Conversion of OEM CDU - Part One

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Completely gutted.  All unnecessary and unusable electronic components have been removed

One of the more advanced projects is the conversion of two OEM Control Display Units manufactured by Smiths.  The two CDUs came from a Boeing 500 series airframe that was retired from service in 2008 due to United Airlines decision to adopt the Airbus A-320.  A chronometer located on the rear of each unit, shows the hours of use - one unit has 5130 hours while the other has 1630 hours.

The Control Display Unit (CDU) is the interface that the flight crew use to access and manipulate 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.  Often the words CDU and FMC are used interchangeably.

In this article I will discuss some of differences between OEM and reproduction CDUs. In addition to explaining some of the advantages that using an OEM unit brings.  A second article will deal with the actual conversion of the units to operate with ProSim737.

Port side of CDU with casing removed to show the electronic boards that are secured by lever clips.  Like anything OEM, the unit is constructed from solid component

Construction and Workmanship

The construction and workmanship that has gone into producing anything OEM is quite astounding. 

The CDU is built like a battleship and no amount of use or abuse can damage the unit.  The unit is quite large and heavy.  I was surprised at the eight, a good 6 kilograms.  Most of the weight is made up by the thick glass display screen  CRT, and other components that reside behind the glass within the sturdy aluminium case. 

A myriad number of small screws hold together the 1 mm thick aluminium casing that protects the internal components.  In addition to screws, there are two special DZUS fasteners, that when unlatched, enable the side of the unit’s casing to be removed for maintenance. 

When the casing of the CDU is removed, the inside is jammed full of components, from the large CRT screen to gold-plated electronic boards that are clipped into one of three internal shelves.

One aspect in using anything OEM is the ease at which the item can be inserted into the flight deck.  DZUS attachments enable the unit, once it has been slid into the CDU bay, to be securely fastened.  I use a MIP manufactured by Flight Deck Solutions and the CDU slides seamlessly into the CDU bay.

Detail of the keyboard and DIM knob.  Interestingly the DIM knob dims the actual CRT screen and not the backlighting

Tactile Differences

Aside from external build quality, one of the main differences you immediately notice between an OEM and reproduction CDU, is the tactile feeling when depressing the keys on the keyboard.  The keys do not wobble in their sockets like reproduction keys, but are firm to press and emit a strong audible click. 

Furthermore, the backlighting is evenly spread across the rear of the keyboard panel with each key evenly illuminated.

Aesthetic Differences – 500 Series and Next Generation

As the CDU dates from 2008, the external appearance isn’t identical to the CDU used in the Next Generation airframe, however, it is very close.

Main Differences:

  • The dim knob is a slightly different shape.

  • The display screen is rounded at the corners od the screen (the NG is more straight-edged).

  • The absence of the horizontal white lines located on the inside edge of the display frame bezel.

  • The display screen is different (cathode ray tube (CRT) in contrast to liquid crystal display (LCD).

  • The illumination is powered by bulbs.

In terms of functionality, as this is controlled by software (ProSim737) the functionality is identical.  This also holds true for the font type and colour.

To an absolute purist, these differences may be important, and if so, you will have to contend with a reproduction CDU, or pay an exorbitant amount for a decommissioned NG unit. 

OEM CDU installed to MIP functioning with ProSim737

Conversion for use with ProSim737

There are many ways to convert a real aircraft part for use in Flight Simulator.  By far the most professional and seamless is the integration of the real part using the ARINC429 protocol language (as used in the real aircraft).  However, using ARINC429 is not a simple process for all applications.  Not too mention that you often must use high voltage AC power.

For the most part I’ve used Phidgets to convert real parts, however, in this conversion I wanted to try a different approach.  I’m going to liaise with an Australian company called Simulator Solutions.  This company specialises in converting high-end electronic components used in commercial flight simulators, and manufactures an interface board that should enable seamless conversion of the CDU.

Glossary

  • ARINC 429 –  A standard used to  address data communications between avionics components.  The most widely used  standard is an avionics data bus.  ARINC 429 enables a single transmitter to communicate data to up to 20 receivers over a single bus.

  • OEM - Original Equipment Manufacture (aka real aircraft part).

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

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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.

ABEAM PTS

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).  

Technique

  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 traveling 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. 

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.

Technique

  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 traveling 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

Final Call

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.

Caveat

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.