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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 Boeing 737-800 Flight Simulator (13)


RNAV, RNP, LNAV and VNAV Operations - Overview 

New flyers to the Boeing 737NG often become confused understanding the various terminology used with modern on-board navigational systems.

Although the concepts are easy to understand, the inter-relationship between systems can become blurred when the various types of approaches and departures are incorporated into the navigational system.

LEFT:  Collins Mode Control Panel (MCP) showing illuminated LNAV annunciation (click to enlarge).

This post will not provide an in-depth review of these systems; such a review would be lengthy, confusing and counterproductive to a new virtual flyer.  Rather, this post will be a ‘grass-roots’ introduction to the concept of RNAV, RNP, LNAV and VNAV.  I will also touch on the concept of Performance Based Navigation (PBN).

In the Beginning there was RNAV  

RNAV is is an acronym for Area Navigation (aRea NAVigation). 

Prior to complex computers, pilots were required to use established on-the-ground navigational aids and would fly directly over the navaid.  Such a navaid may be a VOR, NDB or similar device.  Flying over the various navaids was to ensure that the flight was on the correct route.  Often this entailed a zigzag course as navaids could not be perfectly aligned with each other in a straight line - airport to airport. 

When computers entered the aviation world it became possible for the computer to 'create' an imaginary navigation aid based on a direction and distance from a ground-based navaid.  Therefore, a straight line could be virtually drawn from your origin to destination and several waypoints could be generated along this line.   The waypoints were calculated by the computer based on ground VORs and positioned in such a way to ensure more or less straight-line navigation.

In essence, RNAV can be loosely defined as any 'straight line' navigation method similar to GPS that allows the aircraft to fly on any desired path within the coverage of referenced NAVAIDS.

Required Navigation Performance (RNP) and Performance Based Navigation (PBN)

Simply explained, Required Navigation Performance (RNP) is a term that encompasses the practical application of advanced RNAV concepts using Global Navigation Satellite Systems (GNSS).

However, there is a slight difference between RNP and RNAV although the principles of both systems are very similar. 

RNAV airspace generally mandates a certain level of equipment and assumes you have a 95% chance of keeping to a stated level of navigation accuracy.  On the other hand, RNP is performance based and requires a level of on-board performance monitoring and alerting.  This concept is called Performance Based Navigation (PBN).

RNAV and RNP both state a 0.95 probability of staying within 1 nm of course.  But RNP (through PBN) will let you know when the probability of you staying within 2 nm of that position goes below 0.99999.  In essence, RNP and PBN enable an aircraft to fly through airspace with a higher degree of positional accuracy for a consistently greater period of time. 

To achieve this level of accuracy a selection of navigation sensors and equipment is used to meet the performance requirements.  A further enhancement of this concept is the use of RNP/ANP (Required Navigation Performance and Actual Navigation Performance.  Advanced RNAV concepts use this comparative analysis to determine the level or error between the required navigation (the expected path of the aircraft) and the actual navigation (what path the aircraft is flying.)  This information is then displayed to the flight crew.


LNAV and VNAV are parts of the Flight Guidance System, and are acronyms for Lateral Navigation and Vertical Navigation'.  Both these functions form part of the automation package that the B737NG is fitted with.

LNAV is the route you fly over the ground. The plane may be using VORs, GPS, DME, or any combination of the above. It's all transparent to the pilot, as the route specified in the clearance and flight plan is loaded into the Flight Management System (FMS), of which the Flight Management Computer (FMC) is the interface.

The route shows up as a magenta line on the Navigation Display (ND), and as long as the LNAV mode on the Mode Control Panel (MCP) is engaged and the autopilot activated, the aircraft will follow that line across the ground. LNAV however, does not tell the plane what altitude to fly, VNAV does this.

VNAV is where the specified altitudes at particular waypoints are entered into the FMS, and the computer determines the best way to accomplish what you want.  The inputs from VNAV are followed whenever the autopilot is engaged (assuming VNAV is also engaged).  

The flight crew can, if necessary alter the VNAV constraints by changing the descent speed and the altitude that the aircraft will cross a particular waypoint, and the computer will re-calculate where to bring the throttles to idle thrust and begin the descent, to allow the aircraft to cross the waypoint, usually in the most economical manner.

