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Mission Statement 

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.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.


Note:   I have NO affiliation with ANY manufacturer or reseller.  All reviews and content are 'frank and fearless' - I tell it as I see it.  Do not complain if you do not like what you read.

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 B737-800 Boeing Flight Simulator (17)


Throttle Quadrant Rebuild - Four Speed Stab Trim and Stab Trim Indicator Tabs

This post will document the alterations that have been made to enable the stab trim wheels to utilize four speeds.  The post will also highlight several problems encountered during the conversion and document their solution.  In addition, the post discusses possible reasons for the erratic behavior of the stab trim indicator tabs.

LEFT:  Captain-side stab trim wheel with manual trim handle extended.  The white line on the trim wheel is an aid to indicate that the trim wheels are rotating (click to enlarge).

In the previous throttle unit, the power to rotate the trim wheels was from a inexpensive 12 Volt pump motor.  The forward and aft rotation speed of the stab trim wheels was controlled by an I/O card.  The system worked well, but the single speed was far from realistic.

The upgrade to the throttle quadrant enables the stab trim wheels to rotate at four speeds which are identical to the speeds observed in a Boeing aircraft.  The speed is controlled by three adjustable speed controller cards, five relays and a Phidget 0/0/8 interface card – all of which are mounted within the Throttle Interface Module (TIM).  

To generate the torque required to rotate the trim wheels at varying speeds, the pump motor was replaced with a encoder capable 12 volt dual polarity brush motor.  The replacement motor is mounted on a customized bracket attached to the inside frame of the throttle unit.  This style of motor is often used in the robotics industry.

Boeing Rotation Speed

The speed at which the trim wheels rotate is identical to the Boeing specification for the NG series airframe.  Simply written, it is:

(i)     Manual trim  - speed without flaps (slow speed);
(ii)    Manual trim  - speed with flaps extended (very fast speed);
(iii)   Autopilot trim  - speed without flaps extended (very slow speed); and,
(iv)   Autopilot trim - speed with flaps extended (faster speed than iii but not as fast as ii).

To determine the correct number of revolutions, each trim wheel cycle was measured using an electronic tachometer.  Electronic tachometers are often used in the automobile industry to time an engine by measuring the number of revolutions made by the flywheel.

It is important to understand that it is not the rotation speed of the trim wheels which is important, but more the speed at which the aircraft is trimmed.  With flaps extended, the time taken to trim the aircraft is much quicker than the time taken if the flaps were retracted.

Is There a Noticeable Difference Between the Four Speeds

There is definitely a noticeable difference between the speed that the trim wheels rotate at their slowest speed and fastest speed; however, the difference is subtle when comparing the intermediate speeds.

LEFT:  Electric stab trim switch on Captain-side yoke.  Whenever the trim is engaged the stab trim wheels will rotate with a corresponding movement in the stab trim indicator tabs (click to enlarge).

Design and Perils of Stab Trim

If you speak to any real-world pilot that flies Boeing style aircraft, they all agree upon a dislike for the spinning of the trim wheels.  The wheels as they rotate are noisy, are a distraction, and in some instances can be quite dangerous, especially if your hand is resting on the wheel and the trim is engaged automatically by the autopilot.  This is not to mention the side handle used to manually rotate the trim wheels, which if left extended, can easily damage your knee, during an automatic trimming operation.

If you look at the Airbus which is the primary rival of Boeing, the trim wheels pale by comparison; they are quiet, rotate less often, and are in no way obtrusive.  So why is this case?

Boeing when they deigned the classic and NG series aircraft did not design the throttle unit anew.  Rather, they elected to build upon existing technology which had changed little since the introduction of the Boeing 707.  This saved the company considerable expense.

Airbus, on the other hand, designed their throttle system from the ground-up and incorporated smaller and less obtrusive trim wheels from the onset.

Interestingly, Boeing in their design of the Dreamliner have revamped the design of the stab trim wheels and the new design incorporates smaller, quieter and less obtrusive trim wheels than in the earlier Boeing airframes – no doubt the use of automated and computer controlled systems has removed the need for such a loud and visually orientated system.

Problems Encountered (Teething Issues)

Three problems were encountered when the trim wheels were converted to use four speeds.  They were:

(i)    Excessive vibration when the trim wheels rotate at the fastest speed;
(ii)    Inconsistency with two of the speeds caused when CMD A/B is engaged; and,
(iii)    Fluttering (spiking) of the stab trim indicator tabs when the electric stab trim switch was engaged in the down position.

Point (i) is discussed immediately below while points (ii) and (iii), which are interrelated, have been discussed together.

