust reverser reaches the deployed position,
the Reverse Thrust Lever can be raised to detent No. 2 and 737 NG the REV indication turns green.
This position provides adequate reverse thrust for normal operations. When necessary, the Reverse
Thrust Lever can be pulled beyond detent No. 2, providing maximum reverse thrust.
¡ Downward motion of the Reverse Thrust Lever past detent No. 1 will command the reverser to stow.
Once the thrust reverser is commanded closed, the control valve moves to the stow position allowing
hydraulic pressure to stow and lock the reverser sleeves. After the thrust reverser is stowed, the
isolation valve closes.
¡ The REVERSER Light, located on the aft overhead panel, illuminates when the thrust reverser is
commanded to stow and extinguishes 1 0 seconds later when the isolation valve closes. Anytime the
REVERSER Light illuminates for more than approximately 12 seconds, a malfunction has occurred
and the MASTER CAUTION and ENG System Annunciator Light illuminate.
¡ When the reverser sleeves are in the stowed position, a locking actuator inhibits motion of each
reverser sleeve until reverser extension is selected. Additionally, an auto-restow circuit compares the
actual reverser sleeve position and the command reverser position. In the event of incomplete
stowage or uncommanded movement of the reverser sleeves toward the deployed position, the autorestow circuit will open the isolation valve and command the control valve to the stow position
directing hydraulic pressure to stow the reverser sleeves. Once the auto-restow circuit is activated,
the REVERSER light will illuminate, the isolation valve remains open and the control valve is held in
the stowed position until the thrust reverser is deployed or until corrective maintenance action is
taken.
WARNING: Actuation of the thrust reversers on the ground without suitable precautions is dangerous
to ground personnel.
7. AIR BLEED SYSTEM
COMPRESSOR SECTION
¡ The N1 compressor, or booster section, produces low pressure air and delivers it to the N2
compressor which produces high pressure air. The single stage fan, which is an extension of the first
stage of compression, produces very large volumes of bypass air. Each compressor section is driven
by its own separate turbine at its own best speed. The high pressure compressor (N2) is governed
by the MEC (B737 Classics) or HMU (737 NG) while the fan and low pressure compressor (N1) is
driven by its turbine and is free to select the best speed to ensure optimum airflow. This airflow
matching feature allows the compressor sections to adjust themselves automatically throughout the
operating range of the engine.
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FAN BYPASS / BLEED AIR
Fan bypass air is used for thrust reversal, generator drive and generator cooling. Fifth stage bleed air is
used for the Pneumatic and Engine Anti-ice systems. However, at low thrust settings, fifth stage air
pressure is inadequate, so ninth stage bleed air is used instead.
8. CONTROLS & INDICATORS
Engine start levers IN
IDLE :
- energizes the ignition system
- B737 Classics : Electrically opens engine fuel shutoff valve & mechanically opens the Main Engine
Control (MEC) shutoff valve.
- B737 NG : enables the EEC to open the spar fuel valve & the engine fuel shutoff valve.
CUTOFF :
- B737 Classics : closes the engine fuel shutoff valve in the wing & the MEC shutoff valve.
- B737 Classics : Ignition system is de-energized
- B737 NG : commands the EEC to close the spar fuel valve & the engine fuel shutoff valve.
Engine start switch
GRD (solenoid held, spring-loaded to OFF) :
- opens the starter valve
- provides high energy ignition when the Engine Start Lever is moved from CUTOFF to IDLE
• B737 Classics & B737 NG ground start : the selected igniter(s) is energized & solenoid of the start
valve is held to 46 % N2 (B737 Classics) & 56 % N2 (B737 NG)
• B737 NG air start : both igniters are energized
OFF :
- No ignition
- B737 NG Automatic ignition operates both igniters when engine start lever is in IDLE and :
• an uncommanded rapid engine (N2) decrease or,
• N2 is between 57 % & 50 % or,
• In flight – N2 is between idle & 5 %
CONT :
- Energizes the selected igniter(s) with the Engine Start Lever in IDLE (used during takeoff, landing &
Engine anti-ice ops)
- B737 NG provides (in flight) ignition to both igniters when N2 is < idle & engine start lever is in IDLE
FLT :
- Energizes both igniters when the Engine Start Lever is in IDLE
- The Ignition Select Switch is bypassed when the Engine Start Switch is in FLT
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BOEING 737 SYSTEMS REVIEW
9. FAULTS & INDICATIONS
VALID FOR
LIGHT
ENGINE
CONTROL
LOW
IDLE
LOW OIL
PRESSURE
LOW OIL
PRESSURE
OIL FILTER
BYPASS
OIL FILTER
BYPASS
ON
INOP
ON
ALTN
INDICATION
400
500
ENGINE CONTROL LIGHT (amber)
ILLUMINATED :
- There is a fault in the engine control system
Light operates when engine is operating, aircraft on
Ground, < 80 kt prior to takeoff, approximately 30 sec
after touchdown
LOW IDLE LIGHT (amber)
ILLUMINATED :
- The thrust lever for either engine is near idle and
the MEC on either engine is not commanded to
maintain high idle RPM inflight
- The speed of either engine is below 25 % N1 inflight
X
X
X
LOW OIL PRESSURE LIGHT (amber)
ILLUMINATED :
- Indicates engine oil pressure is at or below the red
radial (13 psi).