VNAV will also function in climb and take into account airspeed restrictions at various altitudes and will fly the aircraft at the desired power setting and angle (angle of attack) to achieve the speed (and efficiency) desired.

There is not a fast rule to whether a flight crew will fly with LNAV and VNAV engaged or not; however, with LNAV and VNAV engaged and the autopilot not engaged, LNAV and VNAV will send their signals to the Flight Director (F/D) allowing the crew to follow the F/D cue display and hand fly the aircraft the way the autopilot would if it were engaged.

Reliance on MCP Annunciators

LNAV and VNAV have dedicated annunciators located on the Mode Control Panel (MCP).  These annunciators illuminate to indicate whether  a particular mode is engaged. 

LEFT:  Flight Mode Annunciator (FMA) showing LNAV and VNAV Path Mode engaged.  The Flight Director provides a visual cue to the attitude of the aircraft while the speed is controlled by the the FMC.  CMD indicates that the autopilot is engaged (ProSim737 avionics suite).

However, reliance on the MCP annunciators to inform you of a mode’s status is not recommended.  Rather, the Flight Mode Annunciator (FMA) which forms part of the upper area of the Primary Flight Display (PFD) should be used to determine which modes are engaged.  Using the FMA will eliminate any confusion to whether VNAV (or any other function) is engaged or not.

This post explains the Flight Mode Annunciators (FMA) in more detail.


In summary, RNAV is a method of area navigation that was derived from the use of VOR, NDBs and other navaids.  RNP through it use of GNSS systems has enabled Area Navigation to evolve to include LNAV and VNAV which are sub-systems of the Flight Guidance System -  LNAV is the course across the ground, and VNAV is the flight path vertically. 

Historically, navigation has been achieved successfully by other methods, however, the computer can almost always do things better, smoother and a little easier – this translates to less workload on a flight crew.  

In my next post, we will discuss RNAV approaches and how they relate to what has been discussed above.


The information for this article came from an online reference for real-world pilots.

Acronyms and Glossary

Annunciator – Often called a korry, it is a light that illuminates when a specific condition is met
DME – Distance Measuring Equipment
FMA - Flight Mode Annunciator
FMC – Flight Management Computer
FMS – Flight Management System
GPS – Global Positioning System
GNSS - Global Navigation Satellite System
LNAV – Lateral Navigation
MCP – Mode Control Panel
ND – Navigation Display
NPA - Non Precision Approach
PBN - Performance-based Navigation
RNAV – Area Navigation
RNP - Required Navigation Performance
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed
VOR – VHF Omni Directional Radio Range


Throttle Quadrant Rebuild - Evolution Has Led to Major Alterations 

Two major changes to the simulator have occurred.  The first concerns the throttle quadrant and the second is the replacement of the trial Interface Master Module with a more permanent modular solution.  The changes will be documented in the near future after final testing is complete.

LEFT:  The throttle quadrant has been completely rebuilt from the ground up.  Although the outside may appear identical to the earlier quadrant, the rebuild has replaced nearly everything inside the quadrant and the end product is far more reliable than its predecessor.

The throttle unit, in its previous revision, worked well, but there were several matters which needed attention.  The automation and functionality was adequate, but could be improved upon.  There were also 'niggling' issues with how the clutch assembly operated - it was somewhat loose which caused several flow-on problems.

Initially, some minor improvements were to be made; however, one thing lead to another and as 'fate would have it' the throttle unit has been rebuilt from the bottom up.


The improvements have primarily been to the automation, the autothrottle and the speedbrake system.  However, during the rebuild other functionality have been improved: the synchronised tracking movement of the thrust levers is now more consistent and reliable,  and an updated system to operate the parking brake has also been devised.  This system replicates the system used in the real aircraft in which the toe brakes must be depressed before the parking lever can set or disengaged.

Furthermore, the potentiometers controlling the movement of the flaps and thrust levers have been replaced with string potentiometers which increases the throw of the potentiometer and improves accuracy.  The calibration of the flaps and speedbrake is now done within the system, removing the need for 'tricky' calibration in FSUIPIC. 

