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

In the previous system, the parking brake lever was controlled by a relay and a 12 volt solenoid.  The mechanical 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.

There has been minimal change to the mechanical system, with the exception that, the solenoid has been replaced by a 12 volt actuator, and to engage the parking brake lever to the UP position the toe brakes must be depressed. 

Navigate to this published post that has explained the earlier conversion of the parking brake: B737 Parking Brake Mechanism.

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 plastic coated 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 unit, operates on 12 volt power and is mounted horizontally on the Captain-side of the quadrant. 

In addition to the actuator, a micro limit switch and relay (on/off) are also used. 

Micro Switch and Relay

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. 

The use of a micro switch facilitates a second line of containment.  What this means is that the circuit will only remain open, when both the relay is open (toe brakes depressed) and the connection from the micro switch is severed - both variables must be triggered for the mechanism to operate.  This can only occur when the toe brakes are depressed whilst simultaneously pulling the parking brake lever to the UP position.

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 has been faithfully replicated in the simulator using a mechanical system.

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. 

Two items are needed to engage the parking brake lever.  A relay to signal open or close (on/off) when the toe brakes are depressed, and the micro limit switch discussed earlier.

Depressing or releasing the toe brakes opens or closes a relay which in turn enables 12 volt power to reach the annunciator.  However, the system is only 'live' (closed system) when the parking brake lever is moved to the UP position, severing the connection between the flange on the vertical control rod and the micro limit switch, 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.

Two Methods of Connection Can be Used - Full Mechanical or Part-Mechanical Software

There are two methods that can be used to connect the actuator to the parking brake mechanism.  The first is a straight physical method - the toe brakes are connected to a relay which in turn is connected to the actuator and micro switch. 

The second method is part-mechanical and software controlled and involves using the ProSim737 avionics suite.  Using a Phidgets 0/0/4 relay, the USER 1 interface in the configuration menu of ProSim737 is programmed to read the movement (offset) for the toe brakes.  When the toe brakes are depressed, the software detects and reads the offset which in turn opens the relay enabling power to flow to the actuator.  The actuator will be engaged (circuit closed) only if the connection between the vertical control rod and the micro switch is severed (parking brake lever is in the raised position).

The power for the actuator 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.

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,

(iv)      Release the toe brakes.  Releasing pressure on the toe brakes causes the actuator to lock into the engaged position.

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.

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 by beginners are open circuit and closed 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.

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