Repair Backlighting on Throttle Quadrant

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The rear of the First Officer side trim lightplate showing one of the two terminals that the wiring loom connects to

During a recent flight, I noticed that the bulbs that illuminate the backlighting for the trim and flaps lightplate (First Officer side) had failed, however, the backlighting on the Captain-side trim lightplate was illuminated.  My first thought was that the 5 volt bulbs that are integrated into the lightplate had burned out; after all, everything has an end life.

Backlighting - Wiring Loom

The wiring loom that supplies the power for the backlighting enters the throttle quadrant via the front firewall, and initially connects with the trim lightplate and parking brake release light on the Captain-side.  A Y-junction bifurcates the wire loom from the Captain-side to the First Officer side of the quadrant, before it snakes its way along the inside edge of the quadrant firewall to connect with the First Officer side trim lightplate, and then the flaps lightplate.  The wiring loom is attached securely to the inside edge of the throttle casing by screwed cable clamps.

The backlighting for all lightplates is powered by 5 volts and the backlighting on the throttle quadrant is turned on/off/dimmed by the pedestal lighting dimmer knob located on the center pedestal. 

Finding the Problem

Ascertaining whether the bulbs are burned out is uncomplicated, however, assessing the terminals on the rear of each lightplate, and the wiring loom the connects to the lightplates, does involve dismantling part of the throttle quadrant.

The upper section of the throttle quadrant must be dismantled (trim wheels, upper and side panels, and the saw tooth flaps arc).  This enables the inside of throttle quadrant to be inspected more easily with the aid of a torch (lamp/flashlight).  When removing the trim wheels, be especially vigilant not to accidently pull the spline shaft from its mount, as doing so will cause several cogs to fall out of position causing the trim mechanism to be inoperable.

After the lightplates have been removed, but still connected to the wiring loom, a multimeter is used to read the voltage of each respective terminal on the lightplate. If the mutlimeter indicates there is power to the terminals, then the bulbs should illuminate. 

What surprised me when this was done, was that the bulbs worked perfectly. Therefore, it was clear the problem was not bulb, but wire related.

Process of Elimination

The process of elimination is the easiest method to solve problems that may develop in complicated systems.  By reducing the components to their simplest form, a solution can readily be attained.

Alligator wire connects power from Captain-side lightplate to the First Officer lightplate.  Note the frayed outer layer of the white aircraft wire.  The gold colour is a thin layer of gold that acts as a fire retardant should the wiring overheat

If you suspect that the wiring is the problem, and don't have a multi meter, then a quick and fool safe method is to connect an alligator cable from the positive terminal of the Captain-side lightplate to the respective terminal on the First Officer lightplate.  Doing this removes that portion of the wiring harness from the circuit. 

In this scenario, the  bulbs illuminated on both trim lightplates.  As such, the problem was not bulb related, but was associated with the wiring loom.

It must be remembered that the wire used to connect the backlighting in the throttle quadrant is OEM wire.  As such, the age of the wire is the same age as the throttle quadrant.  

Inspecting the wire loom, I noticed that one of the wires that connected to the terminal of the lightplate was severed (cut in two).   I also noted that the original aircraft wires had begun to shed their protective insulation layer. 

Aircraft Wire and Insulation Layers

The high voltage and amperages that travel through aircraft wire can generate considerable heat.  This is why aircraft wire is made to very exacting standards and incorporates several layers of insulation that surround the stranded stainless steel wire.  The use of high-grade stainless steel also provides good strength and resistance to corrosion and oxidation at elevated temperatures.  

The green wire has been severed.  A possible scenario was that the wiring loom had been pulled slightly loose from the throttle chassis, and had become caught in the flaps mechanism.  When the flaps lever is moved, the mechanism can easily crimp (and eventually sever) any wire in its path.  If you observe the white wire you can see the insulation that is shedding

Interestingly, one of the insulating layers is comprised of gold (Au).  The gold acts as an effective fire retardant should the wires overheat.

The breakdown of the upper insulating layer is not a major cause for concern, as a 'shedding' wire still has enough insulation to not arc or short circuit.  However, the wire should be replaced if more than one layer is compromised, or the stainless threads of the wire are visible.

Possible Scenario

When inspecting the wiring loom, I noted that one of the screws that holds the cable clamp to the inside of the throttle casing was loose.  This resulted in part of the wire loom to 'hang' near the flaps arc mechanism.    It is possible that during the throttle’s operational use, the movement and vibration of the aircraft had caused the screw to become loose resulting in the wires hanging down further than normal.  It appears that the wire had been severed, because it became caught in the mechanism of the flaps lever.  

