6- FCU and EFIS

Translation by Christian Girardet


With the goal of getting a “flyable” airplane as quickly as possible, we move now to the FCU. To extend the landing gear you can always hit the “G” key on the keyboard but if you want to fly your plane, you must have a FCU and the Throttles.


The FCU is one of the most complex part for the cockpit, with all its LED displays, rotary or push-pull switches.


FCU stands for Flight Control Unit. It allows the pilots to turn on the various flight modes (selected/managed), the Auto-pilot, the Auto-Thrust as well as the visualization of the selected data: SPD, HDG, TRK, ALT, V/S, FPA. The four main switches can be pushed (managed mode) or pulled (selected mode). In the “selected” mode, the values will be displayed as numbers while in “managed” mode, these values are replaced by the underscore symbol (-). Altitude, on the other hand is always displayed as a number on the FCU panel.


The small panels on each side of the FCU are erroneously called “EFIS”. The “EFIS” is actually the system gathering and displaying the flight data on the PFD and the ND screen. Overtime, the misnomer as become so prevalent in the aviation field that we will go along and call these 2 control panels “EFIS” too.

FCU front panel


First thing is to print and back-light the FCU front panel. It is actually difficult to get a consensus on the dimensions for the FCU: each of the suppliers, OpenCockpits, HispaPanels and CockpitSonic have their own idea about what the size of the FCU should be!


I used the Flight Deck Solutions FCI and EFIS panels (thanks to Valerio a.k.a. “Captain Minch”). As FDS does not allow its clients to share the dimensions of the panels, those indicated here-under come from a different source. By the way, FDS no longer supplies individual panels but fully completed cockpits.


As for the other panels, the above listing of drawings contains 3 versions of the FCU panel: one B&W for the flashing of the film, one in RAL7011 color for the visible side of the panel and one reversed for the drilling template.

The whole series is as follows:

Front side : drilling measurements
Front side : drilling measurements
Flashed film
Flashed film
Visible side
Visible side
Reversed image
Reversed image

 One can also have an additional B&W film printed to use as the mask/cut-out for the FCU window displays.

Just as previous panels the FCU front is be made in 3 to 4 mm Plexi. Just follow the same process used before.


This panel does not hold any component. It is screwed onto a fairly thick (6mm) backing panel “E” using four 4 mm screws. The cuts for the backing panel are the same as those of the front panel but for two dimensions: the four 23 mm holes in the front panel are only 6mm wide on the backing plate and the three holes for the push buttons (10mm diam. on the front panel) are 7mm on the backing Plexi. Drill both panels together and then enlarge the wholes on the front panel. That done, the next thing to tackle is the printed circuits: their size will impact that of the other FCU components and their holes will be used as a template to drill the backing panel.

Printed circuit boards

It is prudent to first get the two PCBs needed for the FCU before making all the mechanical parts of the FCU: you won’t be able to change the PCBs’ size, whereas the other parts can be adjusted if required.


OpenCockpits offers some PCBs for 3x or 5x seven segment LED displays, but this is a complex option to go with. The LED blocks used for the fixed displays (SPD, MACH,…) require the 7 segment LED displays to be at least 8 mm above the PCB. This can be done by stacking up some round pins sockets. In that case, the pins for the LED blocks above the displays do not touch the 8 pin connector for the 7 segment LED display. The end product is reliable and strong, but not really elegant…





Here are 2 display installed on a three 7 segment LED display OpenCockpit PCB with a dry fit of the 8 connection pins. The PCB is offset from the Plexi using nylon 3 mm washers.



One of our friends, Thierry, has solved the problem by designing a beautiful double-layer PCB including all the displays, the LED blocks as well as the resistors. The dimensions are 245 mm x 27 mm and the central section is 32mm wide. The right side of the board, as seen on the following drawing allows for cuts depending on the front panel source you will use.

As most of the others found on this site. This PCB was created with the TCI software (Tracé de Circuits Imprimés). The original file, with the .tci suffix is available in the dowloadable plans. To open it, download TCI here and install it on your C:Program Files folder. Very easy to use and to learn, within 5 minutes, you will be running! To open the 6-FCU_afficheurs_v6_TF.tci file, copy it into the “Tracé de CI” folder you created when you installed TCI, start TCI and select the file in the “Open” menu.


