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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 P3D (3)

Thursday
Aug232018

Adding Liveries to ProSim737 Flight Model

Flight simulator enthusiasts enjoy flying the livery of their choice, whether it be a cargo carrier such as FedEX, or a livery from one of the many passenger airlines that fly the Boeing 737 airframe.  

LEFT:  The livery for the JAL TRANSOCEAN Air, which depicts a whale shark is spectacular.  Why would you not want to use liveries when some look like this.  The whale shark inhabits the waters that this particular airline fly to (southern Japan) - © DavE-JetPhotos.

Airlines have unique liveries that identify the carrier.  Often the design is specific to the country or to a particular motif unique to the airlines.  For example, QANTAS depicts a red kangaroo on its tail and Aeroflot always depicts the Russian flag on its tail wing.  Some liveries relate to airline branding, others can be nationalistic (those carrying flags on their tail wings), and others can be just for fun - such as Taiwanese airline's Eva Air 'Hello Kitty' liveryWikipedia has an interesting list of airlines that have liveries that relate floral emblems, animals, flags and the like.

Some software companies, for example PMDG, have developed livery add-ons that can be installed by a self-extractable.exe file;  it’s only a matter of clicking the .exe file and following the prompts, and the information, textures and changes are automatically installed behind the scenes by the software.  

The ProSim737 flight model (developed by ProSim-AR) does not at the time of writing provide a self-executable file for add-on liveries; users must install liveries manually.  Thankfully, the steps to install a livery are generic, and have been more or less the same since FS9 and FSX.

This article will address how to install an aircraft livery to the main aircraft folder for ProSim737 using the ProSim737-800-2018 Professional aircraft (Version 3 flight model) using Prepar3D (P3D).

Note that ProSim-AR produces two flight models – Version 2 and Version 3, and unfortunately, the liveries made for Version 2 (of which there are many) do not work with Version 3.

For those users who use the ProSim737 Version 2 flight model, the process of installing liveries is similar, however, the ProSim737 folder structure is different.

Back-up

Before proceeding with any amendment to the aircraft folder, make a backup of the ProSim737 aircraft folder BEFORE making changes.  It’s also wise to copy the default aircraft configuration file.  This can easily be done by right-clicking the file and saving as a copy.  The copy can reside in the same folder, as it will have the word ‘copy’ annotated to the file name.

It’s good policy to do this just in case a problem is experienced.   If a problem presents itself, it’s an easy matter of deleting the aircraft folder and replacing it with the original, or replacing the aircraft configuration file.

The Basics

We are interested in three components:  

(i)      The ProSim737 default aircraft folder;
(ii)     The add-on livery texture folder; and,
(iii)    The aircraft configuration file (aircraft.cfg).

Note that the default ProSim737 aircraft is installed via a self-executable file that installs the default 738 aircraft to the correct folder.

File and Folder Structure

The ProSim737 aircraft software installs the aircraft to the following folder: D://Documents/Prepar3D V4 Add Ons/ProSim-AR/Simobjects/Airplanes/ProSim737-800-2018 Professional.  

This folder falls outside the main P3D folder architecture, however, various files are automatically linked to P3D so they aircraft can be flown and seen in the game.  In my setup I have two drives, which is why the Prepar3D folder is located on D Drive rather than C Drive.  Your dirve may feature a different drive letter.

LEFT:  One interesting livery is British Airways (BA).  All BA aircraft depict the Union Jack on their tail.  In the 18th Century, England had colonies throughout the world and it was often stated that ‘the sun never set on the Union Jack’.  With the loss of her colonies the sun definitely now sets on the Union Jack, however, it probably never sets on British Airways as there is always a BA aircraft somewhere in the world.  Screen grab of ProSim737 BA livery. © Matthew Fitzjohn (click to enlarge).

Livery Texture Folder

An add-on livery is usually downloaded from the Internet in zip file format.  Once the zip file is extracted, you will see a number of folders and files.  At the very least there will be a texture folder, in which is stored the various bitmaps and images necessary to amend the default aircraft with that livery.   There may also be a thumbnail image of the livery and a ‘read_me’ file.

The ‘read_me’ file is important, as this often will contain the correct edits for the livery that need to be added to the aircraft configuration file.

Non-mipped Images

The developer of the livery may also have included additional folders such as non-mipped images.  Opening this folder will reveal an alternate texture folder.  