Unlike reproduction throttles, the parts used in an OEM throttle are heavy duty and very solid; they are designed to withstand considerable abuse.  The speedbrake lever, when activated can easily cut a pencil in two, and the repeated movement of the flaps lever, when moved quickly between the teeth of the flaps arc, can easily crimp or flatten a wire.

Rather than try to solder the wires together (soldering stainless wire is difficult) and possibly have the same issue re-occur, I routed the wires from both lightplates (trim and flaps) directly to the 5 volt bus bar located in the center pedestal. 

I could have removed the wire loom completely and replaced it with another loom, however, this would involve having to disassemble the complete upper structure of the throttle quadrant to access the wire loom attachment points on the inside of the throttle casing; something I was not keen to do.

Final Call

OEM parts, although used in a static and simulated environment can have drawbacks.  Apart from age, the repeated movement of mechanical parts and the vibration of the spinning trim wheels, can loosen screws and nuts that otherwise should be securely tightened. 

Acronyms

  • OEM – Original Equipment Manufacturer

  • Wire Loom – Several wires bundled together and attached to a fixed point by some type of clamp

Backlighting and Dimming with OEM and Reproduction Panels

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FDS-IBL-DIST-DIM.  A card that makes diming backlighting very easy.  Potentiometer is not shown

Many enthusiasts are now using Original Aircraft Equipment (OEM) panels in their simulators.  These panels are connected to Flight Simulator using a variety of interface cards.  Unless the flight deck uses all OEM panels, or all reproduction panels, there will be a difference in backlighting when the light plates are illuminated.

Reproduction panels, with the exception of expensive very high end types, will have exceptionally bright backlighting.  Manufacturers of reproduction panels want their panel to look good and appeal to a prospective buyer – this is why they have bright backlighting.  In contrast, OEM panels do not have  bright backlighting, and in some cases, depending upon the manufacturer of the panel, the backlighting will appear rather dim.  

Therefore, the brightness of the backlighting when using ‘run of the mill’ reproduction panels is not realistic in comparison to that observed in a real aircraft.

So how does a cockpit builder solve this conundrum of brightness if he or she has a mix of reproduction and OEM panels.  The solution is very simple – install a dimmer switch into your flight deck.

Dimmer Control

There are a number of 5 volt dimmer switches on the market and some are better than others.  For those with electrical knowledge it’s relatively straightforward to make your own dimmer switch, but what about the rest of us?  An excellent solution is the distribution board with built in dimmer control manufactured by Flight Deck Solutions (FDS).  The board keeps with the principle of KIS (keep it simple).  

FDS-IBL-DIST-DIM

The distribution board is well made, small, is fuse protected, and have the capability to connect up to 14 accessory LEDS or bulbs via propriety board connectors.  The board also can be used as a slave, meaning it can be daisy-chained to another board to increase the number items attached.

The distribution board includes a pre-wired metal potentiometer which allows all the LEDS/bulbs attached to the board to be dimmed from on to off or anywhere in-between.  The potentiometer is a standard size and fits the hole located in the panel lights panel on either a reproduction panel or an OEM panel.

One limiting feature that should be noted is that each distribution board will only support 10 amps - the rating of the fuse.  Therefore, depending upon the number of panels that you wish to connect to the board, it may be necessary to use two boards in parallel rather one board or an extension to the board.

Of more importance, the board operates flawlessly and is a very easy solution to maintaining an even brightness across reproduction and OEM panels; adjust the brightness of the reproduction panels to the same level as the OEM panels.

Connection

Connection is straightforward and requires +- 5 volts to be connected to the board.  Each LED (or bulb) that requires dim control is then connected to the board connectors.  If using an FDS panel this is very easy as the FDS panels already use the correct female attachment plugs (FDS also use bulbs and not LEDS).  Failing this, a little extra work is required to source the correct plugs and wire them to the +- wires that connect to the light plate.

Bulbs and LEDS

On another note, with the exception of late model airframes, the Next Generation B737 use 5 volt incandescent bulbs in their panels for backlighting.  This is in contrast to reproduction panels that, for the most part, use LEDS.  

The difference between bulbs and LEDS, other than construction, is the temperature they generate when turned on.  A bulb will generate considerable heat and the colour of the light will appear as a warmer hue.  A LED does not generate heat when turned on.  Therefore, an LED will have a cooler temperature and the colour of the light will be colder and more stark in its appearance.

