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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 Flight Simulator (45)


B737-800 NG Flight Mode Annunciator (FMA)

Automatic Flight System - Background

The Boeing 737-80 has a relatively sophisticated Automatic Flight System (AFS) consisting of the Autopilot Flight Director System (AFDS) and the Autothrottle (A/T).  

LEFT:  Flight Mode Annunciator (FMA).  This is photograph has been take in a real aircraft and provides a good idea to the size, font and position of the FMA.

The Boeing 737-800 NG has a relatively sophisticated Automatic Flight System (AFS) consisting of the Autopilot Flight Director System (AFDS) and the Autothrottle (A/T).   The system is as follows:

  • The N1 target speeds and limits are defined by the Flight Management Computer (FMC) which commands airspeeds used by the A/T and AFDS;
  • The A/T and AFDS are operated from the AFDS Mode Control Panel (MCP), and the FMC from the Control Display Unit (CDU); 
  • The MCP provides coordinated control of the Autopilot (A/P), Flight Director (F/D), A/T and altitude alert functions; and,
  • The Flight Mode Annunciator (FMA), located on the Captain and First Officer side of the Primary Flight Display (PFD),  displays the mode status for the AFS.

If you read through the above slowly and carefully it actually does make sense; however, during in-flight operations it can be quite confusing to determine which system is engaged and controlling the aircraft at any particular time.

Reliance on MCP Annunciations

Without appropriate training, there can be a reliance on the various annunciations and lights displayed on the Mode Control Panel (MCP).  While some annunciations are straightforward and only illuminate when a function is on or off (such as the CMD button), others can be confusing, for example VNAV.

Do not reply on the MCP.  Always refer to the FMA to see what mode is controlling the aircraft.

Flight Mode Annunciator (FMA)

All Boeing aircraft are fitted with an FMA of some type and style.  The FMA on the Next Generation is located on the Captain and First Officer side Primary Flight Display, and is continuously displayed.  The FMA indicates what system is controlling the aircraft and what mode is operational.  All flight crews should observe the FMA to determine operational status of the aircraft and not rely on the annunciators on the MCP that may, or may not indicate a selected function.

The FMA is divided into three columns and two rows. The left column relates to the Autothrottle while the center and right hand column display roll and pitch modes respectively.  The two rows provide space for armed and selected annunciations to be displayed.  Selected modes that are operational are always coloured green while armed modes are coloured white. 

Below the two rows are the Autopilot Status alerts which are in larger green-coloured font, and the Control Wheel Steering (CWS) displays which are coloured yellow.  The Autopilot Status alerts are dependent upon whether a particular system has been installed into that aircraft.  For example, Integrated Approach Navigation (IAN), and various autoland capabilities.

When a change to a mode occurs (either by by a flight crew or by the Automatic Flight System), a mode change highlight symbol (green-coloured rectangle) is displayed around the changed mode annunciation.  The rectangle will be displayed for 10 seconds following the change in mode.

Unfortunately, not all avionics suites have the correct timing (10 seconds) and some displays the rectangle for only 2 seconds.  According to the Boeing manual the default time should be 10 seconds.

Important Points:

  • An annunciation that is green-coloured indicates a selected mode.
  • An annunciation that is white-coloured indicates an armed mode.
  • If there is some confusion to what mode is currently flying the aircraft, the FMA should be what you look at - not the MCP.


The following U-Tube link is a very good video that shows several annunciations being displayed on the Primary Flight Display.

The below image and table (ProSim-AR 737 avionics suite) indicates the more common mode annunciations that the FMA can display.  In the image, the pitch mode column and CWS display are not populated.  Furthermore, the FMA annunciations may differ between airframes depending upon the software installed to the aircraft (and avionics suite used in your simulation).  G, W and Y indicates the colour of the annunciation (green, white, or yellow).

NOTE:  Autoland capability and IAN have not been addressed as not every aircraft has these features installed.

ERRATUM to table: I have failed to mention in this image ILS, SINGLE CH and IDLE (update to come as time permits) - apologies....