(i)    Excessive vibration

When the trim wheels rotate at their highest speed there is considerable vibration generated, which causes the throttle quadrant to shake slightly on its mounts.

One of the reasons for the excessive vibration becomes obvious when you compare the mounting points for the throttle quadrant in a homemade simulator to those found in a real aircraft – the later has several solid attachment points between the throttle unit, the center pedestal, the main instrument panel (CDU Bay), and the rigid floor of the flight deck. 

In a simulator, replicating these attachment points can be difficult.   Also, the throttle is a relatively high yet narrow structure and any vibration will be exacerbated higher in the structure.

LEFT:  Example of a stab trim wheel cog and mechanism (before cleaned) from the First Officer side.  The picture shows some of the internal parts that move (and vibrate) when the trim wheels rotate at very high speeds.  The high and narrow shape of the throttle unit is easily noted  (click to enlarge).

Another reason for the cause of the vibrations is the material used to produce the center pedestal.  In the classic airframe the material used was aluminum; however, in the NG carbon fiber is used, which is far more flexible than aluminum.  Any vibration caused by the rotation of the trim wheels has a tendency to become amplified as it travels to the less rigid center pedestal and then to the floor of the flight deck.


Solving the vibration issue is uncomplicated – provide stronger, additional, and more secure mounting points for the throttle quadrant and the attached center pedestal, or slow the rotation of the trim wheels to a more acceptable speed.  Another option is to replace the platform’s floor with a heavier grade of steel or aluminum.  This would enable the throttle quadrant and center pedestal to be attached to the floor structure more securely.  However, this would add significant weight to the structure.  In my opinion, a heavy steel floor is excessive.

By far the simplest solution, is to reduce the fastest speed at which the trim wheels rotate.  The rotation speed can be altered, by the turn of the screwdriver, on one of three speed controller cards mounted within the Throttle Interface Module (TIM).

For those individuals using a full flight deck including a shell, the excessive vibration is probably not going to be an issue as the shell provides additional holding points in which to secure the throttle quadrant, MIP and floor structure.

(ii)    Inconsistency with two of the speeds caused when CMA A/B is engaged

When the autopilot (CMD A/B) was selected and engaged on the MCP, the rotation of the trim wheels would rotate at an unacceptable very high speed (similar to run-away trim).  

The mechanics of this issue was that when the autopilot was engaged, the electronics was not activating the relay that is responsible for engaging the speed controller card.

(iii)       Fluttering of the stab trim indicators

When the electric stab trim switch was depressed to the down position, it was observed that the stab trim indicator tabs would often flutter.  Although the fluttering was mechanical and had no bearing on the trim accuracy, or speed at which the aircraft was trimmed, it was visually distracting.

A possible cause for the run-away trim was electromagnetic interference (RF) generated by the high torque of the trim motor.  The higher than normal values of RF were being  ‘picked up’ by the relay card, which were causing the relay to not activate when the autopilot was engaged.  Similarly, the fluttering of the stab trim indicator tabs, was thought to have been caused by RF interfering with the servo motor.

There were several possibilities for RF leakage.

(i)    The high torque of the motor was generating and releasing too much RF;
(ii)    The wire lumen that accommodates the cabling for the throttle is mounted proximal to the servo motor.  If the lumen was leaking RF, then this may have interfered with the operation of the servo motor;
(iii)    The servo motor was not digital and did not have an RF shield attached;
(iv)    The straight-through cable from the Throttle Communication Module (TCM) to the Throttle Interface Module (TIM) did not have RF interference nodules attached to the cable.


To counter the unwanted RF energy several modifications were made:

(i)    Three non-polarized ceramic capacitors were placed across the connections of the trim wheel motor;
(ii)    The analogue servo motor was replaced with a higher-end digital servo with an RF shield;
(iii)    The straight-through cable between the TIM and TCM was replaced with a cable that included high quality RF nodes; and,
(iv)    The wires from the servo motor were re-routed and shielded to ensure they were not lying alongside the wire lumen.

Manual Trimming

Manual trimming (turning the trim wheels by hand) is not implemented in the throttle quadrant, but a future upgrade may incorporate this feature.

Cut-out Stab Trim Button (throttle mounted)

In the earlier conversion, the stab trim cut-out toggle was not functional and the toggle had been programmed to switch off the circuit that powers the rotation of the trim wheels.  Having the ability to disconnect the rotation of the trim wheels is paramount when flying at night, as the noisy trim wheels kept family members awake.

LEFT:  Stab trim cut out switches with spring-loaded cover open on main and closed on autopilot (click to enlarge).

