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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.

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


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Journal Archive (Newest First)

Entries in B737 Throttle Quadrant (2)


Throttle Quadrant Rebuild - Parking Brake Mechanism Replacement, Improvement, and Operation

As part of the throttle quadrant rebuild, the parking brake system was enhanced.  In the previous system, the parking brake lever was controlled by a relay and a 12 volt solenoid.  The system worked well, however, there were some minor differences between the simulated system and that of the system used in the real Boeing aircraft.

LEFT:  Parking brake lever in the UP engaged position.  The red incandescent bulb is 28 volts, however, a 12 volt bulb can be used.  Throttle is Boeing OEM (click to enlarge).

Furthermore, as it was predominately a software system, any change to the avionics suite would affect its operation.

To 'get a handle on' the mechanical linkages used, please read the article regarding the previous system B737 Parking Brake Mechanism.

Revamped System

There has been minimal alteration to the mechanical system, with the exception that the solenoid has been replaced by a 12 volt actuator, a micro-switch has replaced the toggle switch, and the system now requires the toe brakes to be depressed to engage the parking brake.

What is an Actuator

An actuator is a type of motor that is responsible for moving or controlling a mechanism or system.  It is operated by a source of energy, typically electric current, hydraulic fluid pressure, or pneumatic pressure, and converts that energy into motion.

Almost every modern automobile has a door lock actuator which is responsible for the locking and unlocking of the door locks.  This website 'How Stuff Works' provides a very good overview of how an actuator works.

The actuator is responsible for maintaining the parking brake lever in the UP position.  This occurs when the circuit is closed and 12 volt power is briefly directed to the actuator to lock the device into the engaged position. 

LEFT:   The actuator is the blue coloured plastic mechanism.  The parking brake vertical control rod, micro limit switch and upper part of the high tensile spring can be to seen to the lower right (click to enlarge).

System Overview

The actuator is the mechanism that enables the parking brake lever to be locked into the UP position.  Without power, the actuator is in the resting position and the parking brake lever is pulled to the DOWN position by a high tensile spring.

The annunciator is mounted horizontally on the Captain-side of the throttle quadrant and is powered by 12 volts.

To connect the actuator to the parking brake system, the following items have been used:

(i)      An actuator;
(ii)     A micro-limit switch;
(iii)    A relay;
(iv)    A 12 volt power supply and busbar;
(iv)    A standard interface card (Leo Bodnar 836 Joystick Controller card); and,
(v)     Depending upon your requirements (mechanical or part mechanical system), a Phidget 0/0/8 card (1017_1).

Registration of Parking Brake Movement

Prior to proceeding, the movement of the parking brake lever must be configured.  This is done by wiring the parking brake to a Leo Bodnar Joystick Controller card and registering the device in Windows.  Following this, the movement of the lever is registered in ProSim-AR (configuration/MCP Throttle Switches), FSX, or via FSUPIC.

Relay and Micro-Switch

Two items are used to control whether power enters the circuit: a double throw relay and a micro-switch.

The relay is connected to the toe brakes, and when the brakes are depressed, the relay will close.  When the brakes are released the relay will open.  The connection of the relay to the toe brakes can be done a number of ways, but probably the easiest way is to install a button or micro-switch to the toe brakes.  A Phidget 0/0/8 card can also be used, but this method is slightly more convoluted.

The relay (open/closed) is triggered by the movement of the toe brakes.

A micro-switch is used to open or close the circuit when the parking brake lever is raised or lowered.

The micro-switch is mounted proximal to the vertical control rod, and when the parking brake is is in the DOWN position, the connectors from the micro switch are touching a flange that has been attached to the rod, however, when the parking brake lever is moved to the UP position, the connection is severed (circuit open or closed). 

The use of a micro-switch facilitates a second line of containment.  What this means is that the mechanism will only function fully when the relay is closed (toe brakes depressed) and the micro-switch is closed (parking lever raised).

The relay, either enables or inhibits 12 volt power to flow into the circuit, and this is dependent upon the whether the toe brakes are depressed.

The reason for this set-up will be understood shortly.

