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Welcome

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).

 

All funds are used to offset the cost of server and website hosting (Thank You...)

No advertising on this website - EVER!

 

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If you see any errors or omissions, please contact me to correct the information. 

Journal Archive (Newest First)

Entries in FMC (9)

Monday
May282018

FMC Software and its Relationship with LNAV and VNAV

The procedure to takeoff in a Boeing 737 is a relatively straightforward process, however, the use of automation, in particular pitch and roll modes (Lateral and Vertical Navigation), when to engage it, and what to expect once it has been selected, can befuddle new flyers.  

In this article I will explain some of the differences between versions of software used in the Flight Management Computer (FMC) and how they relate to when you engage  LNAV and VNAV.  I will also discuss the autopilot and Auto Flight Direction System (AFDS) and explain the Flaps Retraction Schedule (FRS).

It’s assumed the reader has a relatively good understanding of the use of LNAV and VNAV, how to engage this functionality, and how they can be used together or independently of each other.

FMC Software Versions

There are a several versions of software used in the FMC; which version is installed is dependent upon the airline, and it’s not unusual for airframes to have different versions of software.

LEFT:  A mundane photograph of the CDU page displaying the U version of software used by the Flight Management Computer.  The page also displays the current NavData version installed in addition to other information (click to enlarge).

The nomenclature for the FMC software is a letter U followed by the version number.  The version of software dictates, amongst other things, the level of automation available.  For the most part, 737 Next Generation airframes will be installed with version U10.6, U10.7 or later.

Boeing released U1 in 1984 and the latest version, used in the 737 Max is U13.

Later versions of FMC software enable greater functionality and a higher level of automation – especially in relation to LNAV and VNAV.

Differences in Simulation Software

The FMC software used by the main avionics suites (Sim Avionics, Project Magenta, PMDG and ProSim-AR) should be identical in functionality if they simulate the same FMC U number.  

As at 2018, ProSim-AR uses U10.8A and Sim Avionics use a hybrid of U10.8, which is primarily U10.8 with some other features taken from U11 and U12.  Precision Manuals Development Group (PMDG) uses U10.8A.  

Therefore, as ProSim-AR and PMDG both use U10.8A, it’s fair to say that everything functional in PMDG should also be operational in ProSim737.  Unfortunately, as of writing, PMDG is the only software that replicates U10.8A with 97+-% success rate.

To check which version is being used by the FMC, press INIT REF/INDEX/IDENT in the CDU.  

Writing about the differences between FMC U version can become confusing.   Therefore, to minimise misunderstanding and increase readability, I have set out the information for VNAV and LNAV using the FMC U number.   

Roll Mode (LNAV)

U10.6 and earlier

(i)    LNAV will not engage below 400 AGL;

(ii)    LNAV cannot be armed prior to takeoff; and,

(iii)    LNAV should only be engaged  when climb is stabilised, but after passing through 400 feet AGL.

U10.7 and later

(i)    If LNAV is selected or armed prior to takeoff, LNAV guidance will become active at 50 feet AGL as long as the active leg in the FMC is within 3 NM and 5 degrees of the runway heading.  

(i)    If the departure procedure or route does not begin at the end of the runway, it’s recommended to use HDG SEL (when above 400 feet AGL) to intercept the desired track for LNAV capture;

(ii)    When an immediate turn after takeoff is necessary, the desired heading should be preset in the MCP prior to takeoff;  and,

(iii)    If the departure procedure is not part of the active flight plan, HDG SEL or VOR LOC should be used until the aircraft is within range of the flight plan track (see (i) above).

Important Point:

•    LNAV (U10.7 and later) can only be armed if the FMC has an active flight plan.

Pitch Mode (VNAV)

U10.7 and earlier

(i)    At Acceleration Height (AH), lower the aircraft’s nose to increase airspeed to flaps UP manoeuvre speed;

(ii)    At Thrust Reduction Altitude (800 - 1500 feet), select or verify that the climb thrust has been set (usually V2+15 or V2+20);

(iii)    Retract flaps as per the Flaps Retraction Schedule (FRS); and,

(iv)    Select VNAV or climb speed in the MCP speed window only after flaps and slats have been retracted.

