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Mission Statement 

The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.


Note:   I have NO affiliation with ANY manufacturer or reseller.  All reviews and content are 'frank and fearless' - I tell it as I see it.  Do not complain if you do not like what you read.

I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).


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

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

Journal Archive (Newest First)

Entries in Control Display Device (2)


OEM B737 CDU Conversion - Introduction

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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


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

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

Aesthetic Differences

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

Main Differences - 500 series to NG

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

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

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

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

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

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

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


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

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

Converting the CDU

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

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

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


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


Direct-To-Routing, ABEAM PTS and INTC CRS - Review and Procedures

In an earlier post, a number of methods were discussed in which to create waypoints ‘on the fly’ using the Control Display Unit (CDU).  Following on a similar theme, this post will demonstrate use of the Direct-To Routing, ABEAM PTS and Course Intercept (INTC CRS) functionality.

CDU use an appear very convoluted to new users, and by far the easiest way to understand the various functionalities is by ‘trial and error and experimentation’. 

The software (Sim Avionics and ProSim737) that generates the math and formulas behind the CDU is very robust and entering incorrect data will not damage the CDU hardware or corrupt the software.  The worst that can happen is having to restart the CDU software. 

Line Style and Colour

The style and colour of the line displayed on the Navigation Display (ND) is important as it provides a visual reference to the status of a route or alteration of a route.

Dashed white-coloured lines are projected courses whilst solid magenta-coloured lines are saved and executed routes.  Similar colour schemes apply to the waypoints in the LEGS page.  A magenta-coloured identifier indicates that this is the next waypoint that the aircraft will be flying to (it is the active waypoint).

Direct-To Routing

A Direct-To Routing is easily accomplished, by selection of a waypoint from the route in the LEGS page, or by typing into the scratchpad (SP) a NAVAID identifier and up-selecting this to LSK 1L.  Once up-selected, the Direct-To route will be represented on the Navigation Display (ND) by a dashed white-coloured line.  Pressing the EXEC button on the CDU will accept the route modification and precipitate several changes:

  • The route line displayed on the ND, previously a white-coloured dashed line will become solid magenta in colour;
  • The previous displayed route will disappear from the ND;
  • All waypoints on the LEGS page between the aircraft's current position and the Direct-To waypoint in LSK 1L will be deleted; and,
  • The Direct-To waypoint in LSK 1L will alter from white to magenta.

Once executed the FMS will direct the aircraft to fly directly towards the Direct-To waypoint.


Following on from the Direct-To function is the ABEAM PTS function located at LSK 5R. 

ABEAM points (ABEAM PTS) are one or more fixes that are generated between two waypoints from within a programmed route.  The ABEAM PTS functionality is found in the LEGS page of the CDU at LSL 5R and is only visible when a Direct-To Routing is being modified, within a programmed route (the LEGS page defaults to MOD RTE LEGS).  Furthermore, the ABEAM PTS dialogue will only be displayed if the the up-selected fix/waypoint is forward of the aircraft's position; it will not be displayed if the points are located behind the the aircraft.

If the ABEAM PTS key is depressed, a number of additional in-between fixes will be automatically generated by the Flight Management System (FMS), and strategically positioned between the aircraft’s current position and the waypoint up-selected to LSK 1L.  The generated fixes and a white-coloured dashed line showing the modified course will be displayed on the Navigation Display (ND).  

To execute the route modification, the illuminated EXEC button is pressed.  Following execution, the white-coloured line on the ND will change to a solid magenta-coloured line, and the original displayed route will be deleted.  Furthermore, the LEGS page will be updated to reflect the new route.

Nomenclature of Generated Fixes

The naming sequence for the generated fixes is the first three letters of the original waypoint name followed by two numbers (for example, TTR will become TTR 01 and CLARK will become CLA01).  If the fixes are regenerated, for instance if a mistake was made, the sequence number will change indicating the next number (for example, TTR01, TTR02, etc).  


  1. Up-select a waypoint from the route in the LEGS page to LSK 1L, or type into the scratchpad a NAVAID identifier.  This is a Direct-To Routing; when executed the waypoints between the up-selected waypoint and LSL 1L are deleted.
  2. Press ABEAM PTS in LSK 5R to generate a series of fixes along a defined course from the aircraft’s current location to the up-selected waypoint.  The fixes can be seen on the ND.
  3. Pressing the EXEC button will accept and execute the ABEAM PTS route.