In the previous throttle version there was an issue with the speedbrake not reliably engaging on landing.  This in part was caused by a motor that was not powerful enough to push the lever to the UP position with consistent reliability.  This motor has been replaced with a motor more suitable to the power requirement needed.  The speedbrake is mechanical, mimics the real counterpart in functionality, and does not require software to operate.

This throttle conversion has maintained the advanced servo card and motor that was used to control the movement of the stab trim tabs (trim indicators); however, the motor that provides the power to rotate the trim wheels has been replaced with a more reliable motor with greater power and torque.  The replacement motor, in conjunction with three speed controller interface cards, have enabled the trim wheels to be rotated at four independent speeds.  This replicates the four speeds that the wheels rotate in the real B737.

Finally, the automotive fan-belt system/clutch system which was a chapter from the 'Dark Ages' has been replaced with two mechanical clutch assemblies that has been professionally designed to operate within the throttle unit - this will completely remove any of the  'niggles' with the previous clutch assembly becoming loose and the fan belt slipping.  Each thrust lever has a dedicated poly-clutch and separate high powered motor. 

A brief list of improvements and changes is listed below:

  • NG skirt replaced with more accurate skirt (prototype);
  • Reproduction TO/GA buttons replaced with OEM square TO/GA buttons;
  • Fanbelt driven clutch system replaced with slipper clutch system;
  • motors replaced that control lever movement and trim wheels;
  • 95% of wiring re-done to incorporate new interface modules;
  • Replacement interface alert system;
  • Flap potentiometers replaced by string potentiometers;
  • Speedbrake potentiometer replaced by linear potentiometer;
  • Thrust levers potentiometers replaced by dual string potentiometers;
  • Internal mechanism altered to stop noise of chain hitting throttle frame;
  • Thrust lever tracking movement accuracy improved;
  • Thrust reversers now have proportional thrust for each lever 1 and 2; and
  • The parking brake mechanism replaced with a more accurate system that reflects that used in the real aircraft

The conversion of the throttle quadrant has been a learning process, and the changes that have been done improve the unit's functionality and longevity - not too mention accuracy, far beyond what it was previously.

Dedicated Interface Modules

The throttle previously interfaced with the Interface Master Module (IMM).  The IMM was developed as a trial module to evaluate the modular concept.

The throttle quadrant will now directly interface with two dedicated modules called the Throttle Interface Module (TIM) and Throttle Communication Module (TCM).  Both of these modules contain only the interface cards, relays and other components required to operate the throttle and automation.  Additionally, the system incorporates a revised Interface Alert System which evolved from the original concept used in the IMM.

To read more concerning the various interface modules, a new website section  has been produced named Interface Modules.  This section is found in the main menu tabs at the top of each page.

Flight Testing (March 2015)

The throttle and replacement interface modules are currently being evaluated and minor issues rectified.

Once testing is complete, the alterations undertaken during the rebuild process will be documented in separate posts and, to facilitate ease of searching, links will be added to the flight controls/throttle quadrant section.

It should be noted that the work done to rebuild the throttle was done with the help a friend, who has a through knowledge of electronics and robotics.


Navigraph Charts Cloud and Charts Desktop - Review

One aspect of simulation which is identical to the real thing is the use of charts.  Whether a professional real-world pilot approaching Heathrow International or a virtual pilot, the correct approach chart will need to be consulted, interpreted correctly, and followed if a safe landing is to be assured.

LEFT:  The traditional leather-bound binder that contains hundreds of Jeppesen charts.  This particular binder belonged to Gene Mac Farland, a Captain who flew for 30 years with Continental Airlines.

Not so long ago, Jeppesen Charts provided the mainstay for all professional navigation charts and these thin paper charts were carried in a brown leather binder.  Pilots carried a number of binders with them to allow access to the appropriate chart where necessary.  It was the responsibility of the pilot to ensure that the contents were up-to-date and reflected the latest chart; a tedious task.

Later years have witnessed the introduction of computers and several companies, including Jeppesen, have provide electronic charts that can be viewed using laptops, smart phones and apple i-pads.  The days of  lugging binders is now over, and a binder such the one depicted in the above photograph have become, for the most part, keepsakes and door stops.