Primary Flight Display (PFD) - Differences Between Sim Avionics and ProSim737 Avionics Suites

As I work on a slightly more technical post, I thought I would post some images of the Primary Flight Display (PFD) belonging to two of the most popular avionics suites - ProSim737 and Sim Avionics.  I am not going to compare avionics suites in this post as both have their pros and cons and specific features.  What is important, is that the reliability and graphic output of either suite is second to none and exceeds that of many competing avionics suites.

Sim Avionics is owned by Flight Deck Solutions (FDS) in Canada and simulates both the B737 NG and the B777.  ProSim737, developed in the Netherlands, is dedicated solely to the B737 NG.

In the interests of disclosure, I own both suites, however, use ProSim737. 

A post located on the ProSim737 forum discusses the various PFD differences.  The post can be read here: 'Comparing the ProSim PFD' (thanks Jacob for sending this to me).

BELOW:  First image is a screengrab of the PFD from ProSim737.  Second image is a screengrab from Sim Avionics, while the third image is a photograph of a real B737-800 PFD.




Crosswind Landing Techniques Part One - Crab and Sideslip

This video very clearly illustrates my point that landing in a strong crosswind can be a challenging and in some cases downright dangerous (Video © CargoSpotter (with thanks); courtesy U-Tube).

Generally, flight crews use one of two techniques or a combination thereof to approach and land in crosswind conditions.  If winds exceed aircraft tolerances, which in the 737-800 is 33 knots (winglets) and 36 knots (no winglets), the flight crew will divert to their alternate airport (Brady, Chris - The Boeing 737 technical Guide).

Maximum crosswind figures can differ between airlines and often it's left to the pilot's discretion and experience.  Below is an excerpt from the Landing Crosswind Guidelines from the Flight Crew Training Manual (FCTM).  Note that FCTMs can differ depending on date of publication.

LEFT:  Although not as dramatic as the video clip, the screen shot illustrates the ‘crab approach’.  Wind is right to left at 16 knots with aircraft crabbing into the wind to maintain centerline approach course.  Just before flare, left rudder will be applied to correct for drift to bring aircraft into line with centerline of runway.  This technique is called 'de-crabbing’. During such an approach, the right wing may also be lowered 'a tad' (cross-control) to ensure that the aircraft maintains the correct alignment and is not blown of course by a 'too-early de-crab'.  Right wing down also ensures the main gear adheres to the runway during the roll out.
There are several factors that require careful consideration before selecting an appropriate crosswind technique: the geometry of the aircraft (engine and wing-tip contact and tail-strike contact), the roll and yaw authority of the aircraft, and the magnitude of the crosswind component.  Consideration also needs to be made concerning the effect of the selected technique when the aircraft is flared to land.

Crosswind Approach and Landing Techniques

There are four techniques used during the approach and landing phase which center around the crab and sideslip approach.  The crab and sideslip are the primary methods and most commonly used while the de-crab and combination crab-sideslip are subsets that can be used when crosswinds are stronger than usual.

It must be remembered that whatever method is used it is at the discretion of the pilot in command.

1.    Crab (to touchdown).
2.    Sideslip (wing low).
3.    De-crab during flare.
4.    Combination crab and sideslip.

1:  Crab (to touchdown)

  • Airplane maintains a crab during the final approach phase.
  • Airplane touches down in crab.
  • Flight deck is over upwind side of runway (Main gear is on runway center).
  • Airplane will de-crab at touchdown.
  • Flight crew must maintain directional control during roll out with rudder and aileron.

With wings level, the crew will use drift correction to counter the effect of the crosswind during approach.  Drift correction will cause the aircraft to be pointing in a direction either left or right of the runway heading, however, the forward energy of the aircraft will be towards the centerline.  This is called the crab because the aircraft is crabbing at an angle left or right of the aircraft's primary heading.

Most jetliners have the ability to land in a crab, however, it must be remembered that landing in a crab places considerable stress on the main landing gear and tyre side-walls, which in turn can cause issues with tyre and wheel damage, not too mention directional control.