The new conversion does not incorporate this feature as the trim cut-out toggle is fully functional.  Rather, a push-to-engage, green-coloured LED button has been installed to the forward side of the Throttle Interface Module (TIM).  The button is connected to a relay, which will either open or close the 12 volt circuit responsible for directing power to the trim motor.

Stab Trim Indicator Tabs

The method used to convert the stab trim indicators has not been altered, with the exception of replacing the analogue servo with a RF protected digital servo (to stop RF interference).  

LEFT:  Stab trim indicator tabs (Captain side).  The throttle is from  B737-500.  The indicator tabs on the NG airframe are slightly different - they are more slender and pointed (click to enlarge).

To review, a servo motor and a Phidget advanced servo card have been used to enable the stab trim tab indicators to move in synchronization to the revolution and position of the stab trim wheels.  The servo card is mounted within the Throttle Interface Module (TIM) and the servo motor is mounted on the Captain-side of the throttle unit adjacent to the trim wheel.  There is nothing exceptional about the conversion of the stab trim indicator tabs and the conversion is, more or less, a stock standard.

Is Variable Rotation Speed Important to Simulate

As discussed earlier, it is not the actual rotation of the trim wheels that is important, but more the speed at which the aircraft is trimmed.   In other words, the speed at which the trim wheels rotate dictates the time that is taken for the aircraft to be trimmed.  

If the trim wheels are rotating slowly, the movement of the stab trim indicator tabs will be slow, and it will take longer for the aircraft to be trimmed.  Conversely, if the rotation is faster the stab trim indicator tabs will move faster and the aircraft will be trimmed much more quickly.

Final Call - is Four-speed Trim Worthwhile

Most throttle conversions implement only one speed for the forward and aft rotation of the trim wheels with the conversion being relatively straightforward.

Converting the throttle unit to use four speeds has not been without problems, with the main issue being the excessive vibration caused by the faster rotation speed.  Nevertheless, it is only in rare instances, such as when the stab trim is engaged for longer than a few seconds at a time, and at the fastest rotation speed, that the vibration becomes an issue.  If the rotation for the fastest speed is reduced, any vibration issues are alleviated – the downside to this being the fastest speed does not replicate the correct Boeing rotation speed.

For enthusiasts wishing to replicate real aircraft systems, there is little excuse for not implementing four-speed trim, however, for the majority of flight deck builders I believe that two-speed trim, is more than adequate.


Below is a short video, which demonstrates the smooth movement of the stab trim indicator tabs from the fully forward to fully aft position.  The video is only intended to present the functionality of the unit and is not to represent in-flight settings.

Below is short video that demonstrates two of the four rotation speeds used.  In the example, manual trim is has been engaged, beginning with flaps UP, flaps extended, and then flaps UP again.  The rotation speed of the trim wheels with flaps extended (in this case to flaps 1) is faster than the rotation speed with flaps UP.  The video does not reflect in-flight operations and is only to present the functionality of the unit in question.


Electromagnetic Interference (RF) – RF is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation  emitted from an external source (see Wikipedia definition).
MCP – Mode Control Panel.
MIP – Main Instrument Panel.
Stab Trim Indicator Tabs – The two metal pointed indicators located on the throttle unit immediately adjacent to the %CG light plate.  If not using a workable throttle unit, then these tabs maybe located in the lower EICAS as a custom user option.
Servo Motor – Refers to the motor that powers the stab trim indicator tabs.
Trim Motor – Refers to the motor that powers the stab trim wheels.


Throttle Quadrant Rebuild - Clutch, Motors, and Potentiometers

An earlier article, ‘Throttle Quadrant Rebuild – Evolution Has Led to Major Changes’ has outlined the main changes that have been made to the throttle quadrant during the rebuild process. 

LEFT:  Captain-side of throttle quadrant showing an overview of the new design.  The clutch assembly, motors, and  string potentiometer can be seen, in addition to a portion of the revised parking brake mechanism.

This article will add detail and explain the decision making process behind the changes and the advantages they provide.  As such, a very brief overview of the earlier system will be made followed by an examination of the replacement system.


It is not my intent to become bogged down in infinite detail; this would only serve to make the posts very long, complicated and difficult to understand, as the conversion of a throttle unit is not simplistic.

This said, the provided information should be enough to enable you to assimilate ideas that can be used in your project.  I hope you understand the reasoning for this decision.

The process of documenting the throttle quadrant rebuild will be recorded in a number of articles.  In his article I will discuss the clutch assembly, motors, and potentiometers.  The main flight controls page has a several links to articles that relate to the conversion of the throttle quadrant.