Important Points:

•    VNAV cannot be armed prior to takeoff.

•    Remember that prior to selecting VNAV, flaps should be retracted, because VNAV does not provide overspeed protection for the leading edge devices when using U10.7 or earlier.

U10.8 and later 

(i)    VNAV can be engaged at anytime because VNAV in U10.8 provides overspeed protection for the leading edge devices;

(ii)    If VNAV is armed prior to takeoff, the Auto Flight Direction System (AFDS) remains in VNAV when the autopilot is engaged.  However, if another pitch mode is selected, the AFDS will remain in that mode;

(iii)    When VNAV is armed prior to takeoff, it will engage automatically at 400 feet.  With VNAV engaged, acceleration and climb out speed is computed by the FMC software and controlled by the AFDS; and,

(iv)    The Flaps should be retracted as per the flaps retraction schedule;

(v)    If VNAV is not armed prior to takeoff, at Acceleration Height set the command speed to the flaps UP manoeuvre speed; and,

(vi)    If VNAV is not armed prior to takeoff, at Acceleration Height set the command speed to the flaps UP manoeuvre speed.

Important Points:

•    VNAV can be armed prior to takeoff or at anytime.

•    At thrust reduction altitude, verify that climb thrust is set at the point selected on the takeoff reference page in the CDU.  If the thrust reference does not change automatically, climb thrust should be manually selected.

•    Although the VNAV profile and acceleration schedule is compatible with most planned departures, it’s prudent to cross check the EICAS display to ensure the display changes from takeoff (TO) to climb or reduced climb (R-CLB).  

Auto Flight Direction System (AFDS) – Operation During Takeoff and Climb

U10.7 and earlier

If the autopilot is engaged prior to the selection of VNAV:

(i)    The AFDS will revert to LVL CHG;

(ii)    The pitch mode displayed on the Flight Mode Annunciator (FMA) will change from TOGA to MCP SPD; and,

(iii)    If a pitch mode other than TOGA is selected after the autopilot is engaged, the AFDS will remain in that mode.

U10.8 and later

(i)    If VAV is armed for takeoff, the AFDS remains in VNAV when the autopilot is engaged; and,   

(ii)    If a pitch mode other than VNAV is selected, the AFDS will remain in that mode.

Preparing for Failure

LNAV and VNAV have their shortcomings, both in the real and simulated environments.

To help counteract any failure, it’s good airmanship to set the heading mode (HDG) on the MCP to indicate the bearing that the aircraft will be flying.  Doing this ensures that, should LNAV fail, the HDG button can be quickly engaged with minimal time delay; thereby, minimising any deviation from the aircraft’s course.

Autopilot Use, Flap Retraction and Eliminating Unwanted Pitch

When the aircraft is in manual flight (hand flying), the trim setting should be set correctly so that forward and back pressure on the control column is not required.  If the autopilot is engaged when the trim is not correct, the aircraft will suffer unwanted pitch movement as the automated system corrects the out of trim condition.

Adhering to the following recommendations will reduce the likelihood of unwanted or unexpected deviations from the desired flight path.

(i)    The autopilot should not be engaged before passing through 400 feet AGL;

(ii)    The flaps should not be retracted before passing through 400 feet AGL; and,

(iii)   The autopilot should not be engaged before flap retraction is complete, and then only engaged when the aircraft is in trim.  

If this procedure is adhered to, the transition from manual to automated flight will be barely discernible.  

Regarding point (iii).  I have used the word ‘should’ as this is generally a preferred option, however, airline policy may dictate otherwise.  

Flap Retraction Schedule (FRS)

The flaps on the Boeing 737 should be retracted per a defined schedule.  Failure to follow the FRS may cause excessive throttle use and possible flight path deviation.

LEFT:  Flap Retraction Schedule from FCOM (click to enlarge).  Copright FCOM.

Selection of the next flap position should be initiated when reaching the manoeuvre speed for the current flap position. 

Therefore, when the new flap position is selected, the airspeed will be below the manoeuvre speed for that flap position.  For this reason, when retracting the flaps to the next position, the airspeed of the aircraft should be increasing.