This PCB does not require any metallized via: there very few connectors and the wires will be soldered on both sides. Not having via will reduce the cost of the PCB by half, and if a wire is to break, it is as likely to happen on the PCB as it would with a female connector… The choice I made to use wires instead of connectors proved itself a much better choice, as the connectors would have required to cut out the piece “H” and this would has been hindered by the switches.

LED displays and blocks:
LED displays and blocks:

The second PCB holds the 3 square buttons (AP1, AP2 and A/THR) as well as the 3 rectangular buttons (LOC, EXPED and APPR). This .tci file is also available for download.

We now have our 2 PCBs which will be populated later.

Fitting the Display PCB:


 The front and back sides of the FCU panel being ready, we will start with the display PCB. Temporarily place 2 seven segments LED displays and a LED block on the PCB, using PCI sockets:

Line-up the LED displays with the FCU panel window cutout…
Line-up the LED displays with the FCU panel window cutout…
…and flip it around :
…and flip it around :

The front face and the Plexi backing panel “E” can be seen on this picture. The PCB must fit between the mounting screws “A”. It must also be out of the way of the screws we will use later to mount the “Push Pull” switches “C”. The 6mm diameter spacers for these switches will pass just under the PCB. The picture shows a FCU panel from FDS. If you make your own front panel, you can make the “C” holes wherever needed, based on how the PCB fits with your front panel.


Fit the PCB on “E” and solidly hold it in place using some tape. Be careful not to be mistaken about which side goes up as we are working from behind. Looking to the FCU from the front, the group of 3 seven segments LED is on the left, the one with the 5 LED displays, on the right. The “B” holes are first drilled at 1.8mm and enlarged to 2.2mm. At the same time, mark the center of the holes on “E”, without drilling through, as the front panel is just behind! Remove the screws “A” and now drill through “E” with a 3.5mm bit. The front side will be chamfered just enough for the head of the 3mm screws holding the printed circuit. Going back to the printed circuit, drill the four “B” holes with a 3.1mm bit. A fifth hole can be drilled if needed (in the case of a homemade FCU front panel) but not need for that with the FDS FCU panel. The 3.5mm holes in “E” enable a little play: it is important not to force the printed circuit into place (in case of a tight fitting), as it could easily bend.


Push buttons printed circuit

Likewise, screw in the push buttons printed circuit on the front face. Here too, the holes should be drilled at 3.5mm as there are several 3mm screws holding it and some play could be required.



The K6 switches printed circuit on this picture was a prototype. The traces were modified later.





 Front face view



Panels drilling

Both printed circuits are temporarily in their place. At this point we have the printed front panel and the backing plate plate “E”. This Plexi plate “E” is about 6mm, thick enough to guide the encoder axis. For each of the encoders, there are four 3.2mm holes in “E” to hold the “push-pull” system. These 16 holes are obviously not drilled though the front panel and will be chamfered, 3mm deep, on the front side of “E”, to receive a 3mm nut. As they are recessed, these nuts will not interfere with the front panel. This will be detailed later when we build the “push-pull” system.

“Push-pull” on the FCU

The Speed, Heading, Altitude and VS selectors have 3 possible selections: “rotate” to select and display a value, “push” to engage the “managed” mode (the management of that function is transferred to the FMGS) or “pull” to engage the “selected” mode, in which case the plane will change to the value displayed on the FCU.

“Push-pull” selectors functions, from left to right:

SPEED or MACH: encoder with PULL for selected mode and PUSH for managed mode. Round button with a 21.5mm diameter base.

HDG or TRACK: encoder with PULL for selected mode and PUSH for managed mode. Round button with rounded grooves on the side and a triangle on the front face.

ALT: encoder for Altitude selection. PULL for selected mode and PUSH for managed mode. Button with large rounded grooves without triangle. There is no 100/1000 function, as in FS, an increased selector rotation speed switches the increments from 100 to 1,000.

V/S or FPA: encoder with PULL for selected mode and PUSH for levelling off. Round button with 21.5mm base, similar to Speed button (to simplify)

Four CTS288 encoders (OpenCockpits) are required for the FCU

For the PULL functions, you will need (ON) OFF (ON) toggle switches.

