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Mission Statement 

The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.


Note:   I have NO affiliation with ANY manufacturer or reseller.  All reviews and content are 'frank and fearless' - I tell it as I see it.  Do not complain if you do not like what you read.

I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).


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If you see any errors or omissions, please contact me to correct the information. 

Journal Archive (Newest First)

Entries in OEM. Orginal Equipment Manufacturer (2)


Original Equipment Manufacture (OEM) Boeing 737NG Lights Test Toggle Switch - Wired and Installed to MIP

The lights test is an often misunderstood but simple procedure.  The light test is carried out by the crew before each flight to determine if all the annunciators are operating correctly (illuminating).  The crew will toggle the switch upward to lights test followed by a routine scan of each annunciator on the overhead, center pedestal and instrument panel.  An inoperative light may preclude take off.

LEFT: OEM Lights Test Switch (before cleaning...) One switch comprising several switches (click to enlarge).

The lights test switch is a three-way switch which can be placed (and locked) in one of three positions; it is not a momentary switch.  Toggling the switch upwards (lights test) illuminates all annunciators located in the MIP, forward and aft overhead and fire suppression panel (wheel well annunciator may not illuminate), while the central position (BRT) provides the brightest illumination for the annunciators (normal operation).  Toggling the switch downwards activates the DIM function dimming the brightness by roughly half that observed when the toggle is in BRT mode.

Depending upon which manufacturer’s Main Instrument Panel (MIP) you are using, the toggle switch may not function this way.  For example, Flight Deck Solutions (FDS) provide a three-way momentary toggle which is not the correct style of switch.  You should not have to hold the toggle to light test as you make your pre-flight scan.  The real toggle switch in the Boeing 737 aircraft is not a momentary switch.

Anatomy of the Toggle Switch

The OEM Light Test switch may appear to be a ‘glorified’ toggle switch with an aviation-sized price tag; however, there is a difference and a reason for this high price tag.  

The switch although relatively simple in output, encompasses 18 (6+6+6) high amperage individual switches assigned to three terminals located on the rear of the switch.  Each terminal can be used to connect to a particular aircraft system, and then to each other.  This allows the toggle switch to turn on or off multiple aircraft systems during the light test. 

The purpose of these multi-terminals is to allow the toggle switch to cater towards the high amperage flow of several dozen annunciators being turned on at any one time during the lights test, in addition to generators and other aircraft systems that are not simulated in Flight Simulator.  In this way, the switch can share the amperage load that the annunciators draw when activated during the light test.

The switch can control the annunciators (korrys) for the MIP, forward overhead, aft overhead, fire suppression panel and any number of modules located in the center pedestal.  

Terminals, Interfacing and Connection

To determine the correct terminals to be used for the light test is no different to a normal toggle-style switch. 

LEFT:  OEM Lights Test switch.  The appearance of the OEM switch is not dissimilar to a normal toggle switch; however, the functionality is different in that there are a number of terminals on the rear of the switch to allow multi-system connection (click to enlarge).

First, ascertain which of the six terminals correlate to the switch movement (toggle up, center and down).  The three unused terminals are used to connect with other systems in the real aircraft (not used in Flight Simulator).

To determine the correct terminals for wiring, a multimeter is set to conductivity (beep) mode.  Place one of the two multimeter prongs on a terminal and then place the other prong on the earth (common) terminal.  Gently move the toggle.   If you have the correct terminal for the position of the toggle, the multimeter will beep indicating an open circuit. The toggle switch does not require a power source, but power is required to illuminate the annunciators during the lights test.  

For an overview of how to use a multimeter see this post - Flight Deck Builders Toolbox - Multimeter.

Daisy Chaining and Systems

Any annunciator can be connected to the light test function, and considering the number of annunciators that the light test function interrogates, it is apparent that you will soon have several dozen wires that need to be accommodated. 

Rather than think of individual annunciators, it is easier to relate a group of like-minded components as a system.  As such, depending upon your simulator set-up, you may have the MIP annunciators as one system, the overhead annunciators as another and the fire suppression panel and modules mounted in the center pedestal as yet another.  If these components are daisy chained together (1+1+11+1+1=connection), only one power wire will be required to be connected at the end of the array.  This minimises the amount of wire required and makes connection easier with the toggle switch.