Textures developed from non-mipped images are displayed differently by P3D and often provide slightly better detail that standard textures.  This may be advantageous if you often zoom into the aircraft to view close-up detail.  There are many variables that affect the appearance of non-mipped textures, including graphic card settings, computer specifications, and P3D settings.  For most users, the use of non-mipped textures in not necessary.  However, ‘horses for courses’, so test and choose whatever is appropriate to your circumstances.  

If you open the ProSim737 aircraft manual (found with the original default aircraft download) there is a section that addresses the differences between mipped and non-mipped textures.

Aircraft Configuration File

The aircraft configuration file is important as it contains, amongst other things, the necessary instructions to display whatever aircraft has been selected from the P3D aircraft list.  

The configuration file is set out logically with higher-level entries (top of page) identifying the various liveries that have been included in the main ProSim737 aircraft folder.  By default, the ProSim737 flight model installs a number of liveries to the aircraft folder and automatically amends the entries in the configuration file.

BELOW: Text that relates to an aircraft livery in the aircraft configuration file.  Bolded sections need to be edited for each livery.

[fltsim.XX]
title=Prosim_AR_737_800_PRO_2018_Virgin_Australia
sim=Prosim738_Pro
model=
panel=
sound=
//sound=cockpit
texture=VIRGIN
atc_heavy=0
atc_flight_number=209
atc_airline=Velocity

atc_model=737-800
atc_parking_types=GATE,RAMP
atc_parking_codes=VOZ
ui_manufacturer="Prosim_AR"
ui_createdby="ProSim-AR"
ui_type="737-800"
ui_variation="PROSIM_AR_Pro_2018_Virgin_Australia"
ui_typerole="Commercial Airliner"
atc_id=PS209
visual_damage=0

Installing Textures to ProSim737 Aircraft

A: Copy the aircraft texture folder for the livery (from the download) and paste the folder into the ProSim737-800-2018 Professional folder located in simobjects/airplanes.

B: Open the aircraft configuration file (for editing). This file is located in the main aircraft folder.  Make sure you back-up this file or copy it BEFORE making changes.  This will enable to you to revert to the original file if a mistake is made.

C: Copy the aircraft details from the downloaded 'read_me' file and add them to the configuration file.  The correct place to add the details is below the last aircraft listed.  If the ‘read_me’ file does not have this information, then it will be necessary to add the information yourself.

By far the easiest method to do this is to copy/paste the last aircraft listing, and then re-name the segments accordingly.  In the example above, I have bolded the sections that need to be edited.

The most important edits are the texture= ?, title= ? and ui_variation= ?. These three entries directly influence whether you will see the livery in the P3D aircraft list and in the game.  It’s very important that the texture= ? be the exact name of the texture file in the aircraft folder; your livery will not be able to be seen if this is not done.  In some instances, the name of the texture folder may be an airline’s name (texture.virgin) or a three letter aircraft code such as texture.ual (United Airlines).  

D: The FLTSIM number also needs to be edited to reflect the correct sequence order in the configuration file. Make sure each aircraft has a sequential number. If you have three aircraft liveries, the files will be [fltsim.01], [fltsim.02], [fltsim.03].  Be especially vigilant to copy all brackets, equal signs and commas (syntax) as these are necessary to see your aircraft in P3D.

Problems and Troubleshooting

By far the easiest way to troubleshoot a problem, such as the aircraft not being visible in the P3D aircraft folder, is to delete the aircraft configuration file and reinstall the original backed up file.  Then redo your work ensuring there are no mistakes.  If your mistakes relate to the actual texture folders, then delete the complete folder and reinstall the original backed up folder and start again.  Most problems relate to typo errors such as forgetting to include the correct syntax (punctuation marks).

Setting Up the P3D Aircraft Folder for Ease of Use (favourites and type)

When you open P3D to select an aircraft, a graphical user interface (GUI) screen displays  the aircraft and liveries that are installed to the aircraft folder. 

LEFT:  Screen capture showing the P3D aircraft selection folder.  Note the ‘show only favourites’ star, which when selected, will cause that livery to be displayed in the list at the expense of liveries not selected by the star.  Also, note the additional identifier in the vehicle type column (737-800 CARGO) - click to enlarge. 

This list can be long and unwieldy to navigate with the mouse, not to mention time consuming - you want to be able to identify your 738 liveries quickly and not wade through several versions of the aircraft you do not use.  To prune the number of aircraft you need to sort through, you can delete the unwanted aircraft from the aircraft folder, however, an easier method is to use the favourite functionality.