You will also need three round (cylinder) push switches for the FCU (07100 Gotronic type).

On each of the “EFIS”:

ROSE/ARC: one 45 deg rotary switch with 8 positions (OpenCockpits)

RANGE: one 45 degrees rotary switch (8 positions)

BARO: The QFU’s value selection will be done through a CTS288 encoder. The InHg-HP a selection will require either a 30 degrees rotary switch, or inside the EFIS, a switch activated by the InHg-HP’s button rotation. The PULL STD function is the same type as that of the “PULL” on the FCU, but there is no need for a PUSH function (more later on this)

Installing the Push-Pull

Spacers have been precisely cut and matched
Spacers have been precisely cut and matched
The "Push-Pull" washers
The "Push-Pull" washers
Final assembly
Final assembly

Components and material for 4 push-pulls :


Standard tooling, press-drill, new drill bits (3, 6.2, 7 and 8.2mm)

Dremel drill and cutting disks

2 meters of 3mm threaded rod

16 spacer tubes, 15mm long

16 spacer tubes, 10mm long

8 springs, 7mm inner diameter, 1mm wire

4 collars (6mm) used in RC planes to lock the wheels

1 meter of 6mm steel rod

1 meter aluminum tube, 6mm external diameter

1 meter L shape aluminum bracket 15mm x 10mm

1 meter L shape aluminum bracket 10mm x 10mm

8 subminiature micro-switches (GoTronic ref 07186)

2 epoxy printed circuit boards 100mm x 160mm (GoTronic ref 12650)

1 piece of 6mm thick plexi plate (about 100mm x 100mm)

4 CTS288 rotary encoders (OpenCockpits) or Leo Bodnar (no switch)

4 axial coupling collars 6mm-6mm (Conrad ref 183731 or GoTronics ref 24945)

90 washers and nuts (3mm), various screws and bolts

Sub-D 9 pins connector (GoTronic ref 08105 and 08110)

Display PCB assembly:

Wiring a doubled sided printed board of that size can be tricky. You must use a soldering iron with thin conical tips, .5mm soldering wire and a good magnifying glass stand with light. To avoid expenses, these boards do not have metal vias: just use a piece of wire soldered on both sides of the board.


First solder the LED blocks and display stands. Most of them are soldered on the trace side, so no problem there. Two of them (#2 and #5) are soldered on the components side. For those, first put a drop of solder on the components side BEFORE placing the facing LED stands (pins 6 to 10). Do not forget these 2 solders. Same thing with the 12 stand pins for the SPD and MACH LED blocks: leg #6 is connected to the JUMP-DP and soldered on both sides.


Once all solders are done, check continuity with an ohmmeter. For instance, check that all the #2 legs of the displays are connected together. Also make sure that there is no contact between #2 and #1 or #1 for example. Overkill? Maybe, but you never know…

Do not forget to solder the six via holes on both sides of the PCB, even if these are metallized. Pins 2 and 5 of the stands are also soldered on both sides. The resistors on the TCI circuit (R1 to R5) are used for the LED HLMP blocks. These displays (somewhat redundant with the FMA displays) come in 2 “flavors”: yellow 2400 HLMP with 2 LEDs and yellow 2450 HLMP with 4 LEDs. Each individual LED uses 20mA with an average forward voltage drop of 2.2V. On Thierry’s PCB, the LEDs are always installed in parallel: a 2400 HLMP will use 40mA and a 2450 HLMP 80mA. These currents are too high for straight connection to an OpenCockpit Master Card output; we will need a relay card (see OpenCockpit or KMTronics). The relays will send 5 Volts to the LEDs blocks. With these values, calculate the value of the resistors needed for your circuit (Resistor Calculator link). Please note that the HDG and V/S blocks always run together. The same is true of the TRK and FPA blocks. This means that one needs current for 4 LEDs in each case. LAT, ALT and LVL-CH blocks are always on: these 12 LEDS will share the same resistor. Finally, R2 resistor provides current to 4 LEDs if you include the decimal point which is turned “on” in the MACH function. This is achieved with the strap connecting JUMP and DP.