The later is caused by the tandem arrangement of the main landing gear that has a strong tendency to travel in the direction that the nose of the aircraft is pointing at the moment of touchdown.  This can result in the aircraft travelling toward the edge of the runway during the roll out.  To counter this, and align the nose of the aircraft with the centreline of the runway, the pilot flying must apply rudder input when lowering the nose wheel to the runway surface.

A reference to the maximum amount of crab that can be safely applied in the B737 was not found, other than maximum crosswind guidelines must not be exceeded.  The crab touchdown technique is the preferred choice if the runway is wet.

2:  Sideslip (wing low)

  • Upwind wing lowered into wind.
  • Opposite rudder (downwind direction) maintains runway alignment.
  • In a sideslip the aircraft will be directly aligned with the runway centerline using a combination of into-wind aileron and opposite rudder control (called cross-controls) to correct the crosswind drift.

The pilot flying establishes a steady sideslip (on final approach by applying downwind rudder to align the airplane with the runway centerline and upwind aileron to lower the wing into the wind to prevent drift.  The upwind wheels should touch down before the downwind wheels touch down.

The sideslip technique reduces the maximum crosswind capability based on a 2/3 ratio leaving the last third for gusts.  However, a possible problem associated with this approach technique is that gusty conditions during the final phase of the landing may preempt a nacelle or wing strike on the runway.

Therefore a sideslip landing is not recommended when the crosswind component is in excess of 17 knots at flaps 15, 20 and 30, or 23 knots at flaps 40.

The sideslip approach and landing can be challenging both mentally and physically on the pilot flying and it  is often difficult to maintain the cross control coordination through the final phase of the approach to touchdown.  If the flight crew elects to fly the sideslip to touchdown, it may also be necessary to add a crab during strong crosswinds.

3:  De-crab During Flare (with removal of crab during flare)

  • Maintain crab on the approach.
  • At ~100 foot AGL the flight crew will de-crab the aircraft; and,
  • During the flare, apply rudder to align airplane with runway centreline and, if required slight opposite aileron to keep the wings level and stop roll.

This technique is probably the most common technique used and is often referred to as the 'crab-de-crab'.

The crab technique involves establishing a wings level crab angle on final approach that is sufficient to track the extended runway centerline.  At approximately 100 foot AGL and during the flare the throttles are reduced to idle and downwind rudder is applied to align the aircraft with the centerline (de-crab). 

Depending upon the strength of the crosswind, the aircraft may yaw when the rudder is applied causing the aircraft to roll.  if this occurs, the upwind aileron must be placed into the wind and the touchdown maintained with crossed controls to maintain wings level (this then becomes a combination crab/sideslip - point 4).

Upwind aileron control is important, as a moderate crosswind may generate lift by targeting the underside of the wing. Upwind aileron control assists in ensuring positive adhesion of the landing gear to the runway on the upwind side of the aircraft as the wind causes the wing to be pushed downwards toward the ground.

Applied correctly, this technique results in the airplane touching down simultaneously on both main landing gear with the airplane aligned with the runway centerline.

4:  Combination Crab and Sideslip

  • De-crab using rudder to align aircraft with runway (same as point 3 de crab during flare).
  • Application of opposite aileron to keep the wings level and stop roll (sideslip). 

The technique is to maintain the approach in a crab, then during the final stages of the approach and flare increase the into-wind aileron and land on the upwind tyre with the upwind wing slightly low.  The combination of into-wind aileron and opposite rudder control means that the flight crew will be landing with cross-controls.

The combination of crab and sideslip is used to counter against the turbulence often associated with stronger than normal crosswinds.

As with the sideslip method, there is the possibility of a nacelle or wing strike should a strong gust occur during the final landing phase, especially with aircraft in which the engines are mounted beneath the wings.

FIGURE 1:  Diagram showing most commonly used approach techniques (copyright Boeing).

Operational Requirements and Handling Techniques

With a relatively light crosswind (15-20 knot crosswind component), a safe crosswind landing can be conducted with either; a steady sideslip (no crab) or a wings level, with no de-crab prior to touchdown.
With a strong crosswind (above a 15 to 20 knot crosswind component), a safe crosswind landing requires a crabbed approach and a partial de-crab prior to touchdown.