Select the favourite star for those aircraft/liveries you want to be see displayed in the aircraft list.   Once an aircraft /livery has been allocated as a favourite, it will always be displayed in the list, while those aircraft not ‘starred’ will not be displayed.  

If you have both cargo and passenger aircraft (or military versions of the B737), you may also want to segregate these aircraft by type.  This makes it easier to find a particular aircraft type.   This can easily be done by editing the title= ? and the ui_type= ? for that aircraft in the aircraft configuration file.  

BELOW:  An example in which the type has been edited to reflect the aircraft is a cargo aircraft (Aloha Air Cargo).  Editing the title is obvious as this changes the name in the P3D aircraft list.  However, editing the ui_type= ? enables you to change the aircraft type.  In the example below, I have included the word CARGO to differentiate cargo liveries from passenger liveries.  I have bolded the entries that need altering.

[fltsim.XX]
title=Prosim_AR_737_800_PRO_2018_Aloha_Air_Cargo
sim=Prosim738_Pro
model=
panel=
sound=
//sound=cockpit
texture=AAH
atc_heavy=0
atc_flight_number=211
atc_airline=Aloha
atc_model=737-800
atc_parking_types=GATE,RAMP
atc_parking_codes=AF
ui_manufacturer="Prosim_AR 2018"
ui_createdby="ProSim-AR"
ui_type="737-800 CARGO"
ui_variation="PROSIM_AR_Pro_2018_Aloha_Air_Cargo"
ui_typerole="Commercial Airliner"
atc_id=PS211
visual_damage=0

Livery List

Liveries for the Version 3 flight model can be downloaded from the ProSim-AR website:  Version 3 ProSim737 Liveries and Version 3 ProSim 737 Liveries.

Final Call

Adding various liveries can be fun and adds a element of realism, especially if you fly in different regions and enjoy looking at the aircraft, or are a videographer that creates flight simulator videos.   Paring down the aircraft list in P3D to display only the aircraft and liveries you want to see, and then segregating aircraft based on type, can save considerable time and mouse use.


ABOVE:  The livery for the JAL TRANSOCEAN Air – another viewpoint.  There is also a pink coloured livery.  Japan is one of my favourite regions to fly.  © Keishi Nukina - KN Aviation
Sunday
Jul152018

String Potentiometers - Are They Worthwhile

A flight simulator enables us to fly a virtual aircraft in an endless number of differing scenarios.  The accuracy of the flight controls when the aircraft is flown manually (hand flown) comes down to how well the aircraft’s flight controls are calibrated, and what type of potentiometer is being used to enable each control surface to be calibrated.

LEFT:  Custom-made box housing Bournes string potentiometer.  Note cable, dog lead clip, and JR Servo connection wires (click to enlarge).

This article will examine the most common potentiometers used.  It will also outline the advantages in using string potentiometers instead of inexpensive linear and rotary potentiometers.

What is a Potentiometer

A potentiometer (pot for short) is a small sized electronic component (variable resister) whose resistance can be adjusted manually, either by increasing or decreasing the amount of current flowing in a circuit.

The most important part of the potentiometer is the conductive/resistance layer that is attached (printed) on what is called the phenolic strip. This layer of material, often called a track, is usually made from carbon, but can be made from ceramic, conductive plastic, wire, or a composite material.  

The phenolic strip has two metal ends that connect with the three connectors on the potentiometer.  It’s these connectors that the wires from a control device are soldered to.  The strip has a wiper-style mechanism (called a slider) that slides along the surface of the track and connects with two of the potentiometer’s connectors. 

The strip enables the potentiometer to transport current into the circuit in accordance with the resistance as set by the position of the potentiometer on the phenolic strip. 

As the potentiometer moves from one position to another, the slider moves across the carbon layer printed to the phenolic strip.  The movement alters the current (electrical signal) which is read by the calibration software.

Types of Potentiometers (linear, rotary and string)

Potentiometers are used in a number of industries including manufacturing, robotics, aerospace and medical.  Basically, a potentiometer is used whenever the movement of a part needs to be accurately calibrated. 

LEFT:  Inexpensive rotary potentiometer.  This pot used to control the ailerons.  The pot was inserted into the base of the control column and held in place by a fabricated braket (click to enlarge).

For the most part, flight simulators use adjustable type potentiometers which, broadly speaking, are either linear or rotational potentiometers.  Both do exactly the same thing, however, they are constructed differently.  Another type of rotary potentiometer is the string potentiometer.

A linear potentiometer (often called a slider) measures changes in variance along the track in a straight line (linear) as the potentiometer's slider moves either in a left or right direction.  A linear potentiometer is more suitable in areas where there is space available to install the potentiometer. 