IMPORTANT: the LED blocks should be supplied 5Volts and NOT 12 Volts. In the latter case one would need high power resistors to lower the voltage, as much as 4 Watts resistors in the case of R5! Even with 5 Volts, it is preferable to use 1 Watt carbon resistors. The only issue with those is that they are too large to fit on the PCB, on the components side (the whole PCB would have to be pushed further back into the FCU box and the displays would therefore be too far from the front face of the FCU…). The best solution, but not the most elegant is to install these 5 resistors on a small printed circuit and to connect the wires from that small PCB to the corresponding pads on the FCU circuit (R1 to R5).


The LEDs blocks for LAT + ALT + LVL-CH are always “on” (R5 resistor). They can be directly connected to the 5V power supply of the cockpit. There is no need to use a relay, but it would simplify the wiring. Such a relay could be energized par SIOC (J2 output =BAT1)


L1 and L2 LEDs must be soldered on the components side of the circuit. L3 LED had to be connected (using thin wires) to the L3 and COM pads on the left side of the circuit. With a 5V power supply, the resistor is 150 ohms ¼ Watt.


You may wonder why we need a relay card (we already have a USB Output card, perfectly able to provide the current to the blocks)? It is because the USB Output Card uses a positive common ground, which is incompatible with the wiring of the LEDs blocks and with the PCB presented here.


After L1, L2 and L3 LEDs have been installed, place a piece of 6mm inner diameter opaque plastic pipe around each to avoid light leaking to the closest displays.


The 8 relays board has been installed in the central section of the glare-shield, behind the FCU. This board runs on +5V provided from LED outputs on the J2 Master Card. When a LED output on the J2 Master is turned on by a SIOC program command, the corresponding relay on the relay board is triggered, sending 5 volts to the appropriate LEDs block on the FCU Display board. These LEDs blocks can be split into 5 groups requiring the 5 resistors in the table below as well as 5 of the 8 relays on the relay board. The remaining 3 relays are used to power the L1, L2 and L3 LEDs. Each of these could be directly connected to a LED output on the Master board, without the need for a resistor. But as we are going through the relay board, we will need to insert three 150 ohms resistors for each of these LEDs circuits.


Resistors summary for the LEDs and LEDs blocks. Resistors values are for 5 volts power supply.

      Current I Resistor R  (W)
R1 SPD 2 LED 40 mA 82 Ohms
R2 MACH + dp 4 LED 80 mA 39 Ohms
R3 HDG+HDG/VS+VS 8 LED 160 mA 18 Ohms
R4 TRK+TRK-FPA+FPA 8 LED 160 mA 18 Ohms
R5 LAT+ALT+LVL-CH 12 LED 240 mA 12 Ohms
L1 LED L1 1 LED 20 mA 150 Ohms 1/4W
L2 LED 2 1 LED 20 mA 150 Ohms 1/4W
L3 LED 3 1 LED 20 mA 150 Ohms 1/4W

Connections and wiring

7 segments displays connections:

These displays require 2 connectors. The first one is a HE100 16 pins female connector for the 16 cathodes of the 1 to 16 digits. This connector will plugged into the male cathodes connector on the Display board #1. The color for all these wires can be white, apart #1 to avoid connecting #1 on #16 The wires (A to D groups at the bottom of the PCB) are soldered on both sides of the circuit. One has to be careful not to create a short with the trace linking the #5 pins of the displays. That trace is very close and it would be smart to use an ohmmeter at each wire solder. Make sure to remove the displays from the stands while making this check, otherwise your ohmmeter may give you some false values and results.


Practically, it is not possible to keep the connectors on the board: the support plate “H” and mainly some of the switches are in the way… As a result, the wires will be soldered directly on the board. Strip the wires on about 1cm length so that they can be soldered on both sides of the board. Ideally, you want the stripped wire to be tinned on 5mm and to leave the other 5mm bare: this will give the wire some flexibility and will reduce the risk of breaking it. Once each wire group has been soldered, use some heat shrink tubing to avoid any short between the wires and block the whole ensemble by depositing a drop of glue with a glue gun. The end result is the equivalent of a connector. With only limited clearance between the board and the support plate “H”, make sure your wires come out flat on the face of the board. To prevent pulling them out, fasten them with wire zip ties which are attached to the plate “H”. Do not forget that the wires come out on the back side of the board, on the trace side…

First step in the connectors wiring: the 16 pins connector for the common cathodes.
First step in the connectors wiring: the 16 pins connector for the common cathodes.

