E-mail Subscription

Enter your email address:

Delivered by FeedBurner

Syndicate RSS

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...)

No advertising on this website - EVER!


Find more about Weather in Hobart, AU
Click for weather forecast






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

Journal Archive (Newest First)

Entries in Autopilot Flight Director System (1)


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