Toe Brakes

In the real aircraft, the parking brakes can only be engaged or disengaged when the Captain-side or First Officer-side toe brakes are depressed.  This mechanical system has been faithfully replicated by using a relay, micro-switch and actuator.

How It Works

The actuator will only engage when the toe brakes are depressed.  This means that the parking brake cannot be engaged (lever locked in the UP position with red annunciator on) or disengaged (lever in DOWN position with red annunciator off) unless the toe brakes are depressed. 

Depressing or releasing the toe brakes closes or opens the relay which in turn enables 12 volt power to reach the annunciator via the busbar.  However, the system is only 'live' (closed system) when the parking brake lever is moved to the UP position, enabling power to flow unhindered through the circuit.  When the toe brakes are released, the circuit is open and the actuator remains in the engaged locked position with the parking brake lever locked in the UP position.

To release the parking brake lever, the opposite occurs.  When the toe brakes are depressed, the relay opens directing power to the actuator which disengaged the actuator lock.  The parking brake lever is then pulled to the DOWN position by the tensile spring.

How To Engage The Parking Brake

The method used to engage the parking brake is as follows:

(i)        Slightly depress the toe brakes.  This will open the relay and enable 12 volts to engage the actuator;
(ii)       Raise the parking brake lever to the UP position and hold it in this position; and,
(iii)     Release the toe brakes.  Releasing pressure on the toe brakes causes the actuator to lock into the engaged position, as the power ceases to flow to the actuator.

To release the parking brake, the toe brakes are depressed.  This will cause the actuator to unlock and return to its resting position.  The high tensile spring will pull the parking brake lever to the DOWN position with a loud snapping sound.

More Ways To Skin A Cat - Full Mechanical or Part-Mechanical

There are several methods that can be used to connect the actuator to the parking brake mechanism. No one method is better than the other.  I have outlined two methods.

(1)   Mechanical Method: This has been described above.

The toe brakes are connected to a relay (via micro-switches, buttons or whatever) and then connected with a busbar/12 volts power source, micro switch, and finally the actuator. 

Other than  connection of the parking brake lever to an interface card, and registration of the movement of the parking brake lever (either in ProSim-AR, FSX, or via FSUIPC) this method requires minimal software.

(2)  Part-mechanical/Software Controlled:  This involves using the USER section in the configuration menu within ProSim-AR.

A Phidgets 0/0/8 relay card is connected to ProSim-AR and the the USER interface located in the configuration/switches menu of ProSim737 is programmed to read the movement for the toe brakes.  In this example USER 1 has been selected.  This process removes the need to install a micro-switch or button to the toe brakes.

Each USER IN has a corresponding USER OUT and this is located in GATES.  Opening Configuration/Gates, the same USER number is found (USER 1).  In the tab beside USER 1 the output from the Phidgets 0/8/8 card is entered.  Therefore, whenever USER 1 is triggered, there will be a corresponding output.

When the toe brakes are depressed, the software will read the movement and send a signal to the Phidget card to engage the relay.  This in turn will enable the busbar to be powered and the micro-switch to receive power.  Whether the parking brake lever is engaged (UP) or disengaged (DOWN) will open or close the micro-switch (closing or opening the circuit).  

The actuator will be engaged (circuit closed) only if the micro-switch (located on the vertical rod mentioned earlier) connection is severed (parking brake lever is in the raised position closing the circuit).

NOTE:  Since publication, ProSim-AR has incorporated into their software a parking brake release 'command'.  This by-passes the need to use the USER OUT settings mentioned above.  The 'command' is set to the output on the Phidget 0/0/8 card.  The parking brake release can be found in the Configuration/Gates menu.  (MORE TO COME HERE - in construction).

The power for the actuator (in my set-up) is connected from the 12 volt busbar in the Throttle Communication Module (TCM) and then, via a straight-through cable, to the Throttle Interface Module (TIM).  The relay for the parking brake mechanism is located in the TIM.

Actuator Caution LED

The design of an actuator is such, that if power is continuously applied to the mechanism, it will burn out.  If operating correctly, the actuator will onlt receive power when the toe brakes are depressed and the parking brake lever is raised at the same time.