Next is the CEN-G4 connector for the digits. The internal connections of the HDSP7403 segments can be found in the “download” section. For the OpenCockpit display card the 8 pins segments connector order (including the decimal point which we will not use) is from “a” to “g”. In the case of our printed circuit (.tci), it was not possible to follow the same order for the segment connector located in the center of the board. The yellow numbers shown on the circuit drawing go from 1 to 10, bottom to top. There are no 5,6 or 7.


As result, here is the correspondence between the display segments and the numbers on the printed circuit:

C.I. FCU segments
 1  d
2 e
3 g
4 f
8 c
9 b
10 a

common ground power supply 

Prepare an 8 wire cable, making sure that each wire from the board side connects to its assigned connector on the display card: 1 (d segment) is connected to the d connector on the display board. The black ground wire should be connected to the CEN-G1 input connector in the central CEN-G1 block. Same process as described before, only the wire has a different color.


The printed circuit board we use here has only one common connector for the 16 displays cathodes. This means that all the FCU displays have to be controlled through only one Display board; we cannot split them over 2 boards. Pay attention when connecting: remember that displaying a number starts with the digit for the units.

Connection of the segments
Connection of the segments

The LEDS and LEDs blocks connector requires 10 wires: 5 for the LEDs blocks, 3 for the LEDs, 1 for the LEDs blocks ground and 1 for the LEDs ground. Obviously these 2 ground wires could be combined, but things are clearer with 2 wires and thus we avoid having a strap on the PCB.


You could use a HE14 as the connector for the relay board (labelled CENG-2 in our connectors Excel Sheet) into which a female FH100 will be plugged in, going to the relays. The wiring can be a ribbon cable or even stronger, made of 24 AWG wires. Use yellow wires as we are dealing with 12V.


On the FCU board, there is no need to solder the wires directly on the board as we did earlier for the displays. Solder some HE14 connectors as follow: enlarge the holes on the board to .8mm if necessary. Take a piece of HE14 (4 or 6 pins) and push the plastic pin holder all the way to the end of the pins. Push the pins into the holes on the board and solder on both sides. Carefully push the plastic pin holder as close as possible to the board: there should be enough of the pin left exposed to plug in a HE100 female connector.


 Note: if a wire, a pin or any component is not soldered perfectly straight, do not use any type of pliers to correct the alignment: you can be sure that the soldering pad will break off the board. Instead, reheat the solder, correct the position of the component and let the solder cool off.


Once all the connections have been completed, you can check everything is functioning properly by supplying the relay board with 12v and sending 5v to each of the 8 channels one after the other.

Third step : connection to the 8 relays board
Third step : connection to the 8 relays board

Push buttons switches PCB

The .tci circuit board was designed with resistors for each of the K6BL switches LED. This board can be used with a relay board but not with a USB Output as the latter has a positive common ground. If the connection is done directly on the Mater card, the LEDs resistors must be replaced with some thin wire strapping such as resistor wire leads for instance. Note: the connectors are on the opposite side of the board from the push button switches. The circuit board designed by Thierry is available in the “Download” section. It can be fitted with some HE14 connectors. Each time you see a green trace finishing on the same pad as a red trace on the circuit drawing, make sure to put a wire through the via and to solder it on both side of the board.


It is independent from the FCU or from the EFIS and will be described in detail in the “Electricity-Lighting” chapter. It comprises 2 bands of LEDs which have been installed in the glare-shield central block. The light they provide is very uniform and without shadows.


The FCU is one of the most complex and expensive piece of equipment of an A320 cockpit.