A rotary potentiometer uses a rotary motion to move the slider around a track that is almost circular. Because the potentiometer's track is circular, the size of a rotary potentiometer can be quite small and does not require a lot of space to install.

Potentiometer Accuracy

The ability of the potentiometer to accurately read the position of the slider as it moves along the track is vital if the attached control device is to perform in an accurate and repeatable way. 

LEFT: A very inexpensive linear potentiometer ($3.00).  The tracks on this pot are made from carbon and the body is open to dust and grime.  They work quite well, but expect their life to be limited once they begin to get dirty (click to enlarge).

The performance, accuracy, and how long that accuracy is maintained, is governed by the internal construction of the potentiometer; in particular the material used for the track (carbon, cermet, composite, etc).  Of particular importance, is the coarseness of the signal and the noise generated (electrical interference). when the potentiometer has power running through it.

For example, cermet which is composite of metal and plastic produces a very clean low noise signal, where as carbon often exhibits higher noise characteristics and can generate a course output.  It’s the coarseness of the signal that makes a control device easy or difficult to calibrate.  It also defines how accurately the potentiometer will read any small movement.

Potentiometers that use carbon form the mainstay of the less expensive types, such as those used in the gaming industry, while higher-end applications that requite more exacting accuracy use cermet or other materials. 

Essentially, higher end potentiometers generate less noise and produce a cleaner output that is less course.  This translates to more accurate calibration.  This is seen when you trim the aircraft. 

A quality mid to high-end potentiometer, when calibrated correctly, will enable you to easily trim the aircraft, insofar that the trim conditions can be replicated time and time again (assuming the same flight conditions, aircraft weight, engine output, etc).

Simulators, Dust, Grime and Other Foreign Bodies

Flight simulators to control a number of moving parts, generally use a combination of linear and rotary potentiometers.  For example, a rotary potentiometer may be used to control the flight controls (ailerons, elevator and rudder) while a linear potentiometer may be used to control the movement of the flaps lever, speedbrake and steering tiller. 

Any component that has a current running through it will attract dust, and the location of the potentiometer will often determine how much dust is attracted to the unit.  A potentiometer positioned beneath a platform is likely to attract more dust than one located behind the MIP or enclosed in the throttle quadrant.

A rotary potentiometer is an enclosed unit;  it is impervious to dust, grime and whatever else lurks beneath a flight simulator platform.  In comparison, a linear potentiometer is open to the environment and its carbon track can easily be contaminated.  Once the track has become contaminated, the potentiometer will become difficult to calibrate, and its output will become inaccurate.

Sometime ago, I had a linear potentiometer that was difficult to calibrate, and when calibrated produced spurious outputs.  The potentiometer was positioned beneath the platform adjacent to the rod that links the two control columns.  When I removed the potentiometer, I discovered part of the body of a dead cockroach on the carbon track. 

This is not to say that linear potentiometers do not have a place – they do.  But, if they are to be used in a dusty environment, they must have some type of cover fitted.  A cover will minimise the chance of dust adhering to the potentiometer’s track. 

I use linear potentiometers mounted to the inside of the throttle quadrant to control the flaps and speedbrake.  The two potentiometers are mounted vertically on the quadrant’s sidewall.  This area is relatively clean, and the vertical position of the mounted potentiometers is not conducive to dust accumulation.

Ease of Installation

Both linear and rotary potentiometers are straightforward to install, however, they must be installed relatively close to the item they control.  Often a lever or connecting rod must be fabricated to enable the potentiometer to be connected with the control device.

String Potentiometers (strings)

A string potentiometer (also called a string position transducer) is a rotary potentiometer that has a stainless steel cable connected to a spring-loaded spool. 

LEFT:  Cross section diagram showing internals of string potentiometer (click to enlarge).

The string potentiometer is mounted to a fixed surface and the cable attached to a moveable part (such as a control device).  As the control device moves, the potentiometer produces an electrical signal (by the slider moving across the track) that is proportional to the cable’s extension or velocity.  This signal is then read by the calibration software. 

The advantages of using string potentiometers over a standard-issue rotary potentiometer are many:

(i)        Quick and easy installation;

(ii)       Greater accuracy as you are measuring the linear pull along a cable;

(iii)      Greater flexibility in mounting and positioning relative to the control device;

(iv)      No dust problems as the potentiometer is enclosed;

(v)       No fabrication is needed to connect the potentiometer to the control device (only cable and dog clip) and,

(vi)       Greater time span before calibration is required (compared to a linear potentiometer).

