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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).

 

All funds are used to offset the cost of server and website hosting (Thank You...)

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Journal Archive (Newest First)

Entries in Original Equipment Manufacture (5)

Wednesday
Mar082017

OEM B737 CDU Conversion - Introduction

One of the slower projects is the conversion of two B737 CDU units.  The two units were purchased from an aircraft scrap-yard in the US and were formally used in a Boeing 737 operated by United Airlines.  

LEFT:  Straight from United Airlines to me.  Two OEM CDU units.  The rear unit has already had its CRT display removed and is partially  'gutted' (click to enlarge).

The two CDUs came from an airframe of a B737-500, which in 2008 was retired along with other Boeing classics, due to United Airlines decision to adopt the Airbus A-320.

The rear of each unit has a chronometer showing the hours of use - one unit has 5130 hours while the other has 1630 hours.

The CDU presently used in the simulator is manufactured by Flight Deck Solutions (FDS) and although I have been pleased with its operation and reliability, there is little resemblance, other than appearance, to the OEM unit.

LEFT:  Detail of the keyboard and DIM knob.  Interestingly the DIM knob dims the actual screen and not the backlighting (click to enlarge).

The prominent difference is external build quality and the tactile feeling when depressing the keys on the keyboard; the keys don't wobble in their sockets, but are firm to press. 

There is also a strong audible click when a key is depressed.  Furthermore, the backlighting is evenly spread with each key evenly lit.

The OEM CDU is large and VERY heavy.  I was surprised at the weight - a good 6 kilograms.  Most of the weight is made up by the thick glass CRT display screen and other components that reside within the sturdy aluminium case.

LEFT:  The casing removed to show the electronic boards that are secured by lever clips.  Like anything OEM, the unit is made very well from solid components (click to enlarge).

Like the casing, the internal structure is also made from aluminium and has four rails to enable the electronic boards to be installed and secured into place. 

Whenever I look at anything OEM, I am amazed at the workmanship that has gone into producing the item; the CDU does not fall short in this area.

A myriad number of small screws hold together the aluminum casing that protects the internal components.  Not only screws are used, but also special miniature DZUS fasteners than enable the side of the casing to removed easily for maintenance.

Nomenclature

When discussing the CDU there are three similar terms that are often used interchangeably: CDU, FMC and FMS.  In this website, I use the terms CDU and FMC interchangeable which is not quite correct - let me explain.

LEFT:  Protective cover removed to show the main pin-out board, rear of the CRT display, power supply, and electronics.  These parts cause the CDU to be quite heavy.  The two Canon plugs  are just visible at the right of the picture enable connection to the aircraft. (click to enlarge to see detail).

The Control Display Unit (CDU) is the interface that the flight crew use to interrogate the data from the Flight Management Computer (FMC); it's basically a screen and keyboard.  The FMC in turn is but one part of a complex system called the Flight Management System (FMS).  The FMS is capable of four dimensional area navigation.  It is the FMS that contains the navigational database.

CDU vs. MCDU

The older units used in the classic airframes are always referred to as a CDU, while the NG units are called a MCDU.  M stands for multipurpose or multi-function.  Basically, the MCDU has a different key called a menu key.  This key, when pressed, accesses another layer of information that is not available in the earlier CDUs.

For those more military-minded, the CDU in military parlance is called a mission computer.

Aesthetic Differences

The CDU dates from 2008, therefore; it is not exactly identical to the CDU used in the Next Generation airframe, however, it is very close.

Main Differences - 500 series to NG

(i)    The dim knob is a slightly different shape;

(ii)   The display screen is rounded at the edges (the NG is more straight-edged);

(iii)   The absence of the horizontal white lines located on the inside edge of the display frame bezel; and,

(iv)   The display screen is different - cathode ray tube (CRT) verses liquid crystal display (LCD).

(v)   Two of the keys are different.  The NG has a menu and space key whilst the older CDUs have a DIR INTC and a blank key (no lettering on key). 

Other differences, not important in the simulator environment, are the colour of the fonts used; older units have black and white or green font while later model NG units use multi-coloured font.

To a purist, these differences are probably important, and if so, you will have to contend with a reproduction MCDU or pay an exorbitant amount for an NG unit. 

