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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 Real Aviation Part (2)


B737-800 AFDS Unit - Converted and Installed to MIP

The Autopilot Flight Director System (AFDS) is located on the Main Instrument Panel (MIP).  There are two identical units; one situated the Captain-side and the other on the First officer-side. 

LEFT:  The AFDS is a solid piece of enginnering.  it looks like a small 'brick'.  The three angled annunciators can easily be seen in the photograph as can the attachment bracket and screws (click image to enlarge).

The AFDS is one of several components belonging to the Automatic Flight System (AFS) and is also referred to as the autoflight annunciator and autopilot/autothrottle indicator.  The FMC annunciator is often referred to as the FMC alerting indicator.

The purpose of the unit is two-fold; to provide the flight crew with a visual warning of disengagement of the Autopilot and Autothrottle, to an alert on the FMC, and to enable the resetting of and testing of the unit (light test). 

The unit has two annunciation colours, red and amber in either a flashing or steady state which correspond to either an alerting or advisory messages.  Red precedes Amber in the level of importance.  The A/P and A/T annunicators have dual colour capability while the FMC annunciator displays only amber.

This unit was removed from an United Airlines Boeing 737.  On inspection, it was observed that the toggle was slightly bent.  The bent toggle may have been the reason why the part was scrapped; it failed certification. The toggle was easily straightened.

Conditions for Operation

There are four operating conditions:

1:    Autopilot (A/P) Disengage Light

The annunciator will flash RED if either the autopilot or autothrottle is disengaged. The former will also trigger the A/P disengaged tone (whoop, whoop, whoop).  To extinguish the flashing light and reset the unit, the flight crew must push either of the two annunciators (A/P P/RST or A/T P/RST) or press the yoke disengaage switch twice.

The annunciator will illuminate a steady RED in any of the following conditions:

  • The stabilizer is out of trim below 800 feet RA on a duel channel approach
  • The ALT ACQ mode is inhibited during an autopilot go-around (is stabilizer not trimmed correctly)
  • The disengage light test switch is held in position 2, or
  • The automatic ground system fails.

The annunciator will illuminate flashing AMBER when the autopilot automatically reverts to CWS pitch or roll mode while the in Command (CMD).   To extinguish the light, press either the A/P P/RST annunciator or press another mode of the MCP.

The annunciator will illuminate steady AMBER when the light test switch is held in position 1, or when a downgrade in autoland capability occurs.

2:  Autothrottle (A/T) Disengage Light

The annunciator will illuminate flashing RED if the autothrottle (A/T) is disengaged.

The annunciator will illuminate steady RED if the light test switch is held in position 2.

The annunciator will illuminate flashing AMBER to indicate an autothrottle airspeed error exists under either of the following conditions:   

  • Inflight
  • Flaps not up, or
  • Airspeed differs from the commanded value by +10 or -5 knots and is not approaching the commanded value.

The annunciator will illuminate steady AMBER if the light test switch is held in position 1.

3:  Light Test Switch

The AFDS is not connected to the main light test toggle; therefore, it’s equipped with its own light test switch.  The central spring-loaded toggle is used to determine if the unit is operational. 

If the toggle is pushed toward TEST 1, it will illuminate the autopilot, autothrottle and FMC alert annunciator in a steady AMBER colour.  The FMC alert is delayed a few seconds.

If the toggle is pushed toward TEST 2, it will illuminate the autopilot and autothrottle annunciator in a steady RED colour and the FMC alert annunciator will illuminate steady AMBER.  The FMC alert is delayed a few seconds (see last photograph this page).

4:  FMC Alert Light

The FMC P/RST will illuminate steady AMBER when an alerting message exists on the CDU, the fail light on the CDU is illuminated, or the test switch is in position 1 or 2.  To extinguish the annunciation the flight crew can either clear the message from the CDU scratchpad or push the FMC annunciator.

FCOM - Simple yet Confusing

The above information has been interpreted from official documentation from Boeing and whilst straightforward to understand, can appear confusing because of to the repetitious nature of the information and the similar functionality of the unit.

LEFT:  The AFDS is powered by 28 Volts and when illuminated the legends are exceptionally bright and very sharp (click image to enlarge).

Simply put, The AFDS is a caution and advisory panel that illuminates when there is a change from normal flight operations in the autopilot system.  For example, if VNAV disconnects for whatever reason, the A/P annunciator will illuminate (flashing AMBER) to 'cution' the flight crew that something has disengaged in the autopilot system, in this case VNAV.