Collecting Charts

Virtual pilots have a tendency to ‘collect’ charts from innumerable locations.  The collection can become quite large, and often it is difficult to collate the charts in such a way that it is easy to find the wanted chart, let alone know whether the chart is the most accurate up-to-date version.  


Serious simulator enthusiasts have probably heard of the European-based company Navigraph.  For several years the company has been responsible for the production and distribution of AIRAC cycles that are used to update the Flight Management System (FMS) to maintain the accuracy of the navigation database.

AIRAC Cycles

AIRAC is an acronym for Aeronautical Information Regulation And Control.  An AIRAC cycle contains the current aviation regulations, procedures, and charts for airport, runway, airspace, Instrument Approach Procedures (IAP), Standard Terminal Arrival Routes (STAR), and Standard Instrument Departures (SID).  The AIRAC cycle updates the database used by the aircraft's FMC/CDU.

Without this up-to-date data it is not possible to program the FMC/CDU with any degree of accuracy. 

Navigraph provide a subscription service to AIRAC cycles which are updated several times a year (usually there are thirteen cycles per year)

Charts Database

Navigraph, in addition to supplying regular updated AIRAC cycles, has implemented three additional products:  airport charts, video tutorials and en-route charts.  These products are available via an annual subscription from a data cloud database and/or desktop program.

BELOW:  Area of coverage of Navigraph charts (image courtesy of Navigraph).  This link provides an up-to-date coverage area for Navigraph charts.

Airport charts include up-to-date charts for approximately 13,000 airports worldwide. Chart information includes at a minimum: runway data, instrument approach procedures, standard terminal arrival routes and standard instrument departures.  To date, there are approximately 40,000 charts and the number is regularly being expanded with quarterly updates.

Furthermore, several dozen video tutorials instructing in the correct interpretation and use of approach charts are available in addition to dozens of en-route charts which include upper and lower airways.  

The information depicted on the charts originates from suppliers of real-world aviation charts (Navtech) and depicts the latest data, in a format that has been designed by human factor research to be user friendly.

Unlike other companies that have attempted to provide charts for virtual pilots (for example, sim charts), Navigraph charts have been vector scanned in high resolution providing a dataset that can be easily enlarged as required, read, and if required printed in high definition.  Additionally, the information is in colour.  

Ease of Access - Key Feature

In a nutshell, Navigraph has allowed a virtual pilot access to information that otherwise would require considerable collating, revision, and pose difficulties concerning easy access when required. The datasets can be immediately assessed on demand either from a data cloud (charts cloud) or via a desktop program (charts desktop).  

Granted there are many on-line resources to find, read and print approach charts - some better than others.  However, the Navigraph search functionality allows the right chart to be found, quickly and easily, at the appropriate time.  In my opinion, this promotes Navigraph over others programs and on-line resources.

Charts Cloud

The cloud provides an easy to use on-line interface, with an effective search functionality that can be accessed using different platforms, including portable devices such as i-pads and smart phones.  To allow speedier future access, charts can be placed in a favourites list or listed in a paper clip (a separate folder) that is linked to your account.  The charts cloud does not allow printing or permanent downloading of a chart and charts are only available when on-line.  Access to the data sets ceases after the annual subscription has expired.

LEFT:  Screen capture of charts cloud showing list of available charts for Hobart, Tasmania, Australia.  The chart can be viewed full screen and can be enlarged as required.  Note this screen capture is of a very reduced quality (click image to enlarge).

The speed at which charts cloud database can be accessed relates to the Internet connection being used; however, for the most part the server Navigraph uses provides consistent access that should be suitable for most users, with the exception of those that use dial-up.

Charts Desktop

The charts desktop is a program supplied by Navigraph (free of charge), which resides on your computer and allows charts to be downloaded for access when off-line.  This has the obvious benefit of faster access times if the Internet connection is less than optimal.

LEFT: Navigraph Charts 4 desktop opening screen.