Software

The software used in the OEM CDU is not used in the simulator.  The CDU functionality is dictated by the avionics software (ProSim-AR) in use.  This is also true for the font type and colour.

LEFT:  Completely gutted.  All unnecessary and unusable electronic components have been removed.  These two CDU units will soon operate flawlessly with ProSim-AR and flight simulator (click to enlarge).

Converting the CDU

I am liaising with an Australian company that specialises in converting avionics components used in commercial flight simulators.  This company has had considerable experience converting B747 avionics and is keen to see if their expertise will similarly work with the B737.

In a second article, I will explain in more detail how the conversion was done, and examine some of the problems that needed to be resolved.  I also will discuss the mounting of the unit into the CDU bay. 

More photographs of the CDU are located in the image gallery.  Additional images will be added to the gallery in due course.

Glossary

OEM - Original Equipment Manufacture (aka reral aircraft part).

Friday
Aug262016

Assembly of Forward Overhead Panel

Construction of the simulator began in 2011.  It is now 2016 and I am perplexed to why the build has taken so long to complete.   Of course, opting to try and use OEM (Original Equipment Manufacture) parts whenever possible has added significant time to the project -  especially the procurement of parts.

LEFT:  Forward overhead using OEM parts (click to enlarge).

Most of the parts that make up the forward overhead have now been obtained and assembly of the components is well advanced.   Very soon the wiring from the panels to the Phidgets cards will begin.  This will be followed by several hours of testing to check correct functionality and to ensure perfect harmony between components and systems. 

A basic frame has been constructed to enable the overhead to be easily positioned to enable the wiring to be done with a little more ease.  After the forward overhead is completed, work on the aft overhead will commence.  Rome, it seems, was not built in a day.

Certainly, completion of the forward overhead will be the major project over the next few months.

Wednesday
Jul012015

Throttle Quadrant Rebuild - Parking Brake Mechanism Replacement, Improvement, and Operation

In the previous system, the parking brake lever was controlled by a relay and a 12 volt solenoid.  The mechanical system worked well, however, there were some minor differences between the simulated system and that of the system used in the real Boeing aircraft.

LEFT:  Parking brake lever in the UP engaged position.  The red incandescent bulb is 28 volts, however, a 12 volt bulb can be used.

There has been minimal change to the mechanical system, with the exception that, the solenoid has been replaced by a 12 volt actuator, and to engage the parking brake lever to the UP position the toe brakes must be depressed. 

Navigate to this published post that has explained the earlier conversion of the parking brake: B737 Parking Brake Mechanism.

What is an Actuator

An actuator is a type of motor that is responsible for moving or controlling a mechanism or system.  It is operated by a source of energy, typically electric current, hydraulic fluid pressure, or pneumatic pressure, and converts that energy into motion.

Almost every modern automobile has a door lock actuator which is responsible for the locking and unlocking of the door locks.  This website 'How Stuff Works' provides a very good overview of how an actuator works.

The actuator is responsible for maintaining the parking brake lever in the UP position.  This occurs when the circuit is closed and 12 volt power is briefly directed to the actuator to lock the device into the engaged position. 

LEFT:   The actuator is the blue plastic coated mechanism.  The parking brake vertical control rod, micro limit switch and upper part of the high tensile spring can be to seen to the lower right (click to enlarge).

System Overview

The actuator is the mechanism that enables the parking brake lever to be locked into the UP position.  Without power, the actuator is in the resting position and the parking brake lever is pulled to the DOWN position by a high tensile spring. 

The unit, operates on 12 volt power and is mounted horizontally on the Captain-side of the quadrant. 

In addition to the actuator, a micro limit switch and relay (on/off) are also used. 

Micro Switch and Relay

The micro switch is mounted proximal to the vertical control rod, and when the parking brake is is in the DOWN position, the connectors from the micro switch are touching a flange that has been attached to the rod, however, when the parking brake lever is moved to the UP position, the connection is severed. 

The use of a micro switch facilitates a second line of containment.  What this means is that the circuit will only remain open, when both the relay is open (toe brakes depressed) and the connection from the micro switch is severed - both variables must be triggered for the mechanism to operate.  This can only occur when the toe brakes are depressed whilst simultaneously pulling the parking brake lever to the UP position.