Anatomy of the AFDS Unit

The AFDS is a solid piece of engineering that contains it's own logic.  The unit has three buttons (annunciators) that illuminate when specific conditions are met.  Each button can be depressed to either cancel/extinguish a caution.  Interestingly, the buttons on the AFDS are angled downwards and are depressed in this direction - the push to cancel is not a direct push as you would expect with normal style korry (see first photograph).

Each annunciator is fitted with four 28 Volt bulbs and depending upon the ‘caution’ either illuminate an amber of red coloured lens plate in a steady or flashing state.  

Removing the button to replace a bulb or troubleshoot highlights the advanced yet simplistic engineering.  A small insert is located on each side of the button and inserting a flat device such as a blade screwdriver or blunt pen knife bland into the insert allows the button to be slowly loosened. 

LEFT:  AFDS button partially removed showing location of four bullet-style 28 Volt bulbs.  The button when removed from the lightplate hangs by a plastic ball which allows the button to be rotated in either direction.

The complete button when carefully pulled from the unit will hang vertically from a plastic bracket that has been designed with a ball which allows the korry to be turned 360 degrees for bulb access.

Interfacing and Configuration

A Phidget 0/16/16 card is used to interface the unit with the avionics software.   Phidgets Manager 21 (free from Phidgets) is required to interface between the flight avionics suite and the actual analogue inputs from the unit.  

The AFDS annunciators are powered by 28 Volts and like the annunciators on the Master Caution System (six packs) there are exceptionally bright to ensure a flight crew notices them when they are illuminated.  

The AFDS, as with many OEM parts, is fitted with two Canon plugs on the rear of the unit (left image).  These plugs make connecting the unit to the Phidget card very easy – provided you know the plug pin outs.  The benefit of using the default Canon plugs are seven-fold: the connection is very good, they are the plug designed for the unit, they look neat and lastly, the plugs are easy to separate if you need to remove the unit for whatever reason.

I am not going to explain how to determine the pin outs.  This information has been documented several times in earlier posts.  For a detailed review see this link - How To Determine Connectivity.

Another post of interest is Using Interface Cards & Canon Plugs to Convert OEM B737 Parts.

Configuration in ProSim737

It is a two-step process to configure the AFDS unit.  First, the Phidget Manager 21 software must be opened to check the 0/16/16 card designation number and to determine the digital output numbers for the three AFDS switches.  To find the outputs, press any of the switches on the AFDS and note the output number.

Next, open the configuration menu in ProSim737.  You need to configure both switches and indicators (lights).  Find the specific switch in the switches menu and push one of the three switches on the AFDS and assign this to the Phidget 0/16/16 card in the drop down menu.  The output for the switch can be seen at the top of the configuration screen.  ProSim737 also has a very easy to use auto find option.  Press the AFDS switch followed by F and the software automatically assigns this switch to the correct

Interface Card and Outputs 

Then in indicators, use the same card designation used in switches and assign the digital output (found in the Phidget Manager 21 software).  ProSim737 has an automated method for determining the lights/indicators.  Open the configuration menu and selecting the letter F opposite the function required.  The software will then do a sweep of all lights and functions determining the appropriate setting.

Whilst this sounds confusing, it’s very straightforward and comparatively easy to accomplish.  

Installation to MIP

It’s not difficult to mount the unit to the Main Instrument Panel (MIP) as there is already a gap in the MIP where the reproduction unit was fitted.  Depending upon which MIP type you are using, the hole may have to be enlarged with a dremel or a number 2 ‘bastard’ metal file before being finely finished using to remove any sharp edges.

LEFT: Although the hole for the AFDS can be enlarged with the MIP plate in-situ, any filing will result in a fair amount of waste filings.  The AFDS MIP plate should be removed to facilitate easier cutting and enlargement of the hole (if necessary).

The size of the hole should allow the AFDS unit to be firmly placed in the MIP so that the switches and buttons can be firmly pressed without the unit being dislodged. 

The difference in the length of the unit compared with a reproduction unit is obvious, which is why a secure method of attachment is paramount.  There are several methods in which to secure the unit; the best method to use is the original attachment bracket (seen in the first image).  If the bracket is missing, a solid sealant works well.