Example and Figures

The below figures are screen captures using ProSim737 avionics suite.  The programming of the CDU has been done with the aircraft on the ground.  Click any image to enlarge.

FIGURE 1:  The LEGS page shows a route HB-TTR-CLARK-BABEL-DPO-WON.  The route is defined by a solid magenta-coloured line.   

FIGURE 2:  The Route is altered to fly from HB to BABEL.  Note that in the LEGS page, the title has changed from ACT to MOD RTE 1 LEGS.  The ND displays the generated ABEAM PTS and projected course (white-coloured dashed line), beginning from the aircraft’s current position and travelling through HB01, TTR01, CLA01 to BABEL.   The EXEC light is also illuminated.

FIGURE 3:  When the EXEC light is pressed, the ABEAM PTS and altered route (Figure 2) will be accepted.  The former route will be deleted and the white-coloured dashed line will be replaced by a solid magenta-coloured line.  The magenta colour indicates that the route has been executed.  The LEGS page will also be updated and display the new route, with the waypoint HB01 highlighted in magenta.   

The Intercept Course (INTC CRS)

To understand the INTC CRS, it is important to have a grasp to what a radial and bearing is and how they differ from each other.  For all practical purposes, all you need to know is that a bearing is TO and a radial is FROM.  For example, if the bearing TO the beacon is 090, you are on the 270 radial FROM it.  A more detailed explanation can be read by following the ‘radials’ link in the acronyms section at the end of this article.

The Intercept Course (INTC CRS) function is located beneath the ABEAM PTS option in the LEGS page of the CDU at LSK 6R.  Like the ABEAM PTS function, the INTC CRS function is only visible when a when a Direct-To Routing, is being modified within a programmed route (the LEGS page defaults to MOD RTE LEGS).

The function is used when there is a requirement to fly a specific course (radial) to the fix/waypoint.  By default, the INTC CRC displays the current course to the fix/waypoint.  Altering this figure, will instruct the FMS to calculate a new course, to intercept the desired radial towards the fix/waypoint (1)  The radial will be displayed on the ND as a white-coloured dashed line, while the course to intercept the radial (from the aircraft’s current position) will be displayed as a magenta-coloured dashed line.

Visual Cues

An important point to note is that,  if the course (CRS) is altered, is that the displayed (ND) white-coloured line will pass directly through the fix/waypoint, but the line-style will be displayed differently dependent upon what side of the fix/waypoint the radial is, in relation to the position of the aircraft.  The line depicted by sequential long and short dashes (dash-dot-dash) shows the radial TOWARDS the fix/waypoint while the line showing dots, displays the radial AWAY from the fix/waypoint. 

It is important to understand, that for the purposes of the FMS, it will always intercept a course TO a fix/waypoint; therefore, the disparity in how the line-style is represented provides a visual cue to ensure a flight crew does not enter an incorrect CRS direction.

Intercept Heading

However, the flight crew may wish not fly directly to the fix/waypoint, but fly a heading to intercept the radial.  In this case, the flight crew should select the particular heading they wish to fly in the MCP heading selector window, and providing LNAV is armed, the aircraft will fly this heading until reaching the intercept course (radial), at which time the LNAV will engage and the FMS will direct the aircraft to track the inbound intercept course (radial) to the desired fix/waypoint.


  1. Up-select a waypoint from the route in the LEGS page to LSK 1L, or type into the scratchpad a NAVAID identifier and up-select.  This is a Direct-To Routing and will delete all waypoints that the aircraft would have flown to prior to the up-selected identifier.
  2. Type the course required into INTC CRS at LSK 6R.
  3. This will display on the ND a white-coloured long dashed line (course/radial).  Check the line-style and ensure that the course is TOWARDS the waypoint.  The line, closest to the aircraft should display sequential long and short dashes.
  4. Prior to pressing the EXEC button to confirm the route change, check that the intended course line crosses the current course line of the active route (solid magenta-coloured line).
  5. If wishing to fly a heading to intercept the radial, use the MCP heading window.  If LNAV is armed the FMS will direct the aircraft onto the radial.