For the FCU alone (not including the EFIS):



16 yellow HDSP 7403 displays (OpenCokpit)

2 HLMP 2450 and 13 HLMP 2400 lightbars (Farnell),

3 yellow LEDs 5mm

1 printed circuit for displays

1 printed circuit for K6 pushbutton switches

6 K6BL pushbutton switches (ITT or DigiKey)

4 CTS288 encoders (OpenCockpits)

3 momentary pushbuttons with cylindrical actuator (07100 GoTronic)

8 subminiature micro switches (07186 GoTrocnics)

2 bags of RC plane wheels collars diam 6mm

4x6mm aluminum tube (1 meter long)

3mm threaded rod (2 meter length)

8 springs

16 spacers tubes (15mm) GoTronic 11542

1 meter piano wire diam 6mm

3.5 x 6 mm aluminum tube, anodised (1 meter)

Aluminum L profile aluminum 10x10mm (1 meter)

Aluminum L profile aluminum 15x10mm (1 meter)

10 IC sockets (08190)

2 100x160mm Epoxy Printed Circuit boards (12650 GoTronic)

4 axle couplers 6-6mm (Conrad 183731 or GoTronic 24945)

4 Sub D 9 pins connectors 08105 + 08110 GoTronic

1 HE14 straight row PCB header ref 0800

2 FH100 female bars 2.5 euro each

Plexiglass 5mm

1 Flashing and laser printing sheet

1 OpenCockpits Display board

8 relays board (KM Tronic)

4 epoxy cast knobs for FCU

Delivery, screws, nuts, silicone rubber, misc…


This list is indicative only. References can change at any time.


Each EFIS panel comprises 2 switching areas: one related to the PFD (Barometer, FD and ILS) and one for the ND (Display mode selection, scale selection and VOR/ADF selectors).


The EFIS construction follows the same philosophy as the FCU, but with a few changes. Of the 5 seven segments displays on each display board, only the first 4 will be used. We used OpenCockpit boards. There are 2 additional boards, one for the 5 LED K6BL switches for the map data type selection and another one for the 2 LED K6BL at the bottom of the EFIS panel. The drawings for these circuits ( can as usual be found in the Download section.

EFIS Front panel

The EFIS front panel (3 to 4 mm thick) is doubled with a 6 mm Plexi backing plate onto which the various components are populated.


Here are the front and rear panels drawings for each EFIS. The countersunk/routed holes will receive the nuts and/or washers. Other holes will be drilled as needed for the printed circuits.

Drilling holes drawing
Drilling holes drawing

Front panel is painted in RAL 7011 grey. Files for film flashing and negative image can be found in the Download section. The rectangular openings for the map data selection K6BL switches were intentionally not drawn: the cutting will be ad-hoc once put together, without running the risk of having the tracing lines showing up.

Printed circuits for the K6BL switches

These are one-sided circuits, the K6BL mounted on the opposite side of the traces. For the K6BL pins to really stick out on the trace side of the board, cut off the 2 small plastic tabs at the bottom of each switch. Here too, the wires will be soldered directly on the soldering pads, without connectors. With a connector mounted on the soldering side, one would run the risk of pulling them off too easily, tearing up the pads and copper traces. To summarize, the wires are soldered directly to the board on one end and on the other end, to a male connector. The latter will connected to a female connector going to the IO board.


Once the front and backing panels have been cut and drilled following the drawings shown above, drill the holes for the 3 printed circuit boards in the backing panel.


For the switches buttons to travel properly and activate the K6BL switches, the clearance between the front panel and the printed circuits should be about 5mm. (Use two 3mm nuts).


The printed circuit for the displays is a 5 digit display from OpenCockpit. Beware that older and newer version of these circuits have different mounting holes… The 5 pins connector needs to be bent at 90 degrees, at least for the F/O. EFIS.


With the printed circuits in place, measure the available space you have left for the push-pull mechanism. In my case, I got 32 x 50 mm. This gives you the size of the various Plexi boards to prepare.

The pressure setting knobs.

Ideally, you have a lathe and can make these with aluminum…


Short of owning a lathe; make a master from which you will get a silicon mold. The epoxy resin casts will be drilled to the proper diameter. This master can be made using a piece of a large wax candle which will be inserted into the mandrel of a drill press. The shape can be obtained very precisely with small wood chisels resting on an ad-hoc support bloc.


This method can be used for most of knobs, as long as the inner hole is not too large. The small pressure setting knob is a good example of what can be done with this technique.


The 6mm rotary switch axis is glued into the knob with epoxy glue. Another solution, as shown on the drawing, would be a 6 x 10 mm steel tube and a 15 mm knob (cut from a piece of Plexi or a toothpaste tube cap,…). This is stronger than epoxy and there is no risk for the rotary switch axis hole not to be perfectly centered.

Making a master with the candle method.
Making a master with the candle method.