X
X
X
LOW OIL PRESSURE ALERT (amber) > EICAS
ILLUMINATED :
- Indicates engine oil pressure is at or below the red
radial (13 psi).
OIL FILTER BYPASS LIGHT (amber)
ILLUMINATED :
- Indicates an impending or actual bypass of the
scavenge oil filter.
X
X
POWER MANAGEMENT SWITCH (white)
- ON (in view) : Indicates the PMC is selected ON
- INOP (in view) : indicates the PMC is INOP when
engine speed is > 46% N2
POWER MANAGEMENT SWITCH (white)
- ON (in view) : Indicates normal control is selected &
engine ratings calculated by EEC from sensed
atmospheric conditions & bleed air demand.
- ON (blanked)switches have been manually operated
- ALTN (in view) : indicates EEC has automatically
switches to alternate control or it has been selected
manually + EEC provides rated thrust or higher
Note : Both ON & ALTN may be in view if EEC has
automatically switched to soft alternate mode. EGT
limits must be observed in both normal & alternate
control modes.
X
X
600
700
800
900
X
X
X
X
X
X
X
X
X
X
X
X
X
OIL FILTER BYPASS ALERT (amber) > EICAS
ILLUMINATED :
- Indicates an impending or actual bypass of the
scavenge oil filter.
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X
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REVERSER
REVERSER
UNLOCKED
START VALVE
OPEN
START VALVE
OPEN
REVERSER LIGHT (amber) ILLUMINATED :
- Indicates the thrust reverser is unlocked
REVERSER UNLOCKED LIGHT (amber)
ILLUMINATED :
- Indicates the thrust reverser is unlocked
X
X
X
START VALVE OPEN LIGHT (amber)
ILLUMINATED :
- Indicates the engine starter valve is open & air is
being supplied to the starter
X
X
X
START VALVE OPEN ALERT (amber) > EICAS
ILLUMINATED :
- Indicates the engine starter valve is open & air is
being supplied to the starter
Note : in case of uncommanded opening of the
Starter valve, low oil pressure or oil filter bypass
(actual or impending), the associated alert flashes
for 10 seconds & solid amber boxes are displayed
flashing in the other two positions. After 10 seconds,
only the alert remains steady.
Flashing is inhibited :
- During takeoff between 80 kt & 400 ft RA, or 30 sec.
after reaching 80 kt (whichever comes first)
- During landing between 200 ft RA until 30 sec.
after touchdown
If flashing is inhibited, an alert illuminates steady
only.
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BOEING 737 SYSTEMS REVIEW
X
X
X
X
X
X
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BOEING 737 SYSTEMS REVIEW
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BOEING 737 SYSTEMS REVIEW
ENGINE FUEL & OIL SYSTEM
- SCHEMATIC B737 CLASSICS
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BOEING 737 SYSTEMS REVIEW
ENGINE FUEL & OIL SYSTEM
- SCHEMATIC B737 NG
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BOEING 737 SYSTEMS REVIEW
ENGINE START & IGNITION
SYSTEM
- SCHEMATIC B737 CLASSICS
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BOEING 737 SYSTEMS REVIEW
ENGINE START & IGNITION
SYSTEM
- SCHEMATIC B737 NG
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BOEING 737 SYSTEMS REVIEW
FAN BYPASS / BLEED AIR
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