Example and Figures

The below figures are screen captures using ProSim737 avionics suite.  The programming of the CDU has been done with the aircraft on the ground.  Click any image to enlarge.

FIGURE 1:  The LEGS page shows a route HB-TTR-CLARK-BABEL-DPO-WYY-WON.  The route is defined by a solid magenta-coloured line.   ATC request ‘QANTAS 29 fly 300 degrees until intercepting the 345 degree radial of BABEL; fly that radial to BABEL then remainder of route as filed’.

FIGURE 2:  From the LEGS page, locate in the route the waypoint BABEL (LSK 4L).  Recall that the INTC CRS will only function in Direct-To Routing mode. Up-select BABEL to LSK 1L.  Note that a dashed white-coloured line is displayed on the ND showing the new course from HB to BABEL.  The original course is still coloured magenta and the EXEC light is illuminated.

FIGURE 3:  Type the radial required (345) into INTC CRS at LSK 6R.  This action will generate (fire across the page) a white-coloured dashed line displaying the 345 course to BABEL (the 165 radial).  Check the line-style and ensure the radial crosses the aircraft’ current course which is 300.  Recall that this line style indicates that the radial to TO BABEL.

FIGURE 4:   Press EXEC to save and execute the new route.  The dashed line alters to a solid magenta-coloured line and joins with the remainder of the route at BABEL.  The magenta colour indicates this is now the assigned route.  Note that the magenta line continues across the ND away from the aircraft and BABEL.  This is another visual cue that the radial is travelling TO BABEL.

If the aircraft continues to fly on a course of 300 Degrees, and LNAV is armed, the FMS will alter course at the intersection and track the 345 course to BABEL (165 radial).  The LEGS page is also updated to reflect that BABEL is the next waypoint to be flown to (BABEL is coloured magenta).


Direct-To Routings and ABEAM Points are usually used when a flight crew is required to deviate, modify or shorten a route.  Although the use of ABEAM PTS can be debated for short distances, the technology shines when longer routes are selected and several fixes are generated. The Intercept Course function, on the other hand, is used whenever published route procedures (STAR and SID transitions), or ATC require a specific course (radial) or heading to be followed to or from a navigation fix.


The content of this post has been checked to ensure accuracy; however, as with anything that is convoluted minor mistakes can creep in (Murphy, aka Murphy's Law, reads this website).  If you note a mistake, please contact me so it can be rectified.

Acronyms and Glossary

ATC – Air Traffic Control
CDU – Control Display Unit
Direct-To Routing – Flying directly to a fix/waypoint that is up-selected to LSK 1L in the CDU.  All waypoints prior to the u-selected waypoint will be deleted
DISCO – refers to a discontinuity between two waypoints loaded in a route within the LEGS page of the CDU.  The DISCO needs to be closed before the route can be executed
DOWN-SELECT - Means to download from the CDU LEGS page to the scratchpad of the CDU)
FIX – A geographical position determined by visual reference to the surface, by reference to one or more NAVAIDs
FMC – Flight Management Computer
FMS – Flight Management System
Identifiers – Identifiers are in the navigation database and are VORs, NDB,s and published waypoints and fixes
LSK 5L – Line Select: LSK refers to line select.  The number 5 refers to the sequence number between 1 and 6.  L is left and R is right (as you look down on the CDU in plan view)
MCP – Mode Control Panel
NAVAIDS – Any marker that aids in navigation (VOR, NDB, Waypoint, Fix, etc.).  A NAVAID database consists of identifiers which refer to points published on routes, etc
ND – Navigation Display
RADIALS – A line that transects through a NAVAID representing the points of a compass.  For example, the 045 radial is always to the right of your location in a north easterly direction (Bearings and Radials Paper)
ROUTE – A route comprising a number of navigation identifiers (fixes/waypoints) that has been entered into the CDU and can be viewed in the LEGS page
SP - Scratchpad
UP-SELECT – Means to upload from the scratchpad of the CDU to the appropriate Line Select (LSK)

WAYPOINT – A predetermined geographical position used for route/instrument approach definition, progress reports, published routes, etc.  The position is defined relative to a station or in terms of latitude and longitude coordinates.

1:  The FMS will calculate the new course based on great circle course between the aircraft’s current location and the closest point of intercept to the desired course.  This course is displayed on the ND as a white dashed line.