Why Rebuild The Throttle Quadrant

Put bluntly, the earlier conversion had several problems; there were shortfalls that needed improvement, and when work commenced to rectify these problems, it became apparent that it would be easier to begin again rather than retrofit. Moreover, the alterations spurred the design and development of two additional interface modules that control how the throttle quadrant was to be connected with the simulator.

•    Throttle Interface Module (TIM)
•    Throttle Communication Module (TCM)

TIM houses the interface cards responsible for the throttle operation while the TCM provides a communication gateway between TIM and the throttle.

Motor and Clutch Assembly - Poor Design

The previous throttle conversion used an inexpensive 12 volt motor to power the thrust lever handles forward and aft.  Prior to being used in the simulator, the motors were used to power electric automobile windows.  To move the thrust lever handles, an automobile fan belt was used to connect to a home-made clutch assembly.

The system was sourly lacking in that the fan belt continually slipped.  Likewise, the nut on the clutch assembly, designed to loosen or tighten the control on the fan belt, was either too tight or too loose - a happy medium was not possible.   Furthermore, the operation of the throttle caused the clutch nut to continually become loose requiring frequent adjustment.

The 12 volt motors, although suitable, were not designed to entertain the precision needed to synchronize the movement of the thrust levers; they were designed to push a window either up or down at a predefined speed on an automobile.

The torque produced from these motors was too great, and the physical backlash when the drive shaft moved was unacceptable.  The backlash transferred to the thrust levers causing the levers to jerk (jump) when the automation took control (google motor backlash).

This system was removed from the throttle.  Its replacement incorporated two commercial motors professionally attached to a clutch system using slipper clutches.

Clutch Assembly, Connection Bars and Slipper Clutches - New Design

Mounted to the floor of the throttle quadrant are two clutch assemblies (mounted beside each other) – one clutch assembly controls the Captain-side thrust lever handle while the other controls the First officer-side. 

Each assembly connects to the drive shaft of a respective motor and includes in its design a slipper clutch.  Each clutch assembly then connects to the respective thrust lever handle.  A wiring lumen connects the clutch assembly with each motor and a dedicated 12 volt power supply (mounted forward of the throttle quadrant).  See above image.

Connection Bars

To connect each clutch assembly to the respective thrust lever handle, two pieces of aluminium bar were engineered to fit over and attach to the shaft of each clutch assembly. 

LEFT:  Close up image of the aluminium bar and ninety degree flange attachment.  The long-threaded screw connects with the tail of the respective thrust lever handle. An identical attachment at the end of the screw connects the screw to the large cog wheel that the thrust lever handles are attached (click to enlarge).


Each metal bar connects to one of two long-threaded screws, which in turn connect directly with the tail of each thrust lever handle mounted to the main cog wheel in the throttle quadrant. 

Slipper Clutches

A slipper clutch is a small mechanical device made from tempered steel, brass and aluminum.  The clutch consists of tensioned springs sandwiched between brass plates and interfaced with stainless-steel bearings.  The bearings enable ease of movement and ensure a long trouble-free life.

LEFT:  The clutch assembly as seen from the First Officer side of the throttle quadrant.  Note the slipper clutch that is sandwiched between the assembly and the connection rods.  Each thrust lever handle has a dedicated motor, slipper clutch and connection rod.  The motor that powers the F/O side can be seen in the foreground (click to enlarge).

The adjustable springs are used to maintain constant pressure on the friction plates assuring constant torque is always applied to the clutch.  This controls any intermittent, continuous or overload slip.

A major advantage, other than their small size, is the ease at which the slipper clutches can be sandwiched into a clutch assembly.

Anatomy and Key Advantages of a Slipper Clutch

A number of manufacturers produce slipper clutches that are specific to a particular industry application, and while it's possible that a particular clutch will suit the purpose required, it's probably a better idea to have a slipper clutch engineered that is specific to your application. 

LEFT:  The diagram shows a cut-away of a slipper clutch and an image of the actual clutch used (click to enlarge).

The benefit of having a clutch engineered is that you do not have to redesign the drive mechanism used with the clutch motors.

Key advantages in using slipper clutches are:

(i)    Variable torque;
(ii)   Long life (on average 30 million cycles with torque applied);
(iii)  Consistent, smooth and reliable operation with no lubrication required;
(iv)  Bi-directional rotation; and,
(v)   Compact size.

Clutch Motors

The two 12 Volt commercial-grade motors that provide the torque to drive the clutch assembly and movement of the thrust lever handles, have been specifically designed to be used with drives that incorporate slipper clutches.

LEFT:  View of captain-side motor, wiring lumen and string potentiometer (click to enlarge).