The relay, either enables or inhibits 12 volt power to flow into the circuit, and this is dependent upon the whether the toe brakes are depressed.

The reason for this set-up will be understood shortly.

Toe Brakes

In the real aircraft, the parking brakes can only be engaged or disengaged when the Captain-side or First Officer-side toe brakes are depressed.  This has been faithfully replicated in the simulator using a mechanical system.

How It Works

The actuator will only engage when the toe brakes are depressed.  This means that the parking brake cannot be engaged (lever locked in the UP position with red annunciator on) or disengaged (lever in DOWN position with red annunciator off) unless the toe brakes are depressed. 

Two items are needed to engage the parking brake lever.  A relay to signal open or close (on/off) when the toe brakes are depressed, and the micro limit switch discussed earlier.

Depressing or releasing the toe brakes opens or closes a relay which in turn enables 12 volt power to reach the annunciator.  However, the system is only 'live' (closed system) when the parking brake lever is moved to the UP position, severing the connection between the flange on the vertical control rod and the micro limit switch, enabling power to flow unhindered through the circuit.  When the toe brakes are released, the circuit is open and the actuator remains in the engaged locked position with the parking brake lever locked in the UP position.

To release the parking brake lever, the opposite occurs.  When the toe brakes are depressed, the relay opens directing power to the actuator which disengaged the actuator lock.  The parking brake lever is then pulled to the DOWN position by the tensile spring.

Two Methods of Connection Can be Used - Full Mechanical or Part-Mechanical Software

There are two methods that can be used to connect the actuator to the parking brake mechanism.  The first is a straight physical method - the toe brakes are connected to a relay which in turn is connected to the actuator and micro switch. 

The second method is part-mechanical and software controlled and involves using the ProSim737 avionics suite.  Using a Phidgets 0/0/4 relay the USER 1 interface in the configuration menu of ProSim737 is programmed to read the movement offset for the toe brakes.  When the toe brakes are depressed, the software detects and reads the offset which in turn opens the relay enabling power to flow to the actuator.  The actuator will be engaged (circuit closed) only if the connection between the vertical control rod and the micro switch is severed (parking brake lever is in the raised position).

The power for the actuator is connected from the 12 volt busbar in the Throttle Communication Module (TCM) and then, via a straight-through cable, to the Throttle Interface Module (TIM).  The relay for the parking brake mechanism is located in the TIM.

How To Engage The Parking Brake

The method used to engage the parking brake is as follows:

(i)        Slightly depress the toe brakes.  This will open the relay and enable 12 volts to engage the actuator;

(ii)       Raise the parking brake lever to the UP position and hold it in this position; and,

(iv)      Release the toe brakes.  Releasing pressure on the toe brakes causes the actuator to lock into the engaged position.

To release the parking brake, the toe brakes are depressed.  This will cause the actuator to unlock and return to its resting position.  The high tensile spring will pull the parking brake lever to the DOWN position with a loud snapping sound.

Actuator Caution LED

The design of an actuator is such, that if power is continuously applied to the mechanism, it will burn out.  If operating correctly, the actuator will onlt receive power when the toe brakes are depressed and the parking brake lever is raised at the same time.

To combat against the unforeseen event of power being continuously supplied to the actuator, for example by a relay that is stuck in the open (on) position, a coloured LED has been incorporated into the three LEDs that are fitted to the front of the Throttle Communication Module (TCM).  This flashing LED illuminates only when the circuit is closed and the actuator is receiving 12 volt power.

Additional Information

Like many things, there are several ways to accomplish the same or a similar task.  The following posts located in the ProSim737 forum discuss the conversion of the parking brake lever.

How To Make Your Own Parking Brake Release

Parking Brake Logic

Glossary

Two terms often confused by beginners are open circuit and closed circuit.

Any circuit which is not complete is considered an open circuit.  Conversely, a circuit is considered to be a closed circuit when electricity flows from an energy source to the desired endpoint of the circuit.

Thursday
Jun112015

Throttle Quadrant Rebuild - Speedbrake Motor and Clutch Assembly Replacement 

The speedbrake system has been completely overhauled. 