To combat against the unforeseen event of power being continuously supplied to the actuator, for example by a relay that is stuck in the open (on) position, a coloured LED has been incorporated into the three LEDs that are fitted to the front of the Throttle Communication Module (TCM).  This flashing LED illuminates only when the circuit is closed and the actuator is receiving 12 volt power.

Additional Information

Like many things, there are several ways to accomplish the same or a similar task.  The following posts located in the ProSim737 forum discuss the conversion of the parking brake lever.

How To Make Your Own Parking Brake Release

Parking Brake Logic


Two terms often confused are open circuit and closed in relation to an electrical circuit.

Any circuit which is not complete is considered an open circuit.  Conversely, a circuit is considered to be a closed circuit when electricity flows from an energy source to the desired endpoint of the circuit.

Conversely, a closed relay means it allows voltage to travel through it, while an open relay is the opposite.

ProSim-AR - Refers to the ProSim737 avionics suite which has been developed by ProSim-AR.


Throttle Quadrant Rebuild - Speedbrake Motor and Clutch Assembly Replacement 

The speedbrake system has been completely overhauled. 

In the previous conversion a number of problems developed.  In particular, the speed of the speedbrake lever when deployed was either too fast, too slow, or did not move at all.  Furthermore, the clutch mechanism frequently became loose.

Rather than continually ‘tweak’ a problematic system, the motor and clutch assembly was removed and replaced with a more advanced and reliable system.

LEFT:  The motor that provides the power to move the speedbrake lever is attached via a slipper clutch to the speedbrake control rod (click to enlarge).  Below the motor is the Throttle Communication Module (TCM).

To read about the first conversion and learn a little more about closed-loop systems and how the speedbrake works I recommend you read the first portion of my earlier article.

B737 Throttle Quadrant  Speedbrake Conversion and Use

Motor and Clutch Assembly

A 12 volt motor is used to power the speed brake.  The motor is mounted forward of the throttle unit above the Throttle Communication Module (TCM).  The wiring from the motor is routed, in a lumen through the throttle firewall to a 12 volt busbar and relays.  The relays, mounted inside the TCM, are dedicated to the speedbrake. 

Attached to the 12 volt motor is a clutch assembly, similar in design to the slipper clutches used in the movement of the two throttle thrust levers.  The clutch can easily be loosed or tightened to provide the correct torque on speedbrake lever, and once set will not become loose.

The slipper clutch was commercially made.

LEFT:  A linear throw potentiometer has been mounted on the Captain-side of the quadrant.  The potentiometer enables the movement of the speedbrake lever to be finely calibrated (click to enlarge).

Speedbrake Mechanics and Logic

The logic that controls the speedbrake has not be changed from what it was in the previous conversion. 

Micro-buttons have been strategically placed beneath the speedbrake lever arc.  As the speedbrake lever moves over one of the buttons, the button will trigger a relay to either open or close (on/off).  The four relays, which are mounted in the Throttle Communication Module (TCM) trigger whether the speedbrake will be armed, stowed, engaged on landing, or placed in the UP position.

To improve the positional accuracy of the speedbrake lever, a linear throw potentiometer has been mounted to the throttle superstructure on the Captain-side. 

Speedbrake Operation

The speedbrake system is a mechanical closed-loop system, meaning it does not require calibration using FSUIPC or from the avionics suite (ProSim737).  The speedbrake lever, however, does need to be registered and calibrated  as a joystick controller in the Windows operating system.  This enables the avionics suite to see the potentiometer.

The following condtions will cause the speedbrake lever to deploy from the DOWN to the UP position.

(i)  When the aircraft lands and the squat switch is activated;

(ii)  During a Rejected Takeoff (RTO).  Assuming the autobrake selector switch has been set to RTO, there is active wheel spin, and the groundspeed exceeds 80 knots; and,

(iii)  If the reverse thrust (reversers) are engaged with a positive wheel spin and a ground speed in excess of 100 knots.