In the real world, these motors are used in the railway and marine industry to drive high speed components.  As such, their design and build quality is excellent.

Each motor is manufactured from stainless steel parts and has a gearhead actuator that enables the motor to be operated in either forward or reverse.  Although the torque generated by the motor (18Nm stall torque) exceeds that required to move the thrust lever handles forward and aft, the high quality design of the motor removes all the backlash evident when using other commercial-grade motors.  The end result is an extraordinary smooth, and consistent operation when the thrust lever handles move.

A further benefit using this type of motor is its size.  Each motor can easily be mounted to the floor of the throttle quadrant; one motor on the Captain-side and the second motor on the First Officer-side.  This enables a more streamlined build without using the traditional approach of mounting the motors on the forward firewall of the throttle quadrant.

String Potentiometers - Thrust Levers 1/2

Two Bourne dual-string potentiometers have been mounted in the aft section of the throttle unit.  The two potentiometers are used to accurately calibrate the position of each thrust lever handle to a defined %N1 value.  The potentiometers are also used to calibrate differential reverse thrust.

LEFT: Dual Bourne string potentiometer that enables accurate calibration of thrust lever handles and enables differential thrust when reversers are engaged (click to enlarge).

The benefit of using Bourne potentiometers is that they are designed and constructed to military specification, are very durable, and are sealed.  The last point is important as sealed potentiometers will not, unlike a standard potentiometer, ingest dust and dirt.  This translates to zero maintenance.

Traditionally, string potentiometers have been mounted either forward or rear of the throttle quadrant; the downside being that considerable room is needed for the operational of the strings.  

In this build, the potentiometers were mounted on the floor of the throttle housing (adjacent to the motors) and the dual strings connected vertically, rather than horizontally.  This allowed maximum usage of the minimal space available inside the throttle unit.

Reverse Thrust 1/2

Micro-buttons were used in the previous conversion to enable enable reverse thrust - reverse thrust was either on or off, and it was not possible to calibrate differential reverse thrust. 

In the new design, the buttons have been relaced by two string potentiometers (mentioned earlier).  This enables each reverse thrust lever to be accurately calibrated to provide differential reverse thrust.  Additionally, because a string potentiometer has been used, the full range of movement that the reverse thrust is capable of can be used.

Automation, Calibration and Movement

The automation of the throttle remains as it was.  However, the use of motors that generate no backlash, and the improved calibration gained from using string potentiometers, has enabled a synchronised movement of both thrust lever handles which is more consistent than previously experienced.


To correctly position the thrust lever handles in relation to %N1, calibration is done within the ProSim737 avionics software  In calibration/levers, the position of each thrust lever handle is accurately ‘registered’ by moving the tab and selecting minimum and maximum.  Unfortunately, this registration is rather arbitrary in that to obtain a correct setting, to ensure that both thrust lever handles are in the same position with identical %N1 outputs, the tab control must be tweaked left or right (followed by flight testing).

When tweaked correctly, the two thrust lever handles should, when the aircraft is hand-flown (manual flight), read an identical %N1 setting with both thrust levers positioned beside each other.  In automated flight the %N1 is controlled by the interface card settings (Polulu JRK cards or Alpha Quadrant cards).

Have The Changes Been Worthwhile

Comparing the new system with the old is 'chalk and cheese'.  

One of the main reasons for the improvement has been the benifits had from using high-end commercial-grade components.  In the previous conversion, I had used inexpensive potentiometers, unbalanced motors, and hobby-grade material.  Whilst this worked, the finesse needed was not there.

One of the main shortcomings in the previous conversion, was the backlash of the motors on the thrust lever handles.  When the handles were positioned in the aft position and automation was engaged, the handles would jump forward out of sync.  Furthermore, calibration with any degree of accuracy was very difficult, if not impossible. 

The replacement motors have completely removed this backlash, while the use of string potentiometers have enabled the position of each thrust lever handle to be finely calibrated, in so far, as each lever will creep slowly forward or aft in almost perfect harmony with the other.

An additional improvement not anticipated was with the installation of the two slipper clutches.  Previously, when hand-flying there was a binding feeling felt as the thrust lever handles were moved forward or aft.  Traditionally, this binding has been difficult to remove with older-style clutch systems, and in its worst case, has felt as if the thrust lever handles were attached to the ratchet of a bicycle chain.

The use of high-end slipper clutches has removed much of these feeling, and the result is a more or less smooth feeling as the thrust lever handles transition across the throttle arc.

Future Articles

Future articles will address the alterations made to the speedbrake, parking brake lever, and internal wiring, interfacing and calibration.  The rotation of the stab trim wheels and movement of the stab trim indicator tabs will be discussed.