In the previous conversion a number of problems developed.  In particular, the speed of the speedbrake lever when deployed was either too fast, too slow, or did not move at all.  Furthermore, the clutch mechanism frequently became loose.

Rather than continually ‘tweak’ a problematic system, the motor and clutch assembly was removed and replaced with a more advanced and reliable system.

LEFT:  The motor that provides the power to move the speedbrake lever is attached via a slipper clutch to the speedbrake control rod (click to enlarge).  Below the motor is the Throttle Communication Module (TCM).

To read about the first conversion and learn a little more about closed-loop systems and how the speedbrake works I recommend you read the first portion of my earlier article.

B737 Throttle Quadrant  Speedbrake Conversion and Use

Motor and Clutch Assembly

A 12 volt motor is used to power the speed brake.  The motor is mounted forward of the throttle unit above the Throttle Communication Module (TCM).  The wiring from the motor is routed, in a lumen through the throttle firewall to a 12 volt busbar and relays.  The relays, mounted inside the TCM, are dedicated to the speedbrake. 

Attached to the 12 volt motor is a clutch assembly, similar in design to the slipper clutches used in the movement of the two throttle thrust levers.  The clutch can easily be loosed or tightened to provide the correct torque on speedbrake lever, and once set will not become loose.

The slipper clutch was commercially made.

LEFT:  A linear throw potentiometer has been mounted on the Captain-side of the quadrant.  The potentiometer enables the movement of the speedbrake lever to be finely calibrated (click to enlarge).

Speedbrake Mechanics and Logic

The logic that controls the speedbrake has not be changed from what it was in the previous conversion. 

Micro-buttons have been strategically placed beneath the speedbrake lever arc.  As the speedbrake lever moves over one of the buttons, the button will trigger a relay to either open or close (on/off).  The four relays, which are mounted in the Throttle Communication Module (TCM) trigger whether the speedbrake will be armed, stowed, engaged on landing, or placed in the UP position.

To improve the positional accuracy of the speedbrake lever, a linear throw potentiometer has been mounted to the throttle superstructure on the Captain-side. 

Speedbrake Operation

The speedbrake system is a mechanical closed-loop system, meaning it does not require calibration using FSUIPC or from the avionics suite (ProSim737).  The speedbrake lever, however, does need to be registered and calibrated  as a joystick controller in the Windows operating system.  This enables the avionics suite to see the potentiometer.

The following condtions will cause the speedbrake lever to deploy from the DOWN to the UP position.

(i)  When the aircraft lands and the squat switch is activated;

(ii)  During a Rejected Takeoff (RTO).  Assuming the autobrake selector switch has been set to RTO, there is active wheel spin, and the groundspeed exceeds 80 knots; and,

(iii)  If the reverse thrust (reversers) are engaged with a positive wheel spin and a ground speed in excess of 100 knots.

Point (iii) is worth expanding upon.  The speedbrake system (in the real aircraft) has a built-in redundancy in that if the flight crew forget to arm the speedbrake system and make a landing, the system will automatically engage the spoilers when reverse thrust is engaged.  This redundant system was installed into the Next Generation airframe after several occurrences of pilots forgetting to arm the speedbrake prior to landing.  

Therefore, the speedbrake will deploy on landing either by activation of the squat switch (if the speedbrake was armed), or when reverse thrust is applied.

The illumination of the speedbrake annunciators (condition lights) is not part of the system and must be programmed directly within ProSim737 (switches/indicators).

Programmed Variables

The following variables have been programmed into the logic that controls the operation of the speedbrake.

(i)    Rejected Take Off (RTO).  This will occur after 80 knots call-out.  Spoilers will extend to the UP position  when reverse thrust is applied.  The speedbrake lever moves to UP position on throttle quadrant.  RTO must be armed prior to takeoff roll;

(ii)    Spoilers extend on landing when the squat switch is activated.  For this to occur, both throttle thrust levers must be at idle (at the stops).  The speedbrake lever also must be in the armed position prior to landing.  The speedbrake lever moves to UP position on throttle quadrant;

(iii)    Spoilers extend automatically and the speedbrake lever moves to the UP position when reverse thrust is applied;

(iv)    Spoilers close and the speedbrake lever moves to the DOWN position on throttle quadrant when the thrust levers are advanced after landing (auto-stow); and,

(v)    Speedbrakes extend incrementally in the air dependent on lever position (flight detent).