Two Methods to Connect to the Switch

There are two ways to wire the switch; either through the flight avionics software (software-based solution), or as a stand-alone mechanical system.  There is no particular benefit to either system.  The software solution triggers the Lights Test by opening the circuit on the I/O cards that are attached to the computer, while, the mechanical system replicates how it is done in the real Boring aircraft.

Switch in-line (software connection using ProSim737)

The on/off terminal of the toggle switch is connected to a Leo Bodnar card or other suitable card (I use a Flight Deck Solutions System card), and the card’s USB cable connected to the main computer.  Once the card is connected, the avionics suite software (ProSim737) will automatically register the card with to allow configuration.  Depending upon the type of card used, registration of the inputs and outputs for the card may first need to be registered in Windows (if using Windows 7 type into the search bar joystick and select calibration).

To configure the toggle switch in ProSim737, open the configuration/switches tab and scroll downward until you find the lights test function.  Open the tab beside the name; select the appropriate interface card (Leo Bodnar card) from the drop down menu and save the configuration.  

ProSim737 will automatically scan the interface cards that are installed, and if there is a card that has a power requirement, such as a Phidget 0/16/16 card (used to convert OEM annunciators, modules and panels), the software will make a connection enabling the lights test to function.

Considering the connection is accomplished within the ProSim737 software, it stands to reason the lights test will only operate when ProSim737 is open.

To illuminate the annunciators when the switch is thrown, a 28 volt power supply will need to be connected to the annunciators either separately or in a daisy chain array.

Stand-alone (mechanical connection)

The second method, which is the way it is done in the real aircraft, is to use an OEM 50 amp 6 pull/6 throw relay device. 

Depending upon the type of relay device used (there are several types), it may be possible to connect up to three systems to the one relay.

LEFT:  OEM aviation relay mounted in center pedestal (click to enlarge).

Lights Test Busbar

Although the Lights Test switch has the capacity to connect several systems to the switch itself, it would be unmanageable to attempt to connect each panel to the lights test switch.

To solve this issue a centrally-placed aviation-grade relay has been used in association with a busbar.

A benefit of using an OEM relay and busbar is that the relay acts as a central point for all wires to attach.  The wires from the various systems (panels, korrys, etc) attach to the busbar which in turn connects to the various posts on the relay.

The relay will then open or close the relay enabling power to reach the annunciators (via the busbar) when the switch is positioned to Lights Test.

The stand-alone system will enable the lights test to be carried out without ProSim737 being open.

Although the relay is not large (size of a small entree plate), it can be problematic finding a suitable area in which to mount the relay where it is out of the way.  A good location is to mount the relay inside the pedestal bay either directly to the platform floor or to a wooden flat board that is screwed to the lower section of the center pedestal.

Using the DIM Funtionality (toggle thrown downwards)

This post has only discussed the lights test.  The DIM switch is used to dim the OEM annunciators (korrys) for night work.  Another article explains the DIM functionality.

BELOW: Two very basic flow diagrams provide an overview of the two methods of connection (click diagrams to enlarge).













B737 Aural Warning Module (AWM) Installed and Operational

One of the recent upgrades to the simulator has been the installation of an Aural Warning Module (AWM).  This module resides on the first officer side of the flight deck and is attached to the forward bulkhead of the Main Instrument Panel directly beside the throttle quadrant.  The module replaces four of the computer-synthesised warnings with the OEM counterparts.

LEFT:  Front of the AWM.

The purpose of the module is straightforward; to provide a fail-safe mechanical device that delivers loud, clear and concise tones and bells to indicate to the flight crew that major problem or configuration issue exists.  The aural alarms activate in unison with warning lights that are located on the forward overhead panel, fire suppression panel and on the glare shield of the Main Instrument Panel (six pack annunciators and master fire warning (bell cutout) and master caution buttons).

What's in the Grey Box

The aural warning box contains three mechanical devices capable of delivering four aural warnings:

(i)    The fire bell;

(ii)   The clacker; and,

(iii)   The horn (double purpose that activates either in a continuous or intermittent tone).    

The fire bell rings when any number of events relating to a fire on the aircraft occurs.  The fire bell can be silenced by either pushing the master fire warning button located on the glare shield or bellcutout switch located on the fire suppression panel.  I will be discussing at length the fire suppression panel in a future post; therefore, will not discuss the various scenarios that the fire bell operates.