However, before changing out all your bulbs or LEDS to maintain colour consistency, study the flight deck of a real aircraft.  Panels on all aircraft fail or need upgrading from time to time.  Therefore, it is not unrealistic to have a flight deck consisting of both LEDS and bulbs.  Airlines are in the business of making money, and pilots fly.  Neither are particularly interested in whether the ADF radio has a bulb or LED.

Additional Information

Soar-By-Wire has also discussed this subject.  Although his information relates to the Airbus, the same procedure can be done for Boeing OEM panels.

Disclaimer

I do not represent Flight Deck Solutions or any other manufacturer and have no received any fee or reward for discussing one of their interface components.

Further information pertaining to the distribution board can be found on the Flight Deck Solutions website.

A fellow enthusiast has written more information on his website about the distribution board as it relates to Airbus - Soarbywire.  What he has written is well worth the time reading.

Throttle Quadrant Rebuild - New Wiring Design and Rewiring of Center Pedestal

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oem 737-500 center pedestal. the panels change as oem components are purchased and converted

Put bluntly, the wiring in the center pedestal was not to a satisfactory standard.  Several panels were daisy chained together, the wires were not colour coded, and the pedestal looked like a rat’s nest of wires.  Likewise, the wiring of the Master Caution System (MCS) required upgrading as several of the original wires showed signs of fraying.  

A word of thanks goes to a friend (you know who you are...) who helped wade through the labyrinth of wires!

This post shares several links to other pages in the website.

Wiring Redesign (pedestal and panels)

The set-out of the inside of the center pedestal was redesigned from the ground up, and several of the pedestal panels re-wired to ensure conformity to the new design standard, which was neater and more logical than its predecessor.  Additionally, the MCS was rewired using colour-coded wire and the wires labeled accordingly.

New Design (panels must be stand-alone)

The new design called for each panel (module) that was installed into the pedestal to be stand-alone.  Stand-alone means that if removal of a panel was necessary, it would be a simple process of unscrewing the DZUS fasteners, lifting the panel out and disconnecting a D-Sub plug and/or 5 volt backlighting wire.   Doing this with panels that were daisy chained together was impossible.

The following panels have been re-wired:

(i)      EVAC panel;

(ii)     Phone panel;

(iii)   ACP units (2);

(iv)    On/off lighting/flood panel; and,

v)      Radar panel.

737-800 EVAC panel, although not a panel that resides in the pedestal, it demonstrates the 'stand-alone' panel philosophy.  One D-Sub plug with labelled and colour-coded wire.  The mate of the D-sub resides inside the pedestal with the wires connected to the appropriate busbar

All the panels have been retrofitted with colour-coded and labeled D-Sub connections.  Removing a panel is a simple as unfastening a DZUS connector, disconnecting a D-Sub connector, and unscrewing the 5 volt backlighting wire from the 5 volt terminal block (if ued).  If a USB cable is needed for the panel, then this must also be disconnected.

A word concerning the ACP units, which were converted some time ago with an interface card located on a separate board outside of the unit.  As part of the rebuild, the two ACP units were completely re-wired to include the interface card within the unit.  Similar to the fire suppression panel, the ACP units are now stand-alone, and only have one USB cable which is used to connect to the computer.  The First Officer side ACP is daisy chained to the Captain-side unit.

Center Pedestal Flat Board

A flat board 1 cm in thickness and constructed from wood was cut to the same dimensions of the pedestal base.  The board was then attached to the inside bottom of the pedestal by screws.  The wood floor has been installed only to the rear two thirds of the pedestal, leaving the forward third open to allow easy access to the platform floor and area beneath the floor structure..

Attached to the flat board are the following items:

(i)       FDS 5 Volt IBL-DIST panel power card (backlighting for FDS panels);

(ii)      28 Volt busbar;

(iii)     5 Volt busbar (backlighting);

(iv)     12 Volt relay (controls backlighting on/off tp panel knob);

(v)      Terminal block (lights test only);

(vi)     Light Test busbar;

(vii)    OEM aircraft relay; and a,

(viii)    Powered USB hub (NAV, M-COM, ACP & Fire Suppression Panel connection).

The 5, 12 and 28 volt busbars (mounted on the flat board) receive power continuously from the power supplies, mounted in the Power Supply Rack (PSR) via the System Interface Module (SIM). Each panel then connects directly to the respective busbar depending upon its voltage requirement.  