Speedbrake Lever Speed

When the speedbrake lever is engaged, the speed at which lever moves is quite fast.  The term ‘biscuit cutter’ best describes the energy that is generated when the lever is moving; it certainly will break a biscuit in two as well as a lead pencil.  Speaking of lead pencils, I have been told a favorite trick of pilots from yesteryear, was to rest a pencil on the throttle so that when the speedbrake engaged the pencil would be snapped in two by the lever!

In the real Boeing aircraft the movement of the lever is marginally slower and is controlled by an electrically operated actuator (28 volts DC). 

LEFT:  The actuator that controls the movement of the speedbrake.  This image was taken from beneath the floor structure of a Boeing 600 aircraft.  Image copyright to Karl Penrose (click to enlarge).

In theory, the moderate speed that the speedbrake lever moves in the real aircraft should be able to be duplicated; for example, by suppressing the voltage from the 12 volt motor by the use of a capacitor, using a power supply lower than 12 volts, or by using speed controllers.  These alternatives have yet to be trailed.

It is unfortunate, that most throttle quadrants for sale do not include the actuator.  The actuator is not part of the throttle unit itself, but is located in the forward section under the flight deck.  The actuator is then connected to the speedbrake mechanism unit via a mechanical linkage.

In the real Boeing aircraft, the speedbrake lever and actuator provide the input via cables, that in-turn actuate the speedbrakes.  There is no feedback directly from the hydraulics and all operation is achieved via the manual or electric input of the speedbrake lever.

Actuator Sound

The sound of the actuator engaging can easily heard in the flight deck when the speedbrake engages (listen to the below video).  To replicate this sound, a recording of the actuator engaging was acquired.  The .wav sound file was then uploaded into the ProSim737 audio file library and configured to play when the speedbrake is commanded to move.  

The .wav file can be shortened or lengthened to match the speed that the lever moves (download .wav file).

Video

The upper video demonstrates the movement of the speedbrake lever.    The lower video, courtesy of U-Tube, shows the actual movement of the lever in a real Boeing aircraft.

Notice: Not for operational use; video is intended to present the features and functions of the unit in question and not procedures.

If you listen carefully to both videos, you will note a difference in the noise that the actuator generates.  I am lead to believe that the 'whine' noise is slightly different depending upon the aircraft frame.  The actuator in the older classic series Boeing being more of a high whine in comparison to the actuator in the NG airframes.

Glossary

Condition(s) - A term referring to a specific parameter that is required to enable an action to occur.
FSUIPC - Flight Simulator Universal Inter-Process Communication.  A fancy term for software that interfaces between the flight simulator programme and other outside programmes.
Speedbrake Lever Arc - The curved arc that the speedbrake lever moves along.

Saturday
Nov012014

B737-600 NG Fire Suppression Panel (Fire Handles) - Evolutionary Conversion Design

Originally used in a United B737-600 NG and purchased from a wrecking yard, the Fire Supression Panel has been converted to use with ProSim737 with full functionality.

LEFT:  B737-600 NG Fire Suppression Panel installed to center pedestal.  The lights test illuminates the annunciators (click to enlarge).

This is the third fire panel I have owned.  The first was from a Boeing 737-300  which was converted in a rudimentary way to operate with very limited functionality in Flight Simulator. The second unit was from a B737-600 NG; but, the conversion was an ‘intermediate’ design with the relays and interface card located outside the unit within the now defunct Interface Master Module (IMM).  Both these panels were sold and replaced with the current 600 NG series panel.

I am not going to document the functions and conditions of use for the fire panel as this has been documented very well in other literature.  For an excellent review, read the Fire Protection Systems Summary published by Smart Cockpit.

LEFT:  B737-600 NG series Fire Suppression Panel light plate, fire handles, annunciators and installed interface card and relays (click to enlarge).

Nomenclature

Before going further, it should be noted that the Fire Suppression Panel is known by a number of names:  fire protection panel, fire control panel and fire handles are some of the more common names used to describe the unit.

'Plug and Fly' Conversion

What makes this panel different from the previously converted B737-600 NG panel is the method of conversion.  