For most transport category aircraft, touching down with a five-degree crab angle with an associated five-degree wing bank angle is a typical technique in strong crosswinds.

The choice of handling technique is subjective and is based on the prevailing crosswind component and on factors such as; wind gusts, runway length and surface condition, aircraft type and weight, and crew experience.

Touchdown Recommendations

No matter which technique used for landing in a crosswind, after the main landing gear touches down and the wheels begin to rotate, the aircraft is influenced by the laws of ground dynamics.

Effect of Wind Striking the Fuselage, Use of Reverse Thrust and Effect of Braking 

The side force created by a crosswind striking the fuselage and control surfaces tends to cause the aircraft to skid sideways off the centerline.  This can make directional control challenging.

Reverse Thrust

The effects of applying the reverse thrust, especially during a crab ‘only’ landing can cause additional direction forces.  Reverse thrust will apply a stopping force aligned with the aircraft’s direction of travel (the engines point in the same direction as the nose of the aircraft).  This force increases the aircraft’s tendency to skid sideways.

Effects of Braking

Autobrakes operate by the amount of direct pressure applied to the wheels.  In a strong crosswind landing, it is common practice to use a combination of crab and sideslip to land the aircraft on the centerline.  Sideslip and cross-control causes the upwind wing to be slightly down upon landing and this procedure is carried through the landing roll to control directional movement of the aircraft. 

The extra pressure applied to the ‘wing-down’ landing gear causes increased auto-braking force to be applied which creates the tendency of the aircraft to turn into the wind during the landing roll.  Therefore, a flight crew must be vigilant and be prepared to counter this unwanted directional force.

If the runway is contaminated and contamination is not evenly distributed, the anti-skid system may prematurely release the brakes on one side causing further directional movement.

FIGURE 2:  Diagram showing recovery of a skid caused by crosswind and reverse thrust side forces (source: Flight Safety Foundation ALAR Task Force).

Maintaining Control - braking and reverse thrust

If the aircraft tends to skew sideward from higher than normal wheel-braking force, the flight crew should release the brakes (disengage autobrake) which will minimize directional movement.  

To counter against the directional movement caused by application of reverse thrust, a crew can select reverse idle thrust which effectively cancels the side-force component.  When the centerline has been recaptured, toe brakes can be applied and reverse thrust reactivated.

Runway Selection and Environmental Conditions

If the airport has more than one runway, the flight crew should land the airplane on the runway that has the most favourable wind conditions.  Nevertheless, factors such as airport maintenance or noise abatement procedures sometime preclude this.

I have not discussed environmental considerations which come into play if the runway is wet, slippery or covered in light snow (contaminated).  Contaminated conditions further reduce (usually by 5 knots) the crosswind component that an aircraft can land.

Determining Correct Landing Speed (Vref)

Vref is defined as landing speed or threshold crossing speed.

When landing with a headwind, crosswind, or tailwind the Vref must be adjusted accordingly to obtain the optimal speed at the time of touchdown.  Additionally the choice to use or not use autothrottle must be considered. Failure to do this may result in the aircraft landing at a non-optimal speed causing runway overshoot, stall, or floating (ground affect).

This article is part one of two posts.  The second post addresses the calculations required to safety land in crosswind conditions - Crosswind Landing Techniques Part Two Calculations.


OEM Brackets to Secure Gauges and Modules to Boeing 737 MIP

Original Equipment Manufacturer (OEM) parts usually attach to the infrastructure of the flight deck by the use of DZUS fasteners.  The easy to use fasteners allow quick and easy removal of panels and modules.  But what about the gauges that are used in the Main Instrument Panel (MIP); for example, the yaw dampener, brake pressure and AFDS module.

LEFT:  Take your pick – brackets for different sized modules and gauges.  The brackets when tightened provide a snug and secure fit for any OEM gauge or module.

These items do not use DZUS fasteners for attachment to the MIP; rather they are inserted into the MIP from the front and secured from behind by a specially designed bracket.  The different sized brackets are made from lightweight aluminum and are designed to fit particular gauges and modules.   Each bracket incorporates, depending on the style, a number of screws.  These screws are used to loosen or tighten the bracket. 