For a complete list of links that connect to articles that concern the conversion of the throttle quadrant, navigate to the main flight controls page (links at the bottom of this page).


Autobrake System - Review and Procedures

The autobrake, the components which are located on center panel of the Main Instrument Panel (MIP), is designed as a deceleration aid to slow an aircraft on landing.  The system uses pressure, generated from the hydraulic system B, to provide deceleration for preselected deceleration rates and for rejected takeoff (RTO). An earlier post discussed Rejected Takeoff procedures.  This post will discuss the autobrake system.

LEFT:  Ryanair B737-800 -  autobrake set, flaps 30, spoilers deployed, reverse thrust engaged (photograph copyright Pierre Casters).


The autobrake selector knob (rotary switch) has four settings: RTO (rejected takeoff), 1, 2, 3 and MAX (maximum).  Settings 1, 2 and 3 and RTO can be armed by turning the selector; but, MAX can only be set by simultaneously pulling the selector knob outwards and turning to the right; this is a safety feature to eliminate the chance that the selector is set to MAX accidently.  

When the selector knob is turned, the system will do an automatic self-test.  If the test is not successful and a problem is encountered, the auto brake disarm light will illuminate amber.

The autobrake can be disengaged by turning it to OFF, by activating the toe brakes, or by advancing the throttles; which deactivation method used depends upon the circumstances and pilot discretion.  Furthermore, the deceleration level can be changed prior to, or after touchdown by moving the autobrake selector knob to any setting other than OFF.  During the landing, the pressure applied to the brakes will alter depending upon other controls employed to assist in deceleration, such as thrust reversers and spoilers.

The numerals 1, 2, 3 and MAX provide an indication to the severity of braking that will be applied when the aircraft lands (assuming the autobrake is set).

In general, setting 1 and 2 are the norm with 3 being used for wet runways or very short runways.  MAX is very rarely used and when activated the braking potential is similar to that of a rejected take off; passenger comfort is jeopardized and it is common for passenger items sitting on the cabin floor to move forward during a MAX braking operation.  If a runway is very long and environmental conditions good, then a pilot may decide to not use autobrakes favouring manual braking.

Often, but not always, the airline will have a policy to what level of braking can or cannot be used; this is to either minimize aircraft wear and tear and/or to facilitate passenger comfort. 

The pressure in PSI applied to the autobrake and the applicable deceleration is as follows:

•    Autobrake setting 1 - 1250 PSI / 4 ft per second.
•    Autobrake setting 2 - 1500 PSI / 5 ft per second.
•    Autobrake setting 3 - 2000 PSI / 7.2 ft per second.
•    Autobrake setting MAX and RTO - 3000 PSI / 14 ft per second (above 80 knots) and 12 ft per second (below 80 knots).


To autobrake will engage upon landing, when the following conditions are met:

(i)    The appropriate setting on the auto brake selector knob (1, 2, 3 or MAX) is set;
(ii)    The throttle thrust levers are in the idle position immediately prior to touchdown; and,  
(iii)   The main wheels spin-up.

If the autobrake has not been selected before landing, it can still be engaged after touchdown, providing the aircraft has not decelerated below 60 knots.

To disengage the autobrake system, any one of the following conditions must be met:

(i)   The autobrake selector knob is turned to OFF (autobrake disarm annunciator will not illuminate);
(ii)  The speed brake lever is moved to the down detent position;
(iii) The thrust levers are advanced from idle to forward thrust (except during the first 3 seconds of landing); or,
(iv)  Either pilot applies manual braking.

The last three points (ii iii and iv) will cause the autobrake disarm annunciator to illuminate for 2 seconds before extinguishing.

Important Facet

It is important to grasp that the 737 NG does not use the maximum braking power for a particular setting (maximum pressure), but rather the maximum programmed deceleration rate (predetermined deceleration rate).  Maximum pressure can only be achieved by fully depressing the brake pedals or during an RTO operation.  Therefore, each setting (other than full manual braking and RTO) will produce a predetermined deceleration rate, independent of aircraft weight, runway length, type, slope and environmental conditions.

Autobrake Disarm Annunciator

The autobrake disarm annunciator is coloured amber and illuminates momentarily when the following conditions are met:

(i)   Self-test when RTO is selected on the ground;
(ii)   A malfunction of the system (annunciator remains illuminated - takeoff prohibited);
(iii)  Disarming the system by manual braking;
(iv)  Disarming the system by moving the speed brake lever from the UP position to the DOWN detente position; and,
(v)   If a landing is made with the selector knob set to RTO (not cycled through off after takeoff).  (If this occurs, the autobrakes are not armed and will not engage.  The autobrake annunciator remains illuminated amber).