The large knob is more problematic as one of the holes diameter is 13mm, too large to make accurately with a drill. Another method can be used, by inserting into each other some tubes of increasing diameters :

A 6mm axis rod is at the center of the system. This axis slides into a 6 x 8mm steel tube. Re-drill that tube to 6.1 mm or 6.2 mm so that the axis can slide without friction. Onto this tube, a copper 8x10 mm brass or copper tube is glued with epoxy. That gives us a 6 x 10 mm.


The 8 x 10 mm copper tube is inserted and glued into another copper tube (10 x 12 mm dimensions) which in turn, is pushed and glued into a 12 x 14 mm copper tube. To finish up, insert the 14 x 16 mm and 16 x 18 mm short tubes: this gives the outside diameter of the large knob.


Theoretically, on top of this, there is a 18 x 22 mm collar which rotates inside the 23 mm hole drilled in the 2 EFIS plates. This piece, made in Plexi or wood (12 mm thick) cannot be seen. It comes out flush to the front EFIS and we could do without it. Obviously in this case the holes in the front and rear panels will be 18-19 mm instead of 23mm.


The 6mm axis is now guided precisely: it can slide back and forth by pushing or pulling the small knob. Its front travel course is limited by the various tubes inside the large knob. To stop the travel backward, we will need to install a collar, but more on that later.


Finally, fill up the gaps between the tubes where they can be seen, paint the whole thing in black, the small dial knob is grey.

The 2 co-axial knobs are now ready. The 6 x 8 mm tube is 5.5 mm longer than the others. The inHg/hPa switching mechanism will be installed on this tube. In my first version, I used a micro-switch with a styrene cam system. This system was OK, but a bit complicated as in addition to the cam, it needed 2 stops to limit the rotation to an angle identical to that between the inHg and hPa indications on the front panel. The following few pictures describe that first implementation. Still it should not be discarded as it can help us solve a little problem with the SIOC as we will see later.

The micro-switch in the closed position
The micro-switch in the closed position
    The micro-switch is installed between the front panel and the first of the Plexi plates.
The micro-switch is installed between the front panel and the first of the Plexi plates.

The final solution I chose for my EFIS panels is actually a more simple one: I used a potentiometer with an on-off position. Obviously, this is the “on/off” functionality I was interested in, as the potentiometer itself will not be wired. The assembly is quite simple and requires a 20 mm cog wheel (ref 24656 GoTronics). Its center hole will be enlarged to 8mm and pushed in place onto the 6x8 mm tube. Another identical cog wheel is positioned between the spacers. Punch its center and drill a 9.7mm hole on the Plexi plate, where the potentiometer will the screwed in. The cog wheel center hole is enlarged to 6 mm and the wheel will be pushed on the potentiometer axis. That’s all folks!


The on-off switching is controlled by the rotation of the large knob. The rotation is limited on the hPa end by the switch itself. On the other end (inHg) insert a pin on the large knob, sticking out sufficiently to be blocked in its rotation by one of the spacers when reaching the inHg position. You can do without that pin if you want to: only one switch is actually needed as the SIOC will handle the difference between a closed and an open switch.

The “Push” function.

As indicated earlier, the small knob should be pulled to switch to the Std pressure setting, therefore the need for a “pull” switch. Still, this is not what I decided to do… To install such a switch would bring additional complexity when we already have a Push button included in the CTS288. So, after much debate with my fellow A320 cockpit builders, we decided to change the “Pull” into a “Push”….The main reasoning, besides the technical simplicity, is that this is a function which is used only twice during a flight and that the ergonomics for the pilots are about the same and that the switch to “Std” is additionally confirmed on the displays. So while purists may object, this is the route I chose! In addition, this “push” functionality was included in the JeeHell software shortly after: the “long push” option (a few seconds long push) commutes to the pressure to the “Std” setting.


Our system also includes a spring located on the second stage of our contraption (as you cannot rely only on the CTS288 mini-spring to bring the axis back) and a 6mm inner diameter collar limiting the travel course.


The large knob cannot move forward as the cogwheel presses on the first Plexi plate and it cannot go backwards as it presses on the small knob, provided the latter does not move backwards also…This is why the collar was required as it prevents the axis from moving to the back. When pushing, the encoder itself prevents the shift: one would be hard “pressed” to find anything simpler!