LEFT:  Panel with outer casing removed showing installation of Phidget and and relays.  Ferrules are used for easier connection of wires to the Phidget card.  Green tape has been applied to the red lenses to protect them whilst work is in progress  (click to enlarge).

Rather than rewire the internals of the unit and connect to interface cards mounted outside of the unit, it was decided to remove the electronic boards from the panel and install the appropriate interface card and relays inside the unit.  To provide 5 and 28 volt power to illuminate the annunciators and backlighting, the unit uses dedicated OEM (Original Equipment Manufacture) Canon plugs to connect to the power supplies.  Connection of the unit to the computer is by a single USB cable.  The end product is, excusing the pun - ‘plug and fly’.

Miniaturization has advantages and the release of a smaller Phidget 0/16/16 interface card allowed this card to be installed inside the unit alongside three standard relay cards.  The relays are needed to activate the on/off function that enables the fire handles to be pulled and turned.

LEFT:  Rear of panel showing integration of OEM Canon plugs to supply power to the unit (5 and 28 volts).  The USB cable (not shown) connects above the middle Canon plug (click to enlarge).

The benefit of having the interface card and relays installed within the panel rather than outside cannot be underestimated.  As any serious cockpit builder will attend, a full simulator carries with it the liability of many wires running behind panels and walls to power the simulator and provide functionality. Minimizing the number of wires can only make the simulator building process easier and more neater, and converting the fire handles in this manner has followed through with this philosophy.

Complete Functionality including Push To Test

The functionality of the unit is only as good as the flight avionics suite it is configured to operate with, and complete functionality has been enabled using ProSim737. 

One of the positives when using an OEM Fire Suppression Panel is the ability to use the push to test function for each annunciator.  Depressing any of the annunciators will test the functionality and cause the 28 volt bulb to illuminate.  This is in addition to using the lights test toggle located on the Main Instrument Panel (MIP) which illuminates all annunciators simultaneously.

At the end of this post is a short video demonstrating several functions of the unit.

The conversion of this panel was not done by myself.  Rather, it was converted by a gentleman who is debating converting OEM  units and selling these units commercially; as such, I will not document how the conversion was accomplished as this would provide an unfair disadvantage to the person concerned.

Differences - OEM verses Reproduction

There are several reproduction fire suppression panels currently available, and those manufactured by Flight Deck Solutions and CP Flight (Fly Engravity) are very good; however, pale in comparison to a genuine panel.  Certainly, purchasing a panel that works out of the box has its benefits; however the purchase cost of a reproduction panel is only marginally less that using a converted OEM panel.

By far the most important difference between an OEM panel and a reproduction unit is build quality.  An OEM panel is exceptionally robust, the annunciators illuminate to the correct light intensity with the correct colour balance, and the tension when pulling and turning the handles is correct with longevity assured.  I have read of a number of users of reproduction units that have broken the handles from overzealous use; this is almost impossible to do when using a real panel.  Furthermore, there are differences between reproduction annunciators and OEM annunciators, the most obvious difference being the individual push to test functionality of the OEM units.

Classic verses Next Generation Panels

Fire Suppression Panels are not difficult to find; a search of e-bay usually reveals a few units for sale.  However, many of the units for sale are the older panels used in the classic B737 airframes. 

LEFT:  B737-200 Fire Suppression Panel.  The differences between the older 200 and 300 series and the NG style is self evident; however the basic functionality is similar.

Although the functionality between the older and newer units is almost identical, the similarity ends there.  The Next Generation panels have a different light plate and include additional annunciators configured in a different layout to the older classic units.

Video

The video demonstrates the following:

  • Backlighting off to on (barely seen due to daylight video-shooting conditions)
  • Push To Test from the MIP (lights test)
  • Push To Test for individual annunciators
  • Fault and overhead fire test
  • Switch tests; and,
  • A basic scenario with an engine 1 fire.

NOTE:  The video demonstrates one of two possible methods of deactivating the fire bell.  The usual method is for the flight crew to disable the bell warning by depressing the Fire Warning Cutout annunciator located beside the  six packs on the Main Instrument Panel (MIP).  An alterative method is to depress the bell cutout bar located on the Fire Suppression Panel.