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Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Close up photos of internal and external components Illustrated history and description of all variants of 737 Databases and reports of all the major 737 accidents & incidents History and Development of the Boeing 737 - MAX General flightdeck views of each generation of 737's Technical presentations of 737 systems by Chris Brady Detailed tech specs of every series of 737 A collection of my favourite photographs that I have taken of or from the 737 Press reports of orders and deliveries Details about 737 production methods A compilation of links to other sites with useful 737 content Study notes and technical information A compilation of links to major 737 news stories with a downloadable archive A quick concise overview of the pages on this site


B737 New Generation Series

B737-600/700/800 notes. [Mr Meljoe Ferreira].



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*** Updated 14 Nov 2021 ***

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Ice & Rain protection categories:-

1.Wing Thermal Anti-Icing system [WTAI].

2.Inlet Cowl Anti-Icing system.

3.Pitot Tubes, Temperature probe, and Alpha vane Anti-Icing system.

4.Windows, Windshield and Doors.

5.Water and Toilet drain Anti-Icing system.


Windows, Windshields and Doors have:-

A] Control Cabin Window Anti-Icing system.

B] Windshield Wiper system.

C} Window hydrophobic coating.

WTAI system heats the three inboard L.E slats on both wings.


Overheat switches prevent the slats from overheat. Overheat protection only functions when the Aircraft is on ground.

Switches on the Autothrottle switch packs automatically close the WTAI valves when the engine thrust levers are advanced. This conserves engine thrust for Take off. This functions only on ground.

The PSEU [Proximity sensor electronic unit] resets/switches off the WTAI system during Take off.

Slat # 1 & 8 not heated.


Wing Thermal Anti-icing system components:-

A] Wing Anti-Ice Control module [P5-11].

B} Control stand Wing Anti-icing switches Auto throttle switch packs [2].

C] WTAI solenoid valves [2].

D] WTAI shutoff valves [2].

E] WTAI Ground Overheat Thermal switch [2].

WTAI shutoff valve:- 115v AC powered, motor operated butterfly type with manual overide & position indicator lever, held by V-flange clamps.

WTAI Ground overheat Thermal switch located downstream of the WTAI shutoff valves. They are bimetallic, closes at 257 degF [125 degC] causes WTAI control module [P5-11] to close shutoff valves.

Switch has access through Access panel in Wing leading edge.

Wing Anti-Icing telescoping duct [Inner duct] has teflon coating to prevent binding when the two sections slide over each other.

Moving the throttle forward approx 35 deg will activate the control stand WTAI switches on the Auto throttle switch pack to close the WTAI shutoff valve provided the Aircraft is on the ground.

Access to the Auto throttle switch packs are from the Lower nose compartment.

WTAI solenoid valve is located on the top of the compressor section of each engine. Access is from the Left Thrust Reverser Cowl.

WTAI Solenoid valve is a normally closed ball-type poppet valve. It is energized to the open position with 28vdc electric power. This releases the control pressure from the pre-cooler control valve actuator causing it to open fully, to cool the engine bleed air & protect the wing leading edges from overheat damage on the ground.


The WTAI system uses 115vac power to operate the WTAI shutoff valves, & 28vdc for control and indication.

On ground WTAI shutoff valves open when:-

A] The wing anti-ice switch [P5-11] is in the ON position.

B] No overheat conditions exist [Wing Anti-ice ground thermal switch].

C] No engine thrust lever is advanced [Control stand wing anti-ice switches].


The WTAI switches will automatically move to the OFF position during Takeoff. This reduces engine bleed loads and conserves thrust for climb.

Cowl Anti-ice amber lights [P5-11], illuminate if there is an overpressure [>65psi] in the cowl TAI duct.

The Inlet cowl Anti-icing system uses 28vdc power.

The Inlet Cowl Anti-Icing valve controls the flow of air to the engine inlet cowl. It is located on the right side of the engine fan case.