The overspeeed clacker sounds when the aircraft exceeds the maximum allowable airspeed (Vmo /Mno).  The warning clacker can only be silenced by reducing your airspeed below Vmo/Mno.

The intermittent horn is an aural cue for the takeoff configuration alert.  The horn will sound when a configuration problem exists with the aircraft.  For example, advancing the throttles with the parking break set or the flaps not set.  

When the horn is activated a takeoff config warning light (in red) illuminates on the left forward overhead panel.  Deactivation of the alarm is by retarding the throttle levers to idle and then configuring the aircraft correctly.

The continuous horn is activated when specific flight conditions are met. The following are the main scenarios that activate this alarm.

  • The aircraft is below 800 feet radio altitude with flaps set from UP to flaps 10 with either throttle thrust lever set between idle and 20 degrees forward of idle.
  • The aircraft descends below 200 feet radio altitude (any configuration)
  • The aircraft has flaps set 15 through 25 with either throttle thrust lever set between idle and 20 degrees forward of idle.
  • Flaps 15 is selected without the landing gear being in the down position.
  • The aircraft has flaps set greater than flaps 25.
  • The aircraft’s landing gear is not extended.

Silencing the Continuous Tone Horn

The horn can be silenced by depressing the horn cutout switch located on the throttle quadrant; however, if the aircraft descends below 200 feet radio altitude, or the flaps are extended greater than Flaps 15 (without landing gear extended), the horn cutout switch will not silence the horn.  

Lowering the landing gear or ascending to higher altitude will silence and reset the horn.


The grey box is not an OEM part; however, is similar to the module used in the 800 series with the exception of a toggle switch located on the upper part of the unit (the toggle switch is used by maintenance).  The box was replicated (using vacuform technology) to the identical measurements of the OEM counterpart.  The replica box will be replaced when and if I find a 800 series OEM part.

LEFT:  Inside the AWM from left to right: horn, clacker and bell.  The small box houses basic circuitry. 

The mechanical tones and bell have been acquired from a Boeing 737-200 series aircraft and retrofitted into the module. 

In time as OEM NG AWM will be procured.

Difference Between Classic Series and NG Aural Warning Modules

The AWM for the classic series (300, 400, 500) and NG are different.  The 500 is closest to the NG, however, was a transition product.

Earlier AWM were analogue and used circuits to generate (synthesize various sounds), such as chimes, navigation tunes, etc).  These AWM used mechanical devices to generate the mechanical sounds such as the horn and fire bell.

The NG AWM is 100% digital and has no physical mechanical devices to generate sound.  This said, apparently some earlier NG AWM still include the mechanical fire bell.

The 500 series AWM was transitional between analogue and digital.

The NG AWM has a maintenance toggle at the upper part of the unit.  This can be used by maintenance to check the unit and to alter the volume.  However, it’s not possible to alter the volume of an individual sound – adjust one sound’s volume and they all either increase or decrease in volume relative to each other (this is what the engineer told me).  It’s not possible for pilot’s, using the toggle, to alter the volume or to select what sounds they hear.

Table 1:  Excerpt from Boeing maintenance manual explaining conditions necessary for operation.


The aural tones are mechanical and not software generated.  To interface the warnings with ProSim737 a Phidget 0/0/4 card has been used.  This card is located within the SIM interface module (SIM) and is connected to the aural warning module by a custom wired VGA cable.  The relays on the Phidget card are triggered when a specific condition, based on the offsets set within the avionics software, are met.

Authenticity and Volume

Although FSX, ProSim737, Sim Avionics and many other avionics suites include aural warnings within their package, the clarity and volume in sound produced by a mechanical device surpasses that of a computer generated sound.  

"A word of warning".  The horns and bell are loud – very loud… They are loud for a reason – to annoy a flight crew so that will not ignore the "urgency" of the alarm.  The first time the fire bell sounded during testing made me jump out of my skin!  It also activated the “yell” button on my girlfriend…  

The devices do not have a volume control.  To quieten the aural warnings for “inside” simulation use, I’ve installed foam around the mechanical devices and bell.  This has been successful in lowering the volume by around 60%. 

Below is a short video showing the Aural Warning Module and its various sounds (turn volume up).