The gauge is inserted into the MIP from the front.  The bracket is then placed over the gauge from behind the MIP and tightened by one or more of the resident screws.  The screws cause the bracket to clamp tightly to the shaft of the gauge and ‘sandwich’ the MIP between the flanges of the gauge and the edge of the bracket.  Once fitted, the Canon plug is then re-attached to the gauge.

LEFT:  Selection of gauges.  Note the flange on the forward part of the yaw dampener and brake pressure gauge (nearest glass) Click image to enlarge.

Of interest is that some brackets have been designed to fit the differing thicknesses between MIPs.  By turning the bracket end on end the appropriate thickness of the MIP is selected.  

As mentioned above, the brackets are designed to fit specifically sized and shaped gauges and modules; therefore, it is important to purchase the bracket that fits the gauge you are using.  There are several different sized brackets on the market that are used in the Boeing 737 classics and NG airframes.  The 'NG' for the most part incorporates identically sized gauges as the classics, so a bracket is not necessarily NG specific.

One of the benefits in using the OEM brackets is that they are designed for the purpose, are very easy to install, and facilitate the quick removal of a gauge or module should it be necessary.

In the next post we look more at flight training and discuss corsswind landings.


Tools To Assist in Approach: Using the B737-800 Vertical Bearing Indicator, Altitude Range Arc and Vertical Deviation Scale

On 12 February 2012, the flight crew of a Boeing 737 aircraft, registered VH-TJS and operated by Qantas Airways Limited, was conducting a scheduled passenger service from Sydney, New South Wales to Canberra, Australian Capital Territory. Due to scheduled maintenance the instrument landing system at Canberra was not available and the crew prepared for an alternate instrument approach that provided for lateral but not vertical flight path information. The flight was at night with rain showers and scattered cloud in the Canberra area.

Shortly after becoming established on the final approach course with the aircraft’s automatic flight system engaged, the flight crew descended below the minimum safe altitude for that stage of the approach. The crew identified the deviation and levelled the aircraft until the correct descent profile was intercepted, then continued the approach and landed. No enhanced ground proximity warning system alerts were generated, as the alerting thresholds were not exceeded.

During those phases of flight when terrain clearance is unavoidably reduced, such as during departure and approach, situation awareness is particularly crucial. Any loss of vertical situation awareness increases the risk of controlled flight into terrain. This occurrence highlights the importance of crews effectively monitoring their aircraft’s flight profile to ensure that descent is not continued through an intermediate step-down altitude when conducting a non-precision approach (Australian Transport safety Bureau, 2013).

Determining the correct rate of descent (RoD) or vertical speed (V/S) is a critical attribute if an aircraft is to arrive at the correct altitude and avoid excessive descent rates.  Control of the vertical path uses two different methods: the step-down method and the constant descent method.  Both methods assume that the aircraft is being flown in landing configuration at the final approach speed (VaPP) from the final approach fix (FAF) to the landing initiation of the missed approach.

Non Precision Approaches (NPA)

Historically non precision approaches reference ground navigation aids that exhibit a degree of inaccuracy, which is often enhanced by the poorly defined vertical path published on an approach chart; NPA charts typically provide only an assigned altitude at the FAF and the distance from the FAF to the MAP.  Thus, flight crew awareness of the aircraft’s vertical position versus the intended vertical path of the final approach can be quite low when executing traditional style step-down approaches.

To determine the best vertical speed to use during a non precision approach, flight crews use a number of 'back of the envelope' calculations.

Rate of Descent & Glideslope Calculations

There are several calculations that can be used determine rate of descent – some more accurate than others.  Search ‘determine descent rate’ in Google.  Some of the more commonly used rules of thumb are:

  • Divide your ground speed by 2, then add a zero (120 kias / 2 = 60, add 0 = 600 fpm).
  • Rate of descent (RoD) in ft/min should be equal to 5 times the ground speed in knots (same as above but different calculation).
  • To maintain a stabilized approach, add a zero to your indicated air speed and divide by two (150 kias + 0 = 1500 / 2 = 750 fpm).
  • To determine distance from threshold to start a 3 degree glideslope, take the height above ground level and divide by three hundred (600 ft AGL / 300 = 2 nm).
  • To maintain a 3 degree glideslope (ILS), multiply your ground speed by 5.  The resulting number is the rate of descent to fly (110 kias x 5 + 550 fpm on 3 degree glideslope).
  • If the glideslope is not operational on an ILS approach with DME, multiply the distance ‘to go’ by 300.  This will provide the height in feet above the threshold of the runway (4 nm to the threshold; multiply x 300 = 1200 ft).