The Inlet cowl Anti-ice valve is an electrically controlled and pneumatically operated butterfly valve, spring loaded to the closed position. Max pressure limited to 50 psi.


The Inlet cowl TAI valve has a manual override collar. You can manually lock the valve in the Full open or closed position if the valve fails.

The inlet cowl TAI pressure switches is on the Inlet cowl TAI duct, downstream of the Inlet Cowl Thermal anti-Ic valve.The ICTAI pressure switch is an aneroid type switch. The switch closes at pressure >65 psi to illuminate the amber cowl Anti-ice [P5-11] light.

TAI-Indication on the CDS [Common display system].

Green - Valve open and switch ON.

Yellow - Cowl valve not in commanded position.


The Pitot and Static Anti-Icing system supplies heat to three probes:-

A] Alpha vanes [2].

B] Total air temperature probe [1].

C] Pitot probes [5].


P5-9:- The Pitot & static control panel amber lights illuminate when the probe heaters are not drawing current.


If a pitot heater fails, the entire probe must be replaced.

There is one pitot probe on the left fwd fuselage [Capt pitot].

There are two pitot probes on the right fwd fuselage [F/O pitot], [Aux pitot].

There are two pitot probes on the Vertical Stablizer [L ELEV pitot], [R ELEV pitot].




Use hardwood or plastic tools only when you remove or apply the sealant around the probe base plate.

The Pitot probe Anti-Icing system uses resistance type heating elements to warm the probe.

Probe:- 115vac

P5-9:- 28vdc

If the probe heater does not draw current, the logic causes the indication light to come on.


The Alpha vane Anti-icing system prevents Ice formation on the Alpha vanes. This prevents false air data signals that ice can cause.


The Alpha vanes have two integral heaters:-

A] A Vane heater

B] A Case heater

Replacement of the Alpha vanes is from the inside of the flight deck.

If the Elements fail, the probe has to be replaced.


The Indication circuits do not monitor the case heat element, only the vane heat element.

The Total air temperature [TAT] probe is on the left side of the forward fuselage.


If the TAT probe heater does not draw current, the logic causes the indication light to come on.


The Pitot heat A switch controls heat to these system A probes:-

The Captains pitot.

The Left elevator pitot

The Left Alpha vane.

The Total air temperature [TAT] probe.


The Pitot heat B switch controls heat to these system B probes:-

The First Officer pitot.

The Auxiliary pitot.

The Right elevator pitot.

The Right Alpha vane.


The light goes off when the related air data probe has heat.

The light comes on when the related air data probe does not have heat.


The Control cabin window anti-icing system improves window impact strength and prevents ice formation on the Flight compartment windows.

The Control cabin anti-icing system uses electrical power to heat the flight compartment windows.


The Window heat control units do these things:-

A] Monitor Window temperature.

B] Supply ON and OVERHEAT system indication.

C] Do system tests.

D] Program power output to the windows.


The WHCUs control power to these windows:-

No1 Left and right.

No2 Left and right.


Thermal switches monitor window temperature and control power to these windows:-

No4 Left and right.

No5 Left and right.


Windows in the thermal switch control systems are not part of the P5 overhead panel indication and test functions.


The Flight compartment windows are of laminate construction. One layer is made of a conductive coating.


Windows 1 and 2 have resistance type temperature sensors for feedback to the window heat control units. There are two sensors in each window.

A] A primary sensor.

B] A spare sensor.


The WHCU uses only one sensor. If the primary sensor fails, use the spare sensor. This prevents window removal for a single sensor failure.


Functions of the WHCUs:-

A] Sense Window temperature.

B] Apply current to the window heat system when necessary.

C] Control current to the window heat conductive coating to prevent thermal shock.

D] Control the P5-9 window heat status indication.

E] Incorporate circuitry for P5-9 OVHT and PWR test.

D] Incorporate BITE.