Said slightly differently, with airspeed increasing, subsequent flap retraction should be initiated when the airspeed reaches the manoeuvre speed for the current flap position.  

The manoeuvre speed for the current flap position is indicated by the green-coloured flap manoeuvre speed bug.  The bug is displayed on the speed tape of the Primary Flight Display (PFD) and is in increments that replicate the flap settings (UP, 1, 2, 5, etc).

Acceleration Height

Flap retraction commences when the aircraft reaches Acceleration Height, which is usually between 1000 and 1500 feet AGL (this is when the nose of the aircraft is lowered to gain airspeed).

However, often there are constraints that affect the height at which flap retraction commences.  Determining factors are: safety, obstacle clearance, airplane performance, and noise abatement requirements.  At some airports, airlines have a standard climb profile that should be followed for their area of operations

White Carrot

Located on the speed tape of the PFD is a white-coloured marker called a carrot (the carrot looks more like a sideways facing arrow).  The position of the carrot indicates V2+15.

LEFT:  Captain-side PFD showing white carrot.  Image from ProSim-AR 737 avionics suite (click to enlarge).

If you look at the Flap Retraction Schedule in the FCTM (see above image from FCOM), you will note the airspeed that is recommended to begin retracting flaps is V2+15 (the position of the carrot).  The white carrot is a very handy reference reminder.

Important Points:

•    The minimum altitude for flap retraction is 400 feet AGL.  

•    Selection of the next flap position should be initiated when reaching the manoeuvre speed for the current flap position.

•    Airspeed should be increasing when retracting the flaps.

•    The white carrot is a handy reference to V2+15.

•    Acceleration Height can differ between airports.

Summary

I realise that some readers, who only wish to learn the most recent software, will not be interested in much of the content of this article.  Notwithstanding this, I am sure many will have discovered something that may have been forgotten or overlooked.

The content of this short ‘function specific’ article came out of a discussion on a pilot’s forum.  If there is doubt, always consult the Flight Crew Training Manual (FCTM) which provides information specific to the software version used at that particular airline.

Glossary

AFDS – Autopilot Flight Director System
CDU – Computer Display Unit
EFIS – Electronic Flight Instrument System
FMA – Flight Mode Annunciator
FMC – Flight Management Computer
LVL CHG – Level Change
LNAV – Lateral Navigation
MCP – Mode Control Panel
ND – Navigation Display
PFD – Primary Flight Display
 VNAV – Vertical Navigation

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

Wednesday
Mar082017

OEM B737 CDU Conversion - Introduction

One of the slower projects is the conversion of two B737 CDU units.  The two units were purchased from an aircraft scrap-yard in the US and were formally used in a Boeing 737 operated by United Airlines.  

LEFT:  Straight from United Airlines to me.  Two OEM CDU units.  The rear unit has already had its CRT display removed and is partially  'gutted' (click to enlarge).

The two CDUs came from an airframe of a B737-500, which in 2008 was retired along with other Boeing classics, due to United Airlines decision to adopt the Airbus A-320.

The rear of each unit has a chronometer showing the hours of use - one unit has 5130 hours while the other has 1630 hours.

The CDU presently used in the simulator is manufactured by Flight Deck Solutions (FDS) and although I have been pleased with its operation and reliability, there is little resemblance, other than appearance, to the OEM unit.

LEFT:  Detail of the keyboard and DIM knob.  Interestingly the DIM knob dims the actual screen and not the backlighting (click to enlarge).

The prominent difference is external build quality and the tactile feeling when depressing the keys on the keyboard; the keys don't wobble in their sockets, but are firm to press. 

There is also a strong audible click when a key is depressed.  Furthermore, the backlighting is evenly spread with each key evenly lit.

The OEM CDU is large and VERY heavy.  I was surprised at the weight - a good 6 kilograms.  Most of the weight is made up by the thick glass CRT display screen and other components that reside within the sturdy aluminium case.

LEFT:  The casing removed to show the electronic boards that are secured by lever clips.  Like anything OEM, the unit is made very well from solid components (click to enlarge).