The program has the capability to list charts as favourites for easy and fast access, in addition to having a highly responsive search engine.  Unlike the charts cloud, the charts desktop allows access to any chart that has been downloaded after the annual subscription has expired; however, after the subscription has expired the charts cannot be updated.  Another benefit in using the program is that charts can be printed.


Navigraph regularly updates the database with additional charts and changes to prexisiting charts.  The program advises you of an update when you mouse over the chart name.  The program will then allow you to maintain the existing chart or download and replace the chart with the newer version.   Updates are usually half a dozen times a year

Is it a Worthwhile Investment ?

Whether Navigraph chart data is of benefit to you will depend upon how many different airports you fly from and to, how often you fly, and how much money you are prepared to shell out for the convenience and ease of accessed chart information.  Certainly, it is far easier to maintain a collection of charts electronically than store several binders of paper!

A subscription (using the charts cloud or desktop program) is currently 47.92 Euro excluding VAT.  This price allows unlimited access to all charts, and includes the ability to view all instructional videos, which have been professionally produced and run each for approximately 8 minutes duration.  Short of a subscription, individual charts and videos can be purchased separately for a once off fee.  In contrast to purchasing the Jeppesen electronic charts from Jeppesen or an ongoing seller, this fee is reasonable.

Short Review

I elected to not write an in-depth review of Navigraph and their products as the Navigraph interface and their products are constantly being upgraded.  A review may soon be out-of-date!  This review has dealt primarily with the airport charts and has not examined in details the en-route charts or training videos that come packaged with a charts cloud subscription.

Navigraph’s website is very comprehensive and includes several images of their charts that depict the high quality of their product, along with examples of the various programs and how they operate. 

Whilst the charts are not 100% identical to Jeppesen real-world counterparts (various information has been merged and interpolated), the detailed datasets, consistent high quality, and ease of searching and accessibility, make the administrative aspect of virtual flying more enjoyable.


The content in this post is not meant to directly promote or endorse Navigraph.  To trial this software, I purchased a subscription to the charts cloud and charts desktop.  To date, I have been very pleased with the quality of the Navigraph charts and will probably continue to supplement my real-world paper charts with information from this source.

To read more about Navigraph and their products, or to read their active support forum, navigate to Navigraph.com


Main Instrument Panel (MIP) - Seeking Accuracy in Design

I’ve posted this image of the Main Instrument Panel (MIP) of the B737-800NG to briefly examine a few of the differences between a real MIP and a reproduction MIP.   Although a reproduction MIP may appear identical to the real item, there can be subtle differences. 

Let’s look at a few of these differences using the photograph as a reference.

LEFT:  MIP (OEM) from 737-800 NG (click image to enlarge).

Bezel. The bezel is the frame that surrounds the display units (DUs).  In the real aircraft the bezel forms part of the display unit, which is why the bezel breaks open in the lower area; to allow access to and removal of the unit. 

If you carefully look you will note there are no screws that hold the bezel in place to the MIP.  Quite a few manufactures use Phillip head screws in each corner of the bezel to attach the bezel to the MIP

In the real aircraft the bezel is made from machined aluminum.  

Landing Gear Lever.  The real aircraft has a smaller than often seen landing gear knob (the translucent knob).  Further, when the landing gear is in the down and locked position, the red trigger located on the gear shaft completely recesses between the two half-moon protectors. The trigger also is spring-loaded allowing the gear lever to be unlocked by depressing the trigger in specific conditions.

Fuel Flow Reset Switch. The real aircraft incorporates a switch/toggle with a larger defined and bulbous-looking head, rather than a standard-style toggle most manufacturers use.  The OEM toggle is also very specific in operation (3 way pull & release). 

The knobs used on the MIP. These knobs are called general purpose knobs (GPK) and it's uncommon for a reproduction knob to look identical to an OEM knob.  OEM knobs present with curved rather than straight edges and have the grub screw located in a different position.  Many reproduction knobs lack this detail and have the grub screw located at the rear of the knob. 

Furthermore, OEM knobs have an inside metal shroud (circular metal retainer) and a metal grub screw thread, both important to ensure operational longevity of the knob; reproduction knobs usually do not have this.  The shroud can be important as it increases the longevity of the knob as it stops the acrylic from being worn down over time with continual use.