The importance of point (iii) cannot be underestimated.  The string can be extended from the potentiometer within a arc of roughly 60-70 degrees, meaning that the unit can be mounted more or less anywhere.  The only proviso is that the cable must have unimpeded movement. 

Attachment of the string to the control device can be by whatever method you choose.  I have used a small dog lead clip.  As the potentiometer is completely enclosed dust is not an issue, which is a clear advantage in that once the potentiometer calibrated, the calibration does not alter (as dust does not settle on the track).

I have used string Potentiometers to calibrate the axis on the ailerons, elevators and rudder (one potentiometer per item).  I have also used a dual-string potentiometer in the throttle quadrant to calibrate the two thrust levers.

Fabricate Your Own String Potentiometer

Whilst you can purchase ready-made string potentiometers, their cost is not inexpensive.  As a trial, a friend and I decided to fabricate our own string potentiometers.

LEFT:  String potentiometer.  This pot connects to the ailerons.  The stainless cable can be seen leaving the casing that connects with the aileron controls.  An advantage of string pots is that they can installed more or less anywhere, as long as there is unimpeded access for the cable to move (click to enlarge).

The potentiometers used are manufactured by Bourns (3590S series precision potentiometer).  These units are a sealed, wire-wound potentiometer with a stainless steel shaft.  According to the Bourns specification sheet these potentiometers have a tolerance +-5%. 

LEFT:  Diagram showing spring-loaded spool (click to enlarge).

The potentiometer is mounted in a custom-made acrylic box in which a hole the size of the potentiometer's end, has been drilled into the lid.   Similar boxes can be purchased in pre-cut sizes, but making your own custom-sized box enables the potentiometer to be mounted inside the box in a position most advantageous to your set-up. It also enables you to place the mounting holes on the box in strategic positions.

Another small hole has been drilled in the side of the box to enable the stainless steel cable to move freely (see image at beginning of article).  If you want to allow the cable to move through an arc, this hole must be elongated to enable the cable to extend at an angle and move unimpeded. 

The cable (string) is part of a self-ratcheting spool (also called a retractor clip) which is screwed to the inside of the box and connected directly with the stainless steel shaft of the potentiometer.  The cable when attached to a solid point is kept taught by the tension of the self-ratchet spool (an internal spring controls the tension).   Ratchet spools are easily obtainable and come in many sizes and tensions.   Three standard JR servo wires connect the potentiometer to a Leo Bodnar BU0836A 12 bit Joystick Controller card.  A mini dog lead clip is used at attach the cable to the control device.

One of the major advantages when using string potentiometers is that the actual potentiometer does not have to be mounted adjacent to, or even close to the device it controls.  The line of pull on the cable can be anything within roughly a 70 degree arc. 

Additional Information

This website (no affiliation) has an excellent overview on potentiometers.  The video is very interesting.

http://www.resistorguide.com/potentiometer/

Final Call

Previously, I used inexpensive linear and rotary potentiometers to control the main flight controls.  I was continually plagued with calibration issues, and when calibrated the calibration was not maintained for more than few months.  Furthermore, manual flight was problematic as the output from each of the  (cheap) potentiometers was course, which translated to less accuracy when using the ailerons and elevators.  Trimming the aircraft in any condition other than level flight was difficult.

Without doubt, the use of quality string potentiometers have resolved all the earlier calibration and accuracy issues I had been experiencing.  With the replacement potentiometers, the aircraft is easily hand-flown and can be trimmed more accurately.

Perhaps in the future I will ‘up the anti’ and purchase two commercial high-end string potentiometers and dedicate them to the ailerons and elevator, but for the time being the Bourne potentiometers suit my requirements.

Saturday
Mar032018

Wind Correction (WIND CORR) Function - CDU

Wind Correction (WIND CORR)

The approach page in the CDU has a field named WIN CORR (Wind Correction Field or WCF).  Using WIND CORR, a flight crew can alter the Vref+ speed (additive) that is used by the autothrottle to take into account headwinds greater than 5 knots. 

LEFT:  OEM CDU showing WIND CORR display in Approach Ref page (click to enlarge).

The default reading is +5 knots.   Any change will alter how the FMC calculates the command speed that the autothrottle uses.  Any change is reflected in the LEGS page. 