Like the casing, the internal structure is also made from aluminium and has four rails to enable the electronic boards to be installed and secured into place. 

Whenever I look at anything OEM, I am amazed at the workmanship that has gone into producing the item; the CDU does not fall short in this area.

A myriad number of small screws hold together the aluminum casing that protects the internal components.  Not only screws are used, but also special miniature DZUS fasteners than enable the side of the casing to removed easily for maintenance.

Nomenclature

When discussing the CDU there are three similar terms that are often used interchangeably: CDU, FMC and FMS.  In this website, I use the terms CDU and FMC interchangeable which is not quite correct - let me explain.

LEFT:  Protective cover removed to show the main pin-out board, rear of the CRT display, power supply, and electronics.  These parts cause the CDU to be quite heavy.  The two Canon plugs  are just visible at the right of the picture enable connection to the aircraft. (click to enlarge to see detail).

The Control Display Unit (CDU) is the interface that the flight crew use to interrogate the data from the Flight Management Computer (FMC); it's basically a screen and keyboard.  The FMC in turn is but one part of a complex system called the Flight Management System (FMS).  The FMS is capable of four dimensional area navigation.  It is the FMS that contains the navigational database.

CDU and MCDU - Nomenclature

The acronyms CDU and MCDU are often used interchangeably.  The older units used in the classic airframes (up to 500 series) are referred to as a CDU, while the Next Generation airframe (600-900 series) units are called a MCDU.  M stands for multipurpose or multi-function.  Basically, the MCDU has a different key called a menu key.  This key, when pressed, accesses another layer of information that is not available in the earlier CDUs.

For those more military-minded, the CDU in military parlance is called a mission computer.

Aesthetic Differences

The CDU dates from 2008, therefore; it is not exactly identical to the CDU used in the Next Generation airframe, however, it is very close.

Main Differences Between the 500 Series and Next Generation CDUs

(i)    The dim knob is a slightly different shape;
(ii)   The display screen is rounded at the edges (the NG is more straight-edged);
(iii)   The absence of the horizontal white lines located on the inside edge of the display frame bezel;
(iv)   The display screen is different - cathode ray tube (CRT) verses liquid crystal display (LCD);
(v)   Two of the keys are different.  The NG has a menu and space key whilst the older CDUs have a DIR INTC and a blank key (no lettering on key); and,
(vi)  The fonts and colors between units differ.  Earlier units a monochromatic or green in colour while later units are in multicolour.

Fonts and colours are not important in the simulator environment as the avionic suite controls the displasy output.

To a purist, these differences are probably important, and if so, you will have to contend with a reproduction MCDU or pay an exorbitant amount for an NG unit. 

Software

The software used in the OEM CDU is not used in the simulator.  The CDU functionality is dictated by the avionics software (ProSim-AR) in use.  This is also true for the font type and colour.

LEFT:  Completely gutted.  All unnecessary and unusable electronic components have been removed.  These two CDU units will soon operate flawlessly with ProSim-AR and flight simulator (click to enlarge).

Converting the CDU

I am liaising with an Australian company that specialises in converting avionics components used in commercial flight simulators.  This company has had considerable experience converting B747 avionics and is keen to see if their expertise will similarly work with the B737.

In a second article, I will explain in more detail how the conversion was done, and examine some of the problems that needed to be resolved.  I also will discuss the mounting of the unit into the CDU bay. 

More photographs of the CDU are located in the image gallery.  Additional images will be added to the gallery in due course.

Glossary

OEM - Original Equipment Manufacture (aka reral aircraft part).

CDU - Control Display Unit.

MCDU - Multipurpose/multifunction Control Display Unit.

Thursday
Aug272015

10 Mile ARC to VOR 30 Approach - Hobart, Tasmania Australia (YMHB)

Recently, I flew from Brisbane to Hobart and the pilot flying made a different style of approach to what normally is made at this airport.  After landing, I approached the pilots and queried the approach.  The Captain stated that he had decided to fly a semi-automated VOR approach along an arc to land at runway 30. 