The WHCUs are in the EE compartment. Two are on the E4-2 rack and two are on the E2-1 rack.


The windshield sensor switches are on the forward outboard E4 stanchion rack.

The WHCUs are identical and interchangeable. Each WHCU controls the heat to one window.

Electrostatic discharge can cause damage to the window heat control unit.

The WHCU have front face BITE that isolates system faults to the LRU interface level.


For the front windows, you test the resistance of the sensors with the windshield sensor switches on the forward outboard E4 stanchion rack.


The Window heat terminal connections are located behind the window heat control unit in the E & E compartment, You access the connections through access panels in the forward cargo compartment.


The Window heat terminal connection consist of taps on the terminal blocks. On No1 Windows five taps are used. On No2 windows six taps are used.


When a window is replaced, the new window has the resistance identified by a code etched in the windshield glass. The code determines the corresponding transformer tap. If the window does not heat properly, the conductive coating resistance is checked and a proper transformer tap is selected.


The application of power to the window is by ramp function to prevent thermal shock to the window.


Electric current supplied < 100 degF, limits to 110 degF.


When there is current flow to the window. P5 green ON light comes on. In case light is off, it means that the window is warmer than target temperature.


A PWR TEST switch conducts a confidence test of the window heat system when the window is warm. This causes WHCU to send current to the windows and the green P5 overhead panel light to come on. Release the PWR TEST Switch as soon as Green ON light illuminates to prevent overheating the window.


If the WHCU detects both of these conditions, an overheat trip occurs:-A] Window temperature above 145 degF [nominal].B] Electric current to the window heat circuit.The overheat protection circuit operates only while power is applied to the windows, this prevents a lower overheat trip setting, and prevents nuisance system trips during operations under conditions of high ambient heat.


An overheat trip cause the following:-

A] Electric current to the window is cut off.B] The green P5 overhead panel ON light goes out.C] The Amber P5 overhead panel light comes on.D] The P7 Master caution and Anti-Ice annunciator lights come on.


To reset the system, you must move the WINDOW HEAT switch to the OFF and back to ON position.


An Overheat cannot be reset until the window cools.

The thermal switch on No5 windows open at a temperature of > 110degF, and close again at < 90 degF.

No4 & 5 windows are not part of the Anti-Ice panel indication or test systems. These windows do not have an overheat protection.


If Bubbles appear in the window layers, this may be an indication of window overheat [Thermal breakdown and outgassing of Vinyl layers].

The FWD switches turn on and off window heat to their No1 windows.

The SIDE Switches turn on and off window heat to their No2, 4 and 5 windows.


The WHCU BITE circuitry detects failures in these:-

A] WHCU internal faults.

B] Window.

C] Temperature Sensor.

D] Control power input.

E] Bus power input.

F] Associated wiring.


The WHCU has a 10 - Register Fault history memory storage capability.

WHCU BITE Test switches:-






The Lamp test switch does a test of the six BIT indicator lamps. This verifies power and indication availability.

The BIT VERIFY switch starts a system self test. This does a check of system faults.

The Fault history switch shows the last 10 registers, one register at a time.

The BIT LAMP RESET switch clears the fault from the WHCU.


The WHCU has these red fault lamps:-


B] Window sensor.

C] Bus power.

D] Window power.

E] P5-9/Control power.

The BIT TEST OK Lamp shows that a BIT VERIFY Test is complete and found no faults. The lamp stays on for 15 seconds.


The WHCU-LRU lamp shows a failure of the WHCU unit.


The WINDOW SENSOR lamp shows a failed sensor due to opens, shorts, or wiring problems.


The BUS POWER lamp shows that there is no power to the WHCU bus.


The WINDOW POWER lamp shows that there is no window power or there is overcurrent to the window. This is due to either a window, wiring or a connector open or shorted problem.


The P5-9 control power lamp shows that there is no power to the WHCU.


The Windshield wiper system removes rain, sleet and snow from the No1 and No2 flight compartment windows.