Flight crews today, especially those flying in and out of busy intercity hubs, rarely execute step down approaches as computer and GPS-orientated systems have replaced traditional methods of navigation.  However, as the flight into Canberra revealed, the best system may at times be inoperative or fail and it is good airmanship to understand and be able to remember one or more of the above equations. 

Today's systems provide a high level of redundancy and the Boeing 737-800 NG incorporates a number of integrated aids to assist a flight crew during descent and approach.  In this post some of less commonly understood aids will be discussed.

Vertical Bearing Indicator (VBI))

A method often overlooked is to use the Vertical Bearing Indicator (VBI) which is functionality available in the CDU.  The VBI can calculate an accurate rate of descent to a particular spatial point.  It is basically an angle calculator that provides ‘live’ vertical speed information based upon a desired descent angle, the current speed of the aircraft and a end location.

LEFT:  CDU showing DES Page, waypoint/altitude and VBI interface (Key RSK3 & RSK4).

A flight crew enters into the VBI the final altitude that the aircraft should be at (for example, the Final Approach Fix or runway threshold). This figure is determined by consulting the appropriate approach chart for the airport.  The CDU will then calculate the descent rate based on flight variables.  As the aircraft descends, the VBI readout will continually update the descent rate based upon aircraft speed and rate of descent.

The flight crew can either manually fly the descent rate or use part or full automation to maintain the rate of descent.  A common method is to use the Vertical Speed (V/S) function on the MCP.

It is important to underatnd that the VBI has nothing to do with VNAV.  The VBI takes the raw distance between the aircraft and a selected altitude point and calculates a vertical bearing to that point.  If that point is part of a route in the CDU, then the next altitude constraint will be displayed, unless the user changes this.

Accessing the VBI

Navigate to Descent page on the CDU by pressing the DES key.

At lower right hand side of the DES page you will see the following: FPA, V/B, V/S.  This is the Vertical Bearing Indicator.

Key RSK3 (right line select 3) allows manual entry of a waypoint and altitude or altitude restriction.  Type the waypoint and altitude separated by a / slash symbol into the scratchpad of the CDU and upload to the correct line. (for example, MHBWM/200).

The VBI provides three fields:

  1. FPA (Flight Path Angle) is the vertical path in degrees (angle of descent) that the aircraft is currently flying.
  2. V/B (Vertical Bearing) is the computed vertical path in degrees that the aircraft SHOULD be flying to reach the CDU waypoint or altitude restriction.
  3.  V/S (Vertical Speed) is the vertical bearing (V/B) converted into a vertical speed (RoD) for easy input into the MCP.  The V/S is the vertical speed (RoD in feet per minute) required to achieve the displayed vertical bearing.

Observe the V/B.  The idle descent in a B737 is roughly 3.0 degrees.  Wait until the V/B moves between 2.7 and 3.0 degrees (or whatever descent angle you require based upon your approach constraints) and note the descent rate (V/S).  At its simplest level, the V/S can be entered directly into the MCP and is the rate of descent required to achieve the computed vertical path..

The VBI can be used for any waypoint, fix and altitude and acts in conjunction with the AFDS. 

Automation will attempt to follow the vertical bearing indicated by the CDU; for example, if a VNAV descent is activated before the Top of descent (ToD) is reached, the Flight Management System (FMS) commands a 1250 fpm descent rate until the displayed V/B is captured while maintaining VNAV connection. 

The vertical bearing when the aircraft is on final approach calculates data from the Final Approach Fix (FAF) to the runway threshold.

To read an earlier post concerning the Vertical Bearing Indicator.