The two knobs
The two knobs


Using 5mm thick Plexi, cut off three 32 x 50 mm pieces. These are the inside “bulkheads”. Making sure the 3 pieces are perfectly stacked drill a 3.5mm hole in each corner. Temporary attach the 3 plates together with a screw and a nut in each of the 4 corners. Make sure you mark the top and the bottom of each of the pieces with some permanent marker. Place the 3 Plexi ensemble on the front backing plate and drill the mounting holes. Countersink on the front side to a 6.5mm diameter to fit a 3x10mm screw.


Next, drill a 6mm hole for the axis. If the front plate has not been drilled yet, drill through the 4 layers to 2.5 mm diameter, then increase the hole to 6.1mm. To avoid chipping the edges of the holes, countersink the bottom side of the last plate to 6mm, a few mm deep, then drill the four layers from the front. If the front plate was already drilled, set your caliper to the hole’s half diameter. Place one of the inside jaws against the side of the hole and mark an arc by rotating the caliper. Repeat 3 times. The intersection of the arcs is the center of the hole. Drill to 2.5mm, then to 6mm.


Last, enlarge the 6 mm hole on the bottom plate to 8mm for the encoder, and install the first spacers (25mm) on the front back-plate.

Cut a 92 mm length in a 6mm rod. Re-drill the center hole of one of the cogs to 6mm. (ref. 24656 GoTronics). Make sure you are dead center. The cog will be forced onto the axis of the potentiometer. Likewise, re-drill the second cog to 8mm : it will go at the end of the 6x8mm tube sticking out of the largest knob.


Using epoxy, glue the small knob at the end of the 6mm axis. This axis should then stick out by 73mm. Push the small knob into the largest knob and place the #2 cog on the 6x8mm tube. Glue it to the end of the large knob. Make sure the small knob rotates freely inside and that the glue has not created some rotation hard point.


Slide the whole contraption into bulkhead #1 and place the cog for the potentiometer in place. Mark precisely its center and drill a 10mm hole in the bulkhead for the potentiometer. Use the same chipping prevention methods described earlier when drilling.


Cut off the end of the potentiometer axis so that the final length is between 12 to 15 mm. To limit the potentiometer’s rotation, install a shim on the axis part that sticks out of the cogwheel. You can now install the first bulkhead with 25mm spacers.

The second section requires 20 mm female-female spacers. It will also include a 16mm spring system. In my case, I use an 8 mm pull spring which I elongated so that it stayed extended permanently. Then I cut a 16mm length from it. It works quite well as a compression spring. The picture below illustrates the whole system:


The last bulkhead (onto which the encoder will be installed) is very simple. The spacers are 30mm long. There is a 2mm space between the encoder axis and the 6mm axis to prevent them to be in contact with each other. The 2 theoretically could be connected rigidly but if the line-up is not perfect, there will be some hard points when rotating the encoder. As a consequence, I chose to go with a flexible connection, using a piece of 6mm diameter silicon tubing attached to the axis with some nylon collars. It does not introduce any slack in the rotation. It also gives a much better feel when pushing the encoder axis as it collapses by about 1mm before the end of the axes contact. This is simple and very efficient.

Complete system
Complete system
 The left panel
The left panel

Supplies for the 2 EFIS  panels :

8 HDSP 7403 yellow displays

1 printed circuit for the Displays (Opencockpits)

1 printed circuit for 5 K6BL switches

1 printed circuit for 2 K6BL switches

7 K6BL push switches

2 CTS288 encoders (Opencockpits)

4 rotary switches (45 degrees)

4 miniature toggle switches ref. 07011

8 15 mm spacers ref 11568

8 10 mm spacers ref 11567

8 20 mm spacers ref. 11563

8 30 mm spacers ref. 11570

6 cm silicone tubing

1 Display card (Opencockpits)

4 Epoxy specialty molded knobs

7 knobs for push switches

Misc costs, screws and collars


Some of the supplies (flash film, stopping collars,…) were already included in the FCU shopping list. The FCU/EFIS back-lighting is described in the Electrical/Lighting chapter.


This concludes this very long chapter dealing with the FCU and the “EFIS” panels, both key ingredients of the A320 flight management.