You get access to the windshield wiper drive assemblies from panels under the P7 glareshield.


The Windshield wiper assembly does these things:-

A] Moves the windshield wiper.

B] Controls the force the wiper applies on the windshield.

C] Gives rigging adjustments for wiper sweep.


The Wiper arm force adjustment nut sets the force the wiper blades applies to the window.


Do not operate the wipers on dry windshields. This can do these things:-

A] Scratch the window.

B] Decrease wiper blade service life.

C] Remove windshield hydrophobic coating.


A thermal switch protects the wiper drive assembly motor from overheat conditions. The switch actuates at 266 degF. If the switch actuates, it stops the motor operation. The switch resets automatically when the assembly cools.


The windshield wiper system uses 28vdc power.


The Two Wiper switches have four positions:-

PARK:- Stops the wiper operation.

INT:- Intermittent wiper operation, one sweep cycle approximately every 7 seconds.

LOW:- Low speed wiper operation, approximately 160 sweeps per minute.

HIGH:- High speed wiper operation, approximately 250 sweeps per minute.


The Hydrophobic windshield coating improves visibility in heavy rain. The coating is on the outside surface of the left and right #1 Flight deck windows.


Hydrophobic windshield coatings are transparent films. The coatings repel water. This causes water drops to bead up and roll off the windshields. The coatings do not affect windshield strength or optical clarity.


Hydrophobic coating wear depends on the following:-

A] Wiper use.

B] Route structure.

C] Windshield maintenance practices.


Maintenance of hydrophobic coatings require regular cleaning of the windshields. Use a 50% solution of Isopropanol in distilled water and a soft cloth to clean the windshields.

Do not use abrasive cleaning pads or cleaners. Do not use cleaning solutions with fluorides.

Make sure the force the blades put on the window is to specification. Worn or incorrectly set up windshield wipers wear the coatings down. Hydrophobic coatings replace earlier Rainboe spray on systems. Do not Apply Rainboe to hydrophobic coatings. Rainboe can have harmful effects on the coatings.


Ice formation in Water and Toilet systems can cause:-

A] Ice expansion damage.

B] Line blockage that prevents normal system operation.

C] Ice formation on the forward drain mast can break off and damage airplane structure.


These system components have integral heaters:-

Service panel fittings.

Drain masts.

Hoses with Integral heating elements.


Components without integral heaters get heat from these components:-

Heater tape ["Ribbon " heaters].

Heater blankets.


If you park the airplane in freezing conditions with no electric power, drain the water and toilet system to prevent ice formation.


Water and toilet drain Anti-Icing system is divided into:-

A] Potable water anti-Icing system.

B] Gray water Anti-Icing system.

C] Vacuum waste Anti-icing system.


The potable water anti-Icing system prevents Ice formation in these areas:-

A] The potable water fill fitting.[28vdc power].

B] The potable water fill hose.[115vac power].

C] the potable water supply hoses.[115vac power].


Thermostatic switches [control & overheat] in the hoses control heat to the hoses.

Heat to the hoses is automatic when power is on the airplane.


The gray water Anti-Icing system prevents Ice formation in these areas:-

A] The gray water drain lines.

B] The drain masts.


Tape heaters warm the gray water drain valve and the gray water drain lines [115vac power].

An In-line thermostatic switch controls heat to the drain mast inlet line.


The drain mast heating system elements operate on 115vac in flight and 28vdc on ground.

The drain mast heat uses voltage reduction on the ground to prevent a burn hazard to personnel. This also extends the drain mast service life.


Do not overlap the wraps of the tape heaters. If the tape is too long, increase the number of wraps.

The Vacuum waste Anti-Icing systems uses resistance type electric heaters in these areas:-A] The Vacuum waste tank drain [ball] valve.[115vac powered blanket heater].B] The Vacuum waste tank rinse line. [line heater 28vdc].


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