The reason being, that Air Traffic Control (ATC) had warned them of turbulent conditions near the airport.  He commented that in such conditions, he felt more confident using the older style arc approach using LNAV/VNAV with Speed Intervention (SPD INTV) engaged, with a transition to Vertical Speed and VOR once on final.

LEFT: Approach chart depicting VOR 30 Approach to YMHB.  Important points to note are: initial approach courses to intercept the arc (295 & 334), the D10 HB arc, the altitude increments of 4000, 3000 and at 7 miles, 2400, and the Initial Approach Fix (IAF) and speed of 210 kias (click to enlarge).

The First Officer stated that this was the first time he had seen an arc being used to set-up for a VOR approach.  He said that usually they use ILS into RWY 12 or RNAV into RWY 30.  He commented that the only time he had made a VOR approach was during simulator training, and then he would probably only use such an approach, if the ILS was inoperative or there was an issue with RNAV.

The use of this approach is a prime example of the variation offered to pilots in relation to how they fly and land the Boeing 737. 

Screen Images

Several screen captures from the Instructor Station, CDU and Navigation Display (ND) which I hope will make it easier to understand this post.  The avionics suite used is ProSim737 distributed by ProSim-AR.  Note that some of the mages are not sequential as I captured the images over two simulator sessions.

How To Set-Up An Arc

To set-up an approach using an arc is very easy.  

The following example is for Hobart, Tasmania Australia (YMHB).  The instructions assume that you are conversant with operating the CDU and have a basic understanding of its use.  

Essentially, an arc is using a Place/Bearing/Waypoint to define an arc around a point at a set distance.  The distance between each of the generated waypoints along the arc, is at the discretion of the flight crew.

Approach Charts

To determine the correct distance to create the arc, the approach chart for the airport should be consulted.  The chart, in addition to providing this information, will also aid you in decided where to place the final waypoint (if wanted) along the approach course.

In this example, the YMHB VOR 30 approach states that the aircraft must fly an arc 10 miles from the airport between an altitude of 4000 and 3000 feet before descending to be at 2400 feet 7 miles from the runway  threshold.

The approach chart depicted is provided by Lufthansa Systems (LIDO/FMS) distributed by Navigraph

CDU Instructions

(i)    Open the FIX page and type in the scratchpad the airport code (YMHB).  After uploading, type the distance (/10 miles).  This will create a green-dotted citcle around YMHB with a radius of 10 miles.

(ii)    Open the LEGS page and type into the scratchpad the airport code (YMHB).  Immediately following YMHB, type the required radial1 (in degrees) from the airport that you wish the initial waypoint to be generated.  Follow this with a slash and type in the distance from the airport (YMHB340/10).  

This will generate a waypoint 10 miles from YMHB on the 340 radial.  This is the waypoint from which you will begin to build your arc.  

Obviously, the radial you use to define the location of your first waypoint will depend upon the bearing that you are flying toward the airport.

(iii)    To Generate the ARC you must repeat the above process (ii) changing the radial by 10 degrees (or whatever you believe is needed) to generate the required waypoints around the arc at 10 miles from the airport.  As an example: YMHB320/10, YMHB340/10, YMHB000/10 and so forth until the arc is built.

As you upload each of the radials you will note that the name for the waypoint is changed to a sequential number specific to each waypoint.  As an example; the above waypoints will each be named YMH01, YMH02 and YMH03.

If you make a mistake, you can delete a waypoint and start again; however, realize that the sequential numbers will not be in order.  This is not an issue (it is only a number) but it is something be aware of.

In our example, the VOR approach is for runway 30.  Therefore; your final waypoint on the arc will be YMHB121/10.  Prior to reaching this waypoint, if flying manually, begin the right hand turn to intercept the approach on the 121 radial (bearing 300 degrees).

A Note About /-+

The more observant will note that the distances in the example above do not utilise the /+ key before the distance (YMHB340/+10).  When entering the distance it can be with or without the + key.  

Variation

Before going further, there are many ways to fly the B737.  The method selected is at the discretion of the pilot in command and is dependent upon airline preferences, environmental conditions, and pilot experience.  This statement was stressed to me when I spoke with the Captain of the aircraft.