In general, 5 volts is used for panel backlighting while 12 and 28 volts is used to power the fire suppression panel, EVAC, throttle unit, phone panel and other OEM components

The flat board has a fair amount of real-estate available; as such, expanding the system is not an issue if additional items need to be mounted to the board.

Lights Test busbar.  Similar in design to the 5 volt busbar, its use centralizes all wires and reduces  the number of connections to a power supply.  Despite the pedestal rewire, there is still a lot of loose wire that cannot be 'cleaned up'.  The grey coloured object is the flat board

Lighting Panel Knob (backlighting on/off)

All the panels in the center pedestal require 5 volt power to illuminate the backlighting.  The general purpose knob located on the pedestal OEM lights panel is used to turn the backlighting on and off.  

Instead of connecting each panel’s wire to the on/off lights panel knob – a process that would consume additional wire and look untidy, each wire has been connected to a 10 terminal 5 volt busbar.  The busbar in turn is connected to a 12 volt relay which is connected directly with the on/off knob.

When panel lights knob is turned from off to on, the relay closes the circuit and the busbar is energised; any panel connected to the busbar will automatically receive power.

The busbar and relay are mounted to the flat board.

This system has the advantage that it minimizes the number of wires that are connected to the lights panel knob.  It also enables one single high capacity wire to connect from the relay to the knob rather than several smaller gauge wires.  This minimises the heat produced from using several thinner wires.  It is also easier to solder one wire to the rear of the panel knob than it is to solder several wires.

Lights Test and DIM Functionality

The center pedestal also accommodates the necessary components (Lights Test busbar) to be able to engage the Lights Test and DIM functionality.  These functions are triggered by the Lights Test Toggle located on the Main Instrument Panel (MIP).  

All wires have been corrected colour coded to various outputs and wire ends use ferrules to connect to the card

Interface Cards

In the previous throttle quadrant, a number of interface cards were mounted within the center pedestal. 

To ensure conformity, all the interface cards have been removed from the pedestal and are now mounted within one of the interface modules located forward of the simulator. 

Furthermore, all the wiring is colour-coded and the wire ends that connect into the I/O cards use ferrules.

The First Officer-side MCS completely rewired.  The MCS has quite a bit of wiring, and making the wire neat and tidy, in addition to being relatively accessible, was a challenge

The use of ferrules improves the longevity of the wiring, makes wire removal easier, and looks neater.

Wiring and Lumens

Needless to say, the alterations have necessitated rewiring on a major scale.  Approximately 80% of the internal wiring has had to be replaced and/or re-routed to a position that is more conducive to the new design.

The majority of the wiring required by the throttle unit now resides in a lumen which navigates from the various interface modules (located forword of the simulator) to the Throttle Communication Module (TCM).  

From the TCM the lumen routes through the throttle firewall, along the Captain-side of the throttle unit before making its way to the flat board in the center pedestal.  

The exception to the above is the cabling required for a powered USB hub located within the center pedestal, the wires required for the Lights Test (from the Lights Test Toggle located in the MIP), and the various power wires navigating to the pedestal from the Power Supply Rack.  These wires have been bundled into a separate lumen, which resides beneath the floor structure.

Identifying the voltage of wires is an important aspect of any simulation build

Wire Management

Building a simulator using OEM parts, requires an inordinate amount of multi-voltage wiring of various gauges, and it can be challenge to maintain the wire in a neat and tidy manner. 

Running the wire through conduits and lumens does help, but in the end, due to the amount of wire, the number of connections, and the very limited space that is available, the wire is going to appear a little messy.  Probably more important, is that the wire conforms to an established design standard – meaning it is colour-coded and labelled accordingly.

A dilemma often facing builders is whether to use electrical tape to secure or bind wires.  Personally, I have a strong dislike for electrical tape - whilst it does have its short-term usages, it becomes sticky very easily, and becomes difficult to remove if left on wires for a considerable time .

My preferred method is to use simple cable ties, snake skin casing, or to protect the wires near terminals of OEM parts. to use electrical shrink tubing (which can be purchased in different colours for easy identification of wires and terminals).

Final Product

The design and rewiring of many parts in the simulator has been time consuming.  But, the result has been:

(i)     That all the wires are now colour-coded and labelled for easy identification;

(ii)     The wiring follows a defined system in which common-themed items have been centralised.  

(iii)    Panels that were daisy chained have been rewired with separate D-Sub plugs so they are now stand-alone;

(iv)    The  frayed wires from the MCS have been replaced with new wires; and,

(v)    The wires in general are neater and more manageable (the rat's nest is cleaner...).