The annunciator will extinguish in the following conditions:

(i)    Autobrake logic is satisfied and autobrakes are in armed mode; and,
(ii)   Thrust levers are advanced after the aircraft has landed, or during an RTO operation.  (There is a 3 second delay before the annunciator extinguishes after the aircraft has landed).

Preferences for Use of Autobrakes and Anti-skid

When conditions are less than ideal (shorter and wet runways, crosswinds), many flight crews prefer to use the autobrake rather than use manual braking, and devote their attention to the use of rudder for directional control.   As one B737 pilot stated - ‘The machine does the braking and I maintain directional control’.

Anti-skid automatically activates during all autobraking operations and is designed to give maximum efficiency to the brakes, preventing brakes from stopping the rotation of the wheel, thereby ensuring maximum braking efficiency.  Anti-skid operates in a similar fashion to the braking on a modern automobile.

Anti-skid is not simulated in FSX/FS10 or in ProSim737 (at the time of writing).

To read about converting an OEM Autobrake Selector navigate to this post.


Rejected Takeoff (RTO) - Review and Procedures

A takeoff may be rejected for a variety of reasons, including engine failure, activation of the takeoff warning horn, ATC direction, blown tyres, or system warnings.  For whatever reason, Boeing estimates that 1 takeoff in every 2000 will be rejected (Boeing Corporation).

LEFT:  The Rejected Takeoff is part of the Auto Brake Selector Panel located on the Main Instrument Panel (MIP).  RTO can be selected by turning the selector knob to the left from OFF by one click.  

This is an OEM (Original Equipment Manufacture) autobrake assembly that has been converted for use in the simulator.  Note that the selector knob is not NG compliant but is from a 500 series airframe.  In time this knob will be replaced.  (click image to enlarge)

Performed incorrectly, an RTO can be a dangerous procedure; therefore, protocols have been are established that need to be followed.  

This is the first of two consecutive posts that will discuss components of the autobrake system.  In this post RTO procedures will be explained.  In the second post the auto brake will be examined.

Rejected Takeoff (RTO)

The Auto Brake and Rejected Takeoff (RTO) are part of Auto Brake System, the components which are located on center panel of the Main Instrument Panel (MIP).  An RTO is when the pilot in command makes the decision to reject the takeoff of the aircraft.  

The Boeing Flight Crew Training Manual (FCTM) states:

(i)    ‘A flight crew should be able to accelerate the aircraft, have an engine failure, abort the takeoff, and stop the aircraft on the remaining runway'; or,
(ii)    'accelerate the aircraft, have an engine failure, and be able to continue the takeoff utilizing one engine’.  

Two important variables of pre-flight planning need to be established for an RTO to be executed safely - V speeds and runway length.

V Speeds and Runway Length

There are three V speeds that are critical to a safe takeoff and climb out: V1, Vr and V2.  

V1 is the speed used to make the decision to ‘abort or fly’.  Vr is the rotation speed, or the speed used to begin the rotation of the aircraft by smoothly pitching the aircraft to takeoff attitude.  V2 is the speed used for the initial climb-out, and is commonly called the takeoff safety speed.  The takeoff safety speed ensures a safe envelope for single engine operations.

It stands to reason, that the runway must be long enough to cater towards the V speeds calculated from the weight of the aircraft and outside temperature.

Rejected Takeoff - Conditions and Procedure

In general, the protocol used to execute an RTO, is to:

(i)    Abort the takeoff for ‘cautions’ below 80 knots; and,
(ii)   between 80 knots and V1 speed, abort only for ‘bells’ (fire warning) and flight control problems.

If a problem occurs below V1 speed, the aircraft should be able to be stopped before reaching the end of the runway.  After exceeding V1 speed, the aircraft cannot be safely stopped and the only option is to takeoff, and after reaching a safe minimum altitude and speed, troubleshoot the problem.

Before takeoff, a flight crew will position the auto brake selector knob to RTO.  This action will trigger the illumination of the auto brake disarm annunciator, which will illuminate amber for 2 seconds; this is a self-test to indicate that the system is working.  After 2 seconds the annunciator will extinguish.

To arm the RTO prior to takeoff, the following conditions must be met:

(i)    The auto brake and anti-skid systems must be operational;
(ii)   The aircraft must be on the ground;
(iii)  The auto brake selector must be set to RTO;
(iv)  The forward thrust levers must be in the idle position; and
(v)   The wheel speed must be less than 60 knots.