AFDS Bracket and Screws

The bracket is a specialist bracket designed to hold the AFDS unit securely to the MIP.  Once the unit is fitted to the MIP, the bracket is slid over the AFDS unit until snug with the rear of the MIP.  The four screws are then placed through the MIP from the front and tightened against the bracket.  This ensures that the unit will not dislodge.  Note that the screws are of two sizes. 

There is strong possibility that the MIP used will not feature the four holes to secure an OEM AFDS unit to the bracket and MIP.  These holes must be drilled into the MIP.  This task requires a solid eye as if the screw holes are not aligned correctly with the bracket, the unit will not fit correctly.

The AFDS units in these images lack the scews as the bracket has yet to be fitted.

OEM Verses Reproduction

First off, most the reproduction units are very good.  There is not a lot to the ADFS unit - basically three push annunciators and a two-way toggle.  The main difference between OEM and reproduction units is:

  • Brightness of annunciations and spread of light – 28 Volt bulbs verses the lower luminance and light spread of LEDS;
  • annunciator legends are laser engraved and are easy to read;
  • feel of the actual annunciators and toggle;
  • the outside appearance of the unit; being OEM, the unit cannot look any better than what it does…;
  • power Consumption and Heat Generation; and, the
  • the four screws on the front of the MIP which decure the bracket to the MIP.  These are rarely replicated correctly on reproduction AFDS units or MIPs.

As with most OEM parts the AFDS units are not brand new but exhibit the usual expected service wear.  This second-hand look may not 'appeal' to everyone.

LEFT: The AFDS with three annunciators illuminated during daylight by pressing test 2.  The 28 Volts provides ample power to allow the lights to be seen easily during daylight flying.  Note that the four screws are not visible in the photograph as the the bracket still needs to be fitted.  The speedbrake korry and stab-out-of-trim korry in the photograph are reproduction (Flight Deck Solutions) to be  replaced with OEM annunciators.

Power Consumptions (bulbs and LEDS)

It is often said that a benefit of using LEDS is the saving of power and generation of less heat.  Whilst this is definitely true for items that are permanently on and illuminated, such as backlighting, many Korrys only illuminate when a specific event triggers them, and then they are only lit up for a very short period of time.  Therefore; the amount of heat and subsequent power draw is negligible.

Another point in question is the use of bulbs and LEDS in the same airframe.  Whilst it is true that LEDS are replacing bulbs in more modern airframes, it is not unrealistic to have a B737-800 with a collection of bulbs and LEDS.  As modules are replaced with newer units, LED technology will slowly creep into the older style flight decks.  

If you are having difficulty coming to grips with using either bulbs or LEDS be assured that both are realistic.

Acronyms and Glossary

The meaning of the below acronyms are second nature to many of you; however, bear in mind that everyone has to begin somewhere and some readers may not yet understand what each acronym stands for.

AFDS - Autopilot Flight Director System
AFS - Automatic Flight System
ALT ACQ – Altitude Acquisition
A/P - Autopilot
A/T - Autothrottle
CDU – Control Display Unit (used in this website interchangeably with FMC)
CMD - Command A or B engagae button on MCP (autopilot activation)
CWS – Control Wheel Steering
FMC - Flight Management Computer (used interchangeably in this website with CDU)
Korry – See Annunciator.  A brand of annunciator used in the Boeing 737 airframe
Legend – the engraved light plate on the front of a Korry (for example, FMC P/RST)
OEM – original Aircraft Manufacture (real aviation part)
Phidget Manager 21 – Software downloadable from Phidgets website that allows card to be interfaced between OEM part and avionics suite
RA – Radio Altitude


OEM B737 Landing Gear Mechanism - Installed and Functioning

I have replaced the landing gear lever supplied by Flight Deck Solutions (FDS) with the landing gear mechanism (LGM) from a Boeing 737-500 aircraft.  The reason for the replacement of the gear was not so much that I was unhappy with the FDS landing gear, but more in line with wanting to use OEM parts.

LEFT:  OEM landing gear mechanism - large and hefty, but impossible to upgrade.  Note that spacer is attached (click image to view larger).

Before wiring further, there are a number of differing styles of landing gear mechanisms seen on Boeing aircraft depending upon the aircraft series.  For the most part, the differences are subtle and relate to wiring and connectivity between different aged airframes.  However, there is a difference in the size of the gear knob between the Boeing classics (300 through 500) and the NG.  The knob is the opaque knob located at the end of the gear handle.  On the classics this knob is rather large while on the NG the knob is roughly 20% smaller in size.  There is also a difference in the length of the stem - the NG being a little shorter than the classics.

The landing gear mechanism I obtained came from a United Airlines B737-300; therefore, had the larger classic style knob.  I replaced this knob with a NG style knob.

Anatomy of LGM

The landing gear mechanism is quite large, is made from aluminum and weights roughly 2 kilograms.  Most of the weight is the heavy solenoid that can be seen at the front of the unit.  A long tube-like structure provides protection for the wiring that connects the solenoid to the harness and Canon plug at the side of the unit.  The red-coloured trigger mechanism is spring loaded and the landing gear lever can be extended outward (toward you) when raising and lowering the gear.

Installation and Mounting

I am using a Main Instrument Panel (MIP) designed by Flight Deck Solutions which incorporates a very handy shelf.  Determining how to mount the gear mechanism was problematic as the position of the shelf would not allow the mechanism to be mounted flush to the MIP.  After looking at several options, it was decided to cut part of the shelf away to accommodate the gear mechanism. 

Once this had been done (rather crudely as I didn’t want to use a dremel on the actual MIP), it became apparent that, although the mechanism mounted flush to the MIP the landing gear lever was not in the correct position; the lever was too far out from the front surface of the MIP and the trigger was not within the half-moon plates when in the down position.


The solution to this problem was to design and mount a 0.5 cm thick spacer to the front of the landing gear.  This spacer was made from plastic and cut to the exact measurement of the gap that the landing gear lever moves through.  Attaching the spacer to the lightweight aluminum of the landing gear mechanism was straightforward and was done with four small screws.  Once the spacer was attached, the position of the trigger in relation to the half-moon plate was more accurate.

Cutting the FDS Plate

Another minor hurdle needing climbing was to alter the aluminum plate located behind the FDS light plate.  The FDS landing gear secures to two ridges that are at 90 degrees to the MIP.  These two ridges need to be removed to incorporate the flat surface of the front of the OEM landing gear mechanism.  The trusty Dremel was used to cut through the thin aluminum to remove the two ridges

LEFT:  Carefully removing the two ridges from the FDS main backing plate)

Custom Bracket

The next issue was how to attach the landing gear mechanism to the MIP.  I made a bracket that fitted snugly to the upper part of the gear mechanism.  To secure the bracket to the gear mechanism, it was a matter of ensuring that the bracket leg covered two preexisting holes.  A bolt was then used to secure the bracket.  Two further holes at the front of the bracket secure the upper part of the gear mechanism to the MIP.  Once again, I picked up on existing holes in the MIP. 

LEFT:  Custom bracket that is used to secure the upper part of the LGM to the rear of the MIP.

To secure the lower part of the landing gear mechanism to the MIP, I replaced the existing bolts used to attach the half-moon to the MIP, with longer bolts.  I then drilled a small hole to the front plate of the landing gear mechanism and spot welded a nut to the inside of the hole.  The bolts could then be used to secure the gear mechanism to the MIP.  To stop lateral movement of the gear mechanism, I used a standard L bracket to secure the unit to the shelf of the MIP.

The reason for the secure mounting will become obvious later in the post.

Stem Length and Buttons for FS configuration

One aspect to take note is that the NG landing gear lever is one inch shorter than the classics; therefore, one inch of the lever needs to be removed.

To connect the actual up and down functionality to Flight Simulator requires the use of two configurable buttons; one for gear up and the other for gear down.  The two buttons (not pictured) are located inside the unit screwed to the inner side of the housing.

Reproduction or OEM

There are three main reasons for using an OEM landing gear mechanism rather than a reproduction unit.

LEFT:  LGM solenoid.  The LGM does have a handy foot beneath the solenoid for attachment to the MIP shelf; however, this foot sits too far forward of the shelf to be of use when the LGM is flush to the MIP.

The mechanism, as mentioned earlier, includes a solenoid.  This solenoid stops the landing gear from being raised or lowered at certain landing gear lever positions.  Reproduction units rely on software to replicate the function of the solenoid.  Using an OEM unit allows the solenoid to be used.

Another difference is the trigger.  Reproduction units usually do not incorporate a spring-loaded trigger as a solenoid is not connected.  The OEM unit requires a spring-loaded trigger to engage or disengage the solenoid.

Furthermore, reproduction units often do not provide correct positioning of the trigger in relation to the half-moon.  The half-moon and trigger are safety features and the trigger should be partially hidden between each of the two half-moons when the landing gear is in the DOWN position.


A Phidget 0/0/8 relay card was used.   The card interfaces the actions of the solenoid (on/off) and is then read by the avionics suite (ProSim737).  The relay card is mounted in the System Interface Module (SIM) and connection from the card to the landing gear mechanism is via the Canon plug discussed earlier. 

LEFT:  Canon plug on ABS plastic mounting plate.  The use of the Canon plug enables a cleaner wiring configuration.

Mounting the interface cards in the SIM provides a central point for card mounting with the benefit of also being able to use this card for other functions for other OEM parts with minimal wiring.

The solenoid requires 28 volts to enable activation.

Muscle Required!

To use OEM landing gear requires muscle!  Pulling the gear lever from its recess position is not a slight pull.  Likewise, moving the gear lever between down, off and up requires a bit of strength.  This is why mounting the mechanism securely is very important.

Operation and Safety Features

Boeing has incorporated several devices in the aircraft, such as squat switches, computerized probes and mechanical locks (down and up-locks), to ensure that the landing gear cannot be raised when there is weight on the main landing gear.  If weight is registered, then the landing gear lever lock is activated inhibiting the gear lever from being able to be placed in the UP position.  This lock is controlled by the solenoid.   

An override trigger in the lever may be used to bypass the landing gear lever lock.  Depressing the trigger will disengage the lock and allow the gear lever to be moved to the UP position.  The reason for the half-moons should now be obvious.  By partially covering the trigger, the half-moons act as a physical barrier to stop a pilot from easily accessing the trigger mechanism to disengage the landing gear lever lock.

After rotation, the air/ground system energizes the solenoid which opens the landing gear lever lock allowing the gear lever to be raised from the DOWN to the UP position.

How it Works in the Real Aircraft (Hydraulic Pressure)

In the real Boeing aircraft hydraulic pressure is used to raise the landing gear.  This pressure is supplied through the landing gear transfer unit.  

Hydraulic system B supplies the volume of hydraulic fluid required to raise the gear.  Conversely, hydraulic system A, by supplying pressure to release the up-locks, is used to lower the landing gear.  Once the up-locks have been disengaged, the gear will extend by gravity, air loads and to a limited extend hydraulic pressure.  

Moving the landing gear lever to OFF (following take off) will remove all hydraulic pressure from the system.

In-Flight Testing

The solenoid and trigger mechanism operate in the simulator as it does in the real aircraft.  When you start flight simulator and ProSim737 there is an audible clunk as the solenoid receives power.   Immediately after rotation, you hear another audible clunk as the solenoid is energized (to open the landing gear lock).

If you want to raise the gear lever to UP whilst on the ground, the only way to do so if by depressing the trigger to override the landing gear lock.

Obviously, hydraulic pressure is not simulated...


OEM parts have been used; there are rarely new.  Therefore, it's expected that any part removed from an operational airliner may have slight aesthetic damage. 

LEFT:  Scratching to the gear lever shaft.  Note the access pin on the shaft that allows removal of the retractable trigger.  Also note the smaller NG style knob which replaced the larger knob used on the classics.

Often the damage occurs when someone removes the part from the aircraft at the scrap yard.  In the case of this part, the landing gear lever was scratched.  Although it has been repainted, the scratches are still apparent if you look very closely. 

Historical Perspective

It's easy to forget that a separated part actually did one belong to an operational airliner. In this case the landing gear mechanism belonged to United Airlines 737-300 registration number N326UA. 

Although arbitrary, I like the idea that the part I'm using was actually used on a real aircraft, has flown the miles and served the airline.  It makes the simulator "feel" a little more real.

LEFT:  United Airlines N326U landing at KLAX (Los Angeles) airport on 26 October, 2008.  Photograph courtesy of Airliners.Net.

Is the effort of installing an OEM mechanism to the simulator worthwhile?  I believe the answer is yes.  The use of the solenoid provides added realism as does the use of a spring-activated trigger, while the tightness of the movement of the gear handle between a reproduction and OEM unit is obviously different.


OEM - Original Equipment Manufacture
FDS - Flight Deck SolutionsMIP - Main Instrument Panel
LGM - Landing Gear Mechanism
NG - Next Generation (B737-800NG)
Half-moons - the two plates that are positioned either side of the trigger of the landing gear when in the DOWN position