Point (iii) is worth expanding upon.  The speedbrake system (in the real aircraft) has a built-in redundancy in that if the flight crew forget to arm the speedbrake system and make a landing, the system will automatically engage the spoilers when reverse thrust is engaged.  This redundant system was installed into the Next Generation airframe after several occurrences of pilots forgetting to arm the speedbrake prior to landing.  

Therefore, the speedbrake will deploy on landing either by activation of the squat switch (if the speedbrake was armed), or when reverse thrust is applied.

The illumination of the speedbrake annunciators (condition lights) is not part of the system and must be programmed directly within ProSim737 (switches/indicators).

Programmed Variables

The following variables have been programmed into the logic that controls the operation of the speedbrake.

(i)    Rejected Take Off (RTO).  This will occur after 80 knots call-out.  Spoilers will extend to the UP position  when reverse thrust is applied.  The speedbrake lever moves to UP position on throttle quadrant.  RTO must be armed prior to takeoff roll;

(ii)    Spoilers extend on landing when the squat switch is activated.  For this to occur, both throttle thrust levers must be at idle (at the stops).  The speedbrake lever also must be in the armed position prior to landing.  The speedbrake lever moves to UP position on throttle quadrant;

(iii)    Spoilers extend automatically and the speedbrake lever moves to the UP position when reverse thrust is applied;

(iv)    Spoilers close and the speedbrake lever moves to the DOWN position on throttle quadrant when the thrust levers are advanced after landing (auto-stow); and,

(v)    Speedbrakes extend incrementally in the air dependent on lever position (flight detent).

Speedbrake Lever Speed

When the speedbrake lever is engaged, the speed at which lever moves is quite fast.  The term ‘biscuit cutter’ best describes the energy that is generated when the lever is moving; it certainly will break a biscuit in two as well as a lead pencil.  Speaking of lead pencils, I have been told a favorite trick of pilots from yesteryear, was to rest a pencil on the throttle so that when the speedbrake engaged the pencil would be snapped in two by the lever!

In the real Boeing aircraft the movement of the lever is marginally slower and is controlled by an electrically operated actuator (28 volts DC). 

LEFT:  The actuator that controls the movement of the speedbrake.  This image was taken from beneath the floor structure of a Boeing 600 aircraft.  Image copyright to Karl Penrose (click to enlarge).

In theory, the moderate speed that the speedbrake lever moves in the real aircraft should be able to be duplicated; for example, by suppressing the voltage from the 12 volt motor by the use of a capacitor, using a power supply lower than 12 volts, or by using speed controllers.  These alternatives have yet to be trailed.

It is unfortunate, that most throttle quadrants for sale do not include the actuator.  The actuator is not part of the throttle unit itself, but is located in the forward section under the flight deck.  The actuator is then connected to the speedbrake mechanism unit via a mechanical linkage.

In the real Boeing aircraft, the speedbrake lever and actuator provide the input via cables, that in-turn actuate the speedbrakes.  There is no feedback directly from the hydraulics and all operation is achieved via the manual or electric input of the speedbrake lever.

Actuator Sound

The sound of the actuator engaging can easily heard in the flight deck when the speedbrake engages (listen to the below video).  To replicate this sound, a recording of the actuator engaging was acquired.  The .wav sound file was then uploaded into the ProSim737 audio file library and configured to play when the speedbrake is commanded to move.  

The .wav file can be shortened or lengthened to match the speed that the lever moves (download .wav file).


The upper video demonstrates the movement of the speedbrake lever.    The lower video, courtesy of U-Tube, shows the actual movement of the lever in a real Boeing aircraft.

Notice: Not for operational use; video is intended to present the features and functions of the unit in question and not procedures.

If you listen carefully to both videos, you will note a difference in the noise that the actuator generates.  I am lead to believe that the 'whine' noise is slightly different depending upon the aircraft frame.  The actuator in the older classic series Boeing being more of a high whine in comparison to the actuator in the NG airframes.


Condition(s) - A term referring to a specific parameter that is required to enable an action to occur.
FSUIPC - Flight Simulator Universal Inter-Process Communication.  A fancy term for software that interfaces between the flight simulator programme and other outside programmes.
Speedbrake Lever Arc - The curved arc that the speedbrake lever moves along.