It's important to update the WIND CORR field if VNAV is used for the approach or of executing an RNAV Approach, as VNAV uses data from the Flight Management System to fly the approach.   However, if hand flying the aircraft, or executing an ILS Approach, it's often easier to add the Vref additive to the speed window in the MCP.  Indeed, flight crews for the most part, other than when using VNAV, leave the WIND CORR as its default (+5 knots), and change airspeed by using the MCP or by using Speed Intervention (SPD INTV).

WIND CORR Explained

The ability to increase the Vref speed is very handy if a flight crew wishes to increase the safety margin the autothrottle algorithm operates.

Boeing when they designed the autothrottle algorithm programmed a speed additive that the A/T automatically adds to Vref when the A/T is engaged.  The reason for adding this speed is to provide a safety buffer to ensure that the A/T does not command a speed equal to or lower than Vref.   (recall that wind gusts can cause the autothrottle to spool up or down depending upon the gust strength). 

A Vref+ speed higher than +5 can be inputted when gusty or headwind conditions are above what are considered normal.  By increasing the +speed, the  speed commanded by the autothrottle will not degrade to a speed lower than that inputted.

How To Use WIND CORR

WIND CORR is straightforward to use.   

Navigate to the approach page in the CDU (press INIT REF key to open the Approach Reference page).  Then double press the key adjacent to the required flaps for approach (for example, flaps 30).  Double selecting the key causes the flap/speed setting to be automatically populated to the FLAP/SPD line. 

It’s important to understand that this is the Vref.  This calculation ALREADY has the +5 additive added; this is the speed that the aircraft should be at when crossing the runway threshold.  

LEFT:  Virtual CDU (ProSim-AR) showing the difference in Vref between a +5 and +13 Knot Wind Correction change.  Vref altered from 152 knots to 160 knots (click to enlarge).

If the headwind is greater than 5 knots, then WIND CORR can be used to increase the additive from the default +5 knots to anything up to but not exceeding 20 knots. 

Type the desired additive into the scratch pad of the CDU and up-select to the WIND CORR line.  The revised speed will change the original Vref speed and take the headwind component into account.  If you navigate to the LEGS page in the CDU, you will observe the change. 

Note that the Vref speed displayed on the Primary Flight Display (PFD) does not change.  This remains at Vref +5.

For a full review on how to calculate wind speed, review this article: Crosswind landing Techniques - Calculations, or read the cheat sheet below.         

LEFT:  Wind calculation cheat sheet (click to enlarge).

Important Varibles - Aircraft Weight

To obtain the most accurate Vref for landing, the weight of the aircraft must be known minus the fuel that has been consumed during the flight.

Fortunately, the Flight Management System updates this information in real-time and provides access to the information in the CDU.  It's important that if an approach is lengthy (time consuming) and/or involves holds, the Vref data shown will not be up-to-date (assuming you calculated this at time of descent); the FLAPS/Vref display will show a different speed to that displayed in the FLAP/SPD display.  To update this data, double press the key adjacent to the flaps/speed required and the information will update to the new speed.

Interestingly, the difference that fuel burn and aircraft weight can play in the final Vref speed is quite substantial (assuming all variables, except fuel, are equal).  To demonstrate:

  • Aircraft weight at 74.5 tonnes with fuel tanks 100% full – flaps/Vref 30/158.
  • Aircraft weight at 60.0 tonnes with fuel tanks 25% full   – flaps/Vref 30/142.

 Important Points:

  • During the approach, V speeds are important to maintain.  A commanded speed that is below optimal can be dangerous, especially if the crew needs to conduct a go-around, or if winds suddenly increase or decrease.  An increase or decrease in wind can cause pitch coupling.
  • If executing an RNAV Approach, it's important to update the WIND CORR field to the correct headwind speed based on conditions.  This is because VNAV uses the data from the Flight Management System (FMS).
  • If an approach is lengthy, the Vref speed will need to be updated to take into account the fuel used in the aircraft.  

Autoland

Autolands are rarely done in the Boeing 737, however, if executing an autoland, the WIND CORR field is left as +5 knots (default).  The autoland and autothrottle logic will command the correct approach and landing speed.

Functionality

WIND CORR may or may not be functional in the avionics software you use.  It is 100% functional in the ProSim-AR 737 avionics suite (Version 2).

Acronyms

CDU – Control Display Unit
FMC – Flight Management Computer
FMS – Flight Management System (comprising the FMC and CDU)
Vref - The final approach speed is based on the reference landing speed
Vapp – Vapp is your approach speed, and is adjusted for any wind component you might have. You drop from Vapp to Vref usually by just going idle at a certain point in the flare