Often an approach will incorporate a number of automated systems including VNAV, LNAV, Vertical Speed, Level Change, VOR Localizer and old fashioned manual VFR flying.  In most cases the particular approach will be programmed into the CDU, at the very least for situational awareness.  However, the CDU does not have to be used and often a step down approach is a good way to maintain flying skills and airmanship.

Handy Hints

The following hints will assist with situational awareness and in allowing the aircraft to be guided by the autopilot to a point to which manual flight can commence.

If you carefully study the approach chart for YMHB VOR 30, you will note that the altitude the aircraft should be at when at 7 miles from the threshold should be 2000 feet.  The chart also depicts the letter D at this point meaning that a continuous descent can be made this point.

Hint One - visual descent point (VDP)

To make the transition from the arc to the approach easier, create a waypoint at the 7 mile point from the airport along the radial used for the approach (YMHB121/7).  Using a waypoint allows the aircraft’s Lateral Navigation (LNAV) to be used.  This type of waypoint is usually refered to as a Visual Descent Point (VDP).

When the waypoint at 7 miles from the threshold is reached, a transition to manual flying can commence, or Vertical Speed can be used to maintain a 3 degree glidepath (GP) while following the VOR.  Remember to change the EFIS from MAP to VOR so you can use the VOR indicator during the approach.

Hint Two - extend runway line

Assuming you have not inserted an approach into the CDU, an aid to increase situational awareness is to select the correct runway from the CDU and enter a distance that the runway line is to be extended from the threshold.

To do this, select runway 30 from the ARRIVALS (ARR) page in the CDU (RWY30) and type the numeral 7 (or whatever distance you require) into the scratchpad and upload.  This will extend the green line from the runway threshold to the previously generated waypoint at 7 miles.  Ensure you clean up any discontinuity (if observed) in the LEGS page.

This enables three things:

  1. The generation of a 3 degree glidepath (GP) from the distance entered (example is 7 miles) to the runway threshold.
  2. It enables LNAV (even if the autopilot is not engaged) to continue to provide the Flight Director (FD) with heading information during the approach, and 
  3. It enables the Navigation Performance Scales (NPL) on the Pilots Flight Display (PFD) to provide glidepath (GP) guidance (assuming that the correct runway or approach is selected in the CDU and NPL is enabled within the ProSim737 avionics suite).

UPPER LEFT: Screen capture from the instructor station PFD and ND for the approach into YMHB.  The aircraft, after turning right from the 10 mile arc, is aligned with the 121 radial approaching the waypoint YMH07 (the WP entered at the 7 mile point).  LNAV is engaged and the aircraft is being controlled by the autopilot.  As RWY 30 was inserted into the route, the Navigation Performance Scales (NPS) show Glidepath (GP) data in the Primary Flight Display (PFD).  Note that the EFIS is still on MAP and is yet to be turned to VOR.  In real life, VOR would have been selected earlier (click to enlarge).

LOWER LEFT:  The transition from LNAV to VOR has occurred and the autopilot and autothrottle are not controlling the aircraft. The aircraft is on short final with gear down, flaps 30 and the airspeed is slowly decaying to VREF+5.  The EFIS has been changed from MAP to VOR to allow manual tracking using the VOR needle. The NPS show good vertical alignment with a lateral left offset; the VOR indicator confirms this.  The Flight Mode Annunciator (FMA) displays LNAV (although the autopilot is disengaged) and the Flight Director (FD) and NPS show glidepath (GP) data.  The Flight Path Vector (FPV) symbol shows a continuous descent at roughly 3 degrees.  The altitude window and heading on the MCP has been set to the appropriate missed approach (4200/300).  Click image to enlarge.

Do Not Alter Constraints

As alluded earlier, there are many ways to accomplish the same task.  However, DO NOT alter any constraints indicated in the CDU if an approach is selected and executed.  CDU generated approaches have been standardised for a reason.

Finding the Correct Radial/Bearing to Build Your Arc

Finding the correct bearing to use on the arc can be challenging for those less mathematically inclined.  An easy method is to use one of the two MCP course selector knobs.  