The NG also has a backlit black coloured line that runs adjacent to a translucent line on the front of the knob; at night this line is backlit. Most of the replica knobs have a black line which is a transfer (sticker) that has been hand applied to the knob.  Stickers and transfers over time often lift, especially at the ends and hand application is often haphazard with some transfers straight and other a little off center.

In my opinion, any high end MIP of considerable financial outlay should have appropriate knobs that are high fidelity and replicate the OEM item.

If you look carefully at the photograph you will note that the knobs have curved edges and the Used Fuel Reset Switch has a bulbous appearing toggle.

I am currently writing a short article on "knobs" which will be published in the near future.

Annunciators (Korrys).  The annunciators on reproduction MIPs use LED technology, are only lights/lamps/indicators, and may exhibit an incorrect colour hue in contrast to the OEM part.  Reproductions can also be lacking with regard to the legend, as OEM legends are lazer cut and well-defined. 

Annunciators in the real aircraft are illuminated by 28 Volt bulbs contrasting the low brightness LEDs seen in reproduction Korrys - this alone can make a huge difference in aesthetics.  Finally, the push to test function seen in the real item, to my knowledge, is lacking in reproductions.

  • Note that some newer airframes may use LEDs in favour of bulbs.

Colour.   Boeing grey (RAL 7011), has a specific RAL colour number; however, rarely is every MIP or aviation part painted exactly the same grey colour; there are sublime differences in shade, colour and hue.  Inspect any flight deck and you will observe small colour variations.  Type RAL 7011 into Google and note the varying shades for a specific RAL number.

Dimensions & 1:1 Ratio.  High-end MIPs for the most part are very close to the correct 1:1 ratio of the real item and differences, if noticeable, are marginal.  But, less expensive MIPs can have the incorrect dimensions.  It's not only the overall dimensions that are important, but the dimensions of the spaces, gaps and holes in the MIP that allow fitment of the various instruments and modules.

Whilst this may not be a concern if you're using the stock gauges, etc that came packaged with your MIP, it can become problematic if you decide to use OEM parts.  There is nothing worse that using a Dremel to enlarge a hole in a MIP that isn't quite the correct size.  Worse still, is if the hole in larger than it should be.

Musings - Does it Matter ?

If everything fits correctly into whatever shell you're using, then a small difference here and there is inconsequential.  However, if you are striving for 1:1 100% accuracy then it's essential to know what’s reproduced factually and what is fiction (Disneyland). 

I have only mentioned differences based on what can be seen in the photograph.  There are additional nuances that differ between MIP manufacturers. 

System Simulation is a Priority

As I move more into my project, I realize that many items available in the reproduction market are not identical to the real aircraft; a certain artistic license has been taken by many manufacturers.  This said, while it's commendable to have an exact reproduction of a flight deck, keep in mind that a simulator is primarily a simulation of aircraft systems.

Of course this doesn't mean you throw everything to the wind aesthetically.  To do so would mean you would have an office chair, desk and PMDG in front of you.  Aesthetics are important as they stimulate by visual cues, a level of immersion that allows the virtual pilot to believe they are somewhere other than their own home.

If you inspect real-world flight simulators used by aircraft companies, you will quickly note, that many of the simulators do not replicate everything or strive to have everything looking just like the real aircraft.  Simulators are designed for training and whilst a level of immersion must be apparent, replicating aircraft systems takes priority.

B 737-800 NG Project Status

The overheads are my main concern at the moment; however, I am also working on replacing as much as possible on the Main Instrument Panel with OEM items.  Once completed, all that will remain is the FDS MIP skeleton and a few bits and pieces.  A decision has yet to be made concerning replacement of the glareshield.

The question is probably asked - why not replace the FDS MIP with a OEM MIP.  Whilst this is possible, a NG style OEM MIP apart from being difficult to find and expensive, would require consdierable fabrication to use in a Flight Simulator.  It's far easier to use a commerical MIP as a template and then replace as much as possible with OEM items.

As the MIP project progresses updates will be made.