Other Approach Aids

Altitude Range Arc (ARA)

A handy feature often overlooked is the Altitude Range Arc (ARA).  The ARA is a green coloured half semicircle which can be viewed on the Navigation Display (ND).  The ARA indicates the approximate map position where the altitude, as set on the mode control panel is expected to be reached.  Once the aircraft is well established on the vertical bearing (V/B) calculated by the CDU, the ARA semicircle should come to rest on the targeted waypoint.  

LEFT:  Altitude Range Arc and Vertical Deviation Scale and Pointer B737-800NG

Vertical Deviation Scale and Pointer (VDS)

The Vertical Deviation Scale is another feature often misunderstood.  The scale can be found on the lower right hand side of the Navigation Display (ND).

The VDI will be displayed when a descent and approach profile is activated in the CDU (such as when using VNAV).  However, the tool can be used to aid in correct glideslope for any type of approach (RNAV, VNAV, VOR, etc).  To display the VDI, an appropriate approach be selected in the CDU; however, the flight crew fly a different type of approach without VNAV engaged).

The Vertical Deviation Scale presents the aircraft’s vertical deviation from the flight management computer’s determined descent path (vertical bearing) within +- 400 feet.  It operates in a similar way to the Glideslope Deviation Scale on the Instrument Landing System (ILS).

The VDS is a solid white-coloured vertical line with three smaller horizontal lines at the upper, lower and middle section, on which a travelling magenta-coloured diamond is superimposed.  The middle horizontal line represents the aircraft’s position and the travelling diamond represents the vertical bearing (V/B). 

When the aircraft is within +- 400 feet of the vertical bearing the diamond will begin to move, indicating whether you are above, below or on the V/B target.  When the aircraft is on target (middle horizontal line) with the indicated vertical bearing, the FMA will annunciate IDLE thrust mode followed by THR HLD as the aircraft pitches downwards to maintain the V/B.

In some literature this tool is referred to as the Vertical Track Indicator (VTI).

Vertical Development (VERT DEV)

The Vertical Development (VERT DEV) is the numerical equivalent of the vertical deviation scale and is found on the Descent Page of the CDU.  The VERT DEV allows a flight crew to cross check against the VBI in addition to obtaining an accurate measurement in feet above or below the targeted vertical bearing. The VERT DEV will display HI or LO prefixed by a number which is the feet the aircraft is above or below the desired glideslope.

The Vertical Deviation Scale and pointer (VDS) will remain visible on the Navigation Display (ND) throughout the approach, and in association with the Vertical Development display on the CDU are important aids to use for Non Precision Approaches (NPA). 


The traditional method of a step down approach, which was the mainstay used in the 1970s has evolved with the use of computer systems and GPS.  In the 1980s RNAV (area navigation) approaches with point to point trajectories began to be used, and in the 1990s these approach procedures were further enhanced with the use of Required Navigational Performance (RNP) in which an aircraft is able to fly the RNAV approach trajectory and meet specified Actual Navigation Performance (ANP) and RNP criteria.  From the 1990s onward with the advent of GPS, the method that non precision approaches are flown has allowed full implementation of the RNP concept with a high degree of accuracy.

Although the nature of non precision approaches has evolved to that of a 'precision-like' approach with a constant descent angle, their are operators that widely use these techniques, despite their flaws, weaknesses and drawbacks. Even if modern navigational concepts are used in conjunction with traditional methods, aids such as the VBI, ASR and VDI should not be overlooked.  Appropriate cross checking of the data supplied by these aids provides an added safety envelope and avoids having to remember, calculate and rely on ‘back of the envelope’ calculations.

The flight crew landing in Canberra, Australia did not use all the available aids at their disposal.  If they had, the loss of vertical situational awareness may not have occurred.


ANP - Actual Navigation Performance
- Altitude Range Arc
CDU – Control Display Unit (used by the flight crew to interface the with the FMC)
FAF - Final Approach Fix
FMS – Flight Management System
FMA – Flight Mode Annunciation
FMC – Flight Management Computer (connects to two CDU units)
ILS – Instrument Landing System
KIAS - Knots Indicated Air Speed
MAP - Missed Approