B737-800 AFDS Unit - Converted and Installed to MIP

OEM AFDS and bracket is a solid piece of engineering.  it looks like a small 'brick'.  The three angled annunciators can easily be seen in the photograph as can the attachment bracket and screws

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. 

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 disengage 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.

The AFDS is powered by 28 Volts and when illuminated the legends are exceptionally bright and very sharp

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 caution 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).

oem 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

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. 

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 737 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.

Matching OEM AFDS units.  The marks on the glass are scuff marks only and were subsequently cleaned

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)

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.

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 brightness 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 secure the bracket to the MIP.  These are rarely replicated correctly on reproduction AFDS units or MIPs.

oem 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. 

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.

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

B737 Center Pedestal Completed and Installed - Flight Testing Begins

oem 737-500 center pedestal and custom panels.  The center pedestal from the 500 series is very similar to that of the next generation

After spending the best part of two weeks wiring the various panels into the center pedestal I am now pleased with the result. 

The center pedestal is from a Boeing 737-500 and is made from fibreglass.  The earlier series two-bay pedestals were made from aluminium.  The three bay pedestal allows much more room inside the pedestal to mount interface cards and house the wiring for the various panels (modules). 

However, as with every positive there often is a drawback.  In this case there are two drawbacks.  The first is a few spare holes must be covered with OEM blanking plates, and the second is the three bay pedestal is considerably wider than a two bay pedestal.  Whilst climbing into the flight deck is easy at the moment, once a shell is fitted, J-Rails will need to be fitted to the seats to allow easy access. 

Space

Taking advantage of the extra internal space of a three bay, I have constructed a small shelf that fits inside the lower section.  The shelf is nothing fancy - a piece of wood that fits securely between the two sides of the pedestal.  Attached to this shelf are bus bars, a Leo Bodnar interface card and a FDS interface card.  A Belkin powered hub also sits on the shelf.  The power supply for the hub resides beneath the platform to the rear ( for easy access).

The bus bars provide power for the various OEM panels and backlighting, while the Leo Bodnar card provides the interface functionality for the two ACP units.  The FDS card is required for operation of the three FDS navigation and communication radios I am currently using.

My aim was to minimise cabling from the pedestal forward to the throttle unit.  The reason for this is the throttle is motorized and moving parts and USB cables do not work well together.  I have two cables that go forward of the pedestal to the computer; one USB cable from the powered Belkin hub and the other the cable required to connect the CP Flight panels.  Both cables have been carefully routed along the inner side of the throttle quadrant so as to not snag on moving internal parts.

Pedestal Colour

The original pedestal was painted Boeing grey which is the correct colour for a B737-500.  The unit was repainted Boeing white to bring it into line with the colour of the B737-800 NG pedestal.

oem 737-500 center pedestal illuminated by 5 volt incandescent bulbs

Backlighting

The backlighting for the throttle quadrant and center pedestal is turned on or off by the panel knob located on the center pedestal.  Power is from a dedicated S-150 5 Volt power supply rated to 30 amps. 

On the Seventh day, GOD created backlighting and the backlighting was said to be good”.

The light plates are mostly aircraft bulbs; however, a few of the panels, such as the phone and EVAC panel, are LEDS and operate on 28 Volts rather than the standard 5 Volts.

Size Does Matter...

It's important when you install the wiring for backlighting that you use the correct gauge (thickness) wire.  Failure to do this will result in a voltage drop (leakage), the wire becoming warm to touch, and the bulbs not glowing at their full intensity.  Further, if you use a very long wire from the power supply you will also notice voltage drop; a larger than normal wire (thickness) will solve this problem.  There is no need to go overboard and for average distances (+-5 meters) standard automotive or a tad thicker wiring is more than suitable to cater to the amp draw from incandescent bulbs.

To determine the amperage draw, you will need to determine how many amps the bulbs are using.  This can be problematic if you're unsure of exactly how many light plates you have.  There are several online calculators that can be googled to help you figure out the amperage draw.  Google "calculation to determine wire thickness for amps".

At the moment, I am not using a dimmer to control the backlighting, although a dimmer maybe installed at a later date.

Minor Problem - Earth Issue

A small problem which took considerable time to solve was an earth issue.  The problem manifested by arcing occurring and the backlighting dimming.  I attempted to solve the problem by adding an earth wire from the pedestal to the aluminium flooring; however, the issue persisted.  The issue eventually was tracked down to an OEM radar panel which was "earthing" out on the aluminum DZUS rails via the DZUS fasteners.  To solve the problem, I sealed the two metal surfaces with tape.

Panels

The panels I am currently using are a mixture of Flight Deck Solutions (FDS), CP Flight, 500 and Next Generation:

  • NAV 1/2 (FDS)

  • M-COM (FDS)

  • ADF 1/2 (CP Flight) - replaced with FDS

  • Light Panel (OEM)

  • Radar Panel (OEM)

  • EVAC Panel (OEM)

  • Phone Panel (OEM)

  • Rudder Trim Panel (CP Flight) - replacd with OEM

  • ATC Transducer Radio (OEM)

  • ACP Panel x 2 (OEM)

  • Fire Suppression Panel (OEM)

In time a ACARS printer will be added and some of the non NG style panels (namely the ACP panels) will be replaced with OEM NG style ACP panels.  The OEM panels installed are fully operational and have been converted to be used with Flight Simulator and ProSim737.  I will discuss the conversion of the panels, in particular the Fire Suppression Panel, in separate journal posts.

The more observant readers will note that I am missing a few of the "obvious" panels, namely the cargo fire door panel and stab trim panel.  Whilst reproduction units are readily available, I'm loathe to purchase them preferring to wait; eventually I'll source OEM panels.  Rome was not built in a day.

Panel Types

If you inspect any number of photographs, it will become apparent that not all aircraft have exactly the same type or number of panels installed to the pedestal.  Obviously, there are the minimum requirements as established by the relevant safety board; however, after this has been satisfied it's at the discretion of the airline to what they order and install (and are willing to pay for...).  It's not uncommon to find pedestals with new and old style panels, incandescent and LED backlighting, colour differences and panels located in different positions.

oem 737-500 center pedestal telephone. although not next generation it completes the pedestal

Telephone Assembly

Purists will note that the telephone is not an NG style telephone and microphone.  I have keep the original B737-500 series telephone and microphone as the pedestal looks a little bare without them attached. 

If at some stage I find a NG communications assembly I'll switch them, but for the time being it will stay as it is.

Flight Testing - Replication

The throttle quadrant and center pedestal are more or less finished.  The next few weeks will be spent testing the unit, it's functionality, and how well it meshes with ProSim737 in various scenarios.  This process always takes an inordinate amount of time as there are many scenarios to examine, test and then replicate. 

Replication is very important as, oddly, sometimes a function will work most times; however, will not work in certain circumstances.  It's important to find these gremlins and fix them before moving onto the next level. 

KIS - Keep It Simple

Although everything is relatively simple in design (OEM part connects to interface card then to ProSim737 software), once you begin to layer functions that are dependent on other functions working correctly, complexity can develop.   It's important to note that the simulator is using over a dozen interface and relay cards, most mounted within the Interface Master Module (IMM) and wired to an assortment of OEM parts configured to operate with ProSim737's avionics suite.