Rotate the knob until the green dotted line on he Navigation Display (ND) lies over the area of the arc that you wish the waypoint to be created.  Consult the MCP course selector window - this is the figure you place in the CDU.  Next, rotate the knob a set number of degrees and repeat the process.  You can also consult the data displayed along the course indicator line on the Navigation Display (ND). 

When you build the arc, ensure you have set the EFIS to PLN (plan).  PLN provides more real estate to visualize the approach on the Navigation Display (ND).  You can use STEP in the LEGS page to cycle through the waypoints to ensure you have an appropriate view of the surrounding area.

Important Points

  • Always double check the Place/Bearing/Waypoint entries in the CDU and in the ND (PLN) before executing.  It is amazing how easy discrepancies can occur.
  • Always check the approach plate for the approach type you are intending to make.  Once again, mistakes are easy to make.
  • If using VNAV, double check all speed and altitude constraints to ensure compliance with the approach chart and situation (some airlines promote the use of the speed intervention button (SPD INTV) to ensure that appropiate speeds are maintained).
  • If need be, select the approach (ARR) in the CDU to provide added situational awareness.

Final Call

I rarely use automated systems during landing, unless environmental conditions otherwise dictate.  I prefer to hand fly the aircraft where possible during the approach phase, and often disengage the autopilot at 5000 feet.  If flying a STAR and when VNAV/LNAV is used, I always disengage the autopilot no later than 1500 feet.  This enables a safe envelope in which to transition from automated flight to manual flight.

Using an arc to fly a VOR approach is enjoyable, with the added advantage that it provides a good refresher for using the Place/Bearing/Waypoint functionality of the CDU.

Other posts that deal with similar subjects are:

Glossary

CDU – Control Display Unit (aka Flight Management Computer (FMC).
EFIS – Electronic Flight Instrument System.
LNAV – Lateral navigation.
RADIAL/BEARING – A radial radiates FROM a point such as a VOR, whilst a bearing is the bearing in degrees TO a point.  The bearing is the direction that the nose of the aircraft is pointing.
VNAV – Vertical Navigation.

Images

The following are screen captures from the instructor station CDU and Navigation Display (ND).  Ignore altitude and speed constraints as these were not set-up for the example.

LEFT: Circular FIX ring has been generated around YMHB at 10 mile point.  The arc waypoints will be constructed along this line (click to enlarge).

 

 

 

 

 

 

LEFT:  Various waypoints have been generated along the 10 mile fix line creating an arc.  The arc ends at the intersection with the 212 radial for the VOR 30 approach into YMHB.  The route is in plan (PLN) view and is yet to be executed (click to enlarge).

 

 

 

 

 

LEFT:  The constructed arc as seen in MAP view.  From this view it is easy to establish that the aircraft is approaching TTR and once reaching the 10 mile limit  defined by the 10 mile FIX (green-coloured dotted circle), the aircraft will turn to the left to follow the arc waypoints until it intersects with the 121 radial (click to enlarge).

 

 

 

 

LEFT:  This image depicts the waypoint generated at 7 mile from the threshold (YMHB121/7).  This waypoint marks the point at which the aircraft should be on the 121 radial to VOR 30 and at 2400 feet altitude (according to the VOR 30 approach plate  (click to enlarge).

 

 

 

 

 

LEFT:  In the example, RWY 30 has been selected from the arrivals (ARR) page.  In addition to situational awareness, the selection of the runway in the CDU provides glidepath (GP) assistance.

The result of this is that the runway line is extended from the threshold to 7 miles out; the same distance out from the threshold that the final waypoint was generated.

The course line is coloured pink indicating that LNAV is enabled and the aircraft is following the programmed route. 

At the final waypoint (YM10) the autopilot (if used) will be disengaged and the aircraft will be flown manually to the runways using the VOR approach instrumentation and visual flight rules (VFR).  The EFIS will be changed from MAP to VOR.  LNAV will remain engaged on the MCP to ensure that the NPL indications are shown on the PFD.  The NDL indicators provide glidepath (GP) guidance that is otherwise lacking on a VOR approach (click to enlarge).

Monday
Jan192015

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.

ABEAM PTS

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).  

Technique

  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.

Technique

  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).

Summary

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.

Caveat

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.