Acronyms & Glossary

Annunciator - A single coloured light or group of lights used as a central indicator of status of equipment or systems in an aircraft. Usually, the annunciator panel includes a main warning lamp or audible signal to draw the attention of operating personnel to the annunciator panel for abnormal events or conditions.  To annunciate means to display or to become audible.  Annunciators are often called Korrys; Korry is a manufacturer of annunciators.
FDS - Flight Deck Solutions
Korry – See Annunciator.  A brand of annunciator used in the Boeing 737 airframe.
Legend - The plastic lens plate that clips to the annunciator.  the legend is the actual engraved writing on the lense.
MIP - Main Instrument Panel.
OEM - Original Aircraft Manufacture (aka real aircraft part).
RAL - International colour matching system.


B737-800 NG Fuel Flow Reset Switch - OEM Switch Installed and Functional

I have replaced the reproduction Fuel Flow Reset Switch (FFRS) with an OEM part (genuine).  I was not happy with the reproduction switch, which did not function or look anything like the real switch used in the aircraft; the genuine switch is spring-loaded, quite large and has a bulbous head.  The FFRS is a new switch which was probably destined to be installed into a Boeing NG series aircraft.

LEFT:   The FFRS switch can clearly be identified by its bulbous head.  I have observed that on some air frames this switch has a cross hatch design.

FFRS Functionality

The Fuel Flow Reset Switch resides on the center forward panel immediately above the central display unit on the Main Instrument Panel (MIP).  The function of the FFRS is to provide information on the fuel flow and fuel used.  The fuel flow/used indications are displayed on the lower display unit (depending on your set-up preferences). 

The switch is a one-pole spring-loaded two-stage three-way momentary toggle switch.  The normal "resting" position of the switch is in the central (RATE) position.  In this position the display unit indicates the fuel currently being used.  Pushing the switch downwards to (USED) changes the display indication to read the fuel that has been used.  Pulling the bulbous knob towards you whilst simultaneously pushing the switch upwards (RESET) resets the fuel used to zero.  The downward and upward throw of the switch is momentary which means that when the switch is released it will automatically return to its central "resting" position.

The reason the switch is two stage for upwards deployment (pull and push upwards) is for safety; a flight crew cannot inadvertently push the switch to the upwards position resetting the fuel used.

Installation and Wiring

Installation may involve enlarging the circular hole in the MIP, to allow the shaft of the OEM switch to fit through the MIP and the light plate of the Center Forward Panel.  If enlargement is required, care must be taken to not damage the light plate.  If the MIP you are using is 1:1 ratio, then the switch should fit through the hole perfectly.  The switch is secured behind the light plate with a hexagonal nut.  This switch fits the FDS MIP without need for enlarging the hole.

The rear of the FFRS has three standard-style screw post connections, each connection being either positive, negative or common (earth).  To determine which throw of the switch does what, it’s necessary to use a multimeter set to conductivity (beep mode).  Place the black probe of the multimeter on the central screw post and then place the red probe on either of the other two screw posts.  When you move the switch you will hear an audible beep indicating that function is “active” for that screw post.


An interface card is required for the switch to interface with Flight Simulator.  A PoKeys card will suffice; however, I have used a Phidget 0/16/16 card, installed in the SMART module.  This card has been used primarily because it had unused inputs.

LEFT:  Copy of FCOM diagram showing display indications for FFRS.

Establishing the correct functionality is done within the flight avionics software.  If using ProSim737 it’s a matter of finding the fuel flow switch functions within the switches section of the configuration menu and assigning them.  Failing this FSUIPC can be used.

The FFR is but a small item; however, many small items make a sum.  I believe that using an OEM switch improves the aesthetics and function of the flight simulator threefold.

The serial number for the switch is: MS24659-27L

An information sheet concerning the switch can be downloaded from the Training & Documents section - Honeywell  Switch Series Information.

Acronyms and Glossary

FFRS – Fuel Flow Reset Switch (also known as the Used Fuel Toggle)
OEM – Original Equipment Manufacturer
MIP – Main Instrument Panel
Momentary Switch - a switch which can be pushed downwards or upwards and when released returns to a central "resting" position
Two-Stage Switch - A switch that requires two events to activate the switch.  For example, simultaneously pulling and pushing upwards on the switch