Once armed, the RTO system only becomes operative after the aircraft reaches 80 knots ground speed (some manuals state 90 knots).  If an ‘abort’ is indicated below 80 knots, the aircraft will need to be stopped using manual braking power.  

The auto brake will remain in the armed mode if the RTO abort was executed prior to 80 knots (the auto brake disarm annunciator does not illuminate).

To engage the RTO the following conditions must be met:

(i)    The auto brake must be set to RTO;
(ii)   The thrust levers must be retarded to idle position;
(iii)  The aircraft must have reached 80 knots; and,
(iv)  The autothrottle must be disconnected.

When an RTO is executed and the auto brake system engages, the system will apply 3000 PSI to the brakes to enable the aircraft to stop.  Additionally, if the aircraft has reached a wheel speed in excess of 60 knots, and one or two of the reverse thrust levers are engaged, the spoiler panels will extend automatically to the UP position (deploy), and the speed brake lever on the throttle quadrant will move to the UP position.

The auto brake will disengage, if during the RTO either pilot:

(i)    Activates the toe brakes;
(ii)   Turns the selector knob of the auto brake from RTO to off.   

If the reversers have been engaged and the speed brake lever is in the UP position, then the lever will abruptly move to the DOWN detente position.  When this occurs, the speed brake annunciator will illuminate amber for 2 seconds before extinguishing.  Braking will then need to be accomplished manually.

RTO Procedure

  1. Pilot flying calls ‘STOP’, ‘ABANDON’ or ‘ABORT’.
  2. Pilot flying closes thrust levers and disengages autothrottle.
  3. Pilot flying verifies automatic RTO braking is occurring, or initiates manual braking if deceleration is not great enough, or autobrake disarm light is illuminated.
  4. Pilot flying raises speed brake lever (if not already in UP position).
  5. Pilot flying applies maximum reverse thrust or thrust consistent with runway and environmental conditions.
  6. Once stopped, pilot flying engages parking brake and completes RTO checklist.

Point 4 is important as although the spoilers deploy automatically when the reversers are engaged, they must be extended manually to minimise any delay in spoilers extension, as extension is necessary for efficient wheel braking.

What Circumstances Trigger An RTO

Prior to 80 knots, the takeoff should be rejected for any of the following:

  • Activation of the master caution system;
  • Unusual noise and vibration;
  • Slow acceleration;
  • Takeoff configuration warning;
  • Tyre failure;
  • Fire warning;
  • Engine failure;
  • Bird strikes;
  • Windshear warning;
  • Window failure; and/or,
  • If the aircraft is unsafe or unable to fly.

After 80 knots and prior to V1, the takeoff should be rejected for any of the following:

  • Fire warning;
  • Engine failure;
  • Windshear warning; and/or,
  • If the aircraft is unsafe or unable to fly.

After V1 has been reached, takeoff is mandatory.

Important Points To Remember

Important points to remember when performing a Rejected Takeoff are:

  1. Engage the RTO selector knob before takeoff;
  2. Retard throttles to idle;
  3. Disengage the autothrottle (A/T);
  4. Engage one or both reverse thrust levers;
  5. Monitor RTO system performance, being prepared to apply manual braking if the auto brake disarm light annunciates;
  6. Raise speed brake lever if not already in the UP position BEFORE engaging reverse thrust; and,
  7. Remember that RTO functionality engages only after the aircraft has reached 80 knots ground speed, and remains armed if the RTO has been executed below 80 knots.

Procedural Variations

A successful RTO is dependent upon the pilot flying making timely decisions and using proper procedures.  Whether an RTO is executed fully or partly is at the discretion of the pilot flying (reverse thrust engaged to deploy spoilers).

It should be noted that If the takeoff is rejected before the THR HLD annunciation, the autothrottles should be disengaged as the thrust levers are moved to idle. If the autothrottle is not disengaged, the thrust levers will advance to the selected takeoff thrust position when released. After THR HLD is annunciated, the thrust levers, when retarded, remain in idle.

For procedural consistency, disengage the autothrottles for all rejected takeoffs.

Figure 1 provides a visual reference indicating the distance taken for an aircraft to stop after various variations of the Rejected Takeoff are executed (copyright, Boeing Flight Crew Training Manual FCTM).

This post has explained the basics of a Rejected Takeoff.  Further information can be found in the Flight Crew Training Manual (FCTM) or Quick Reference Handbook (QRH).

In the next post the autobrake system will be discussed.


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

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.


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).  


  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 travelling 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.  A more detailed explanation can be read by following the ‘radials’ link in the acronyms section at the end of this article.

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.


  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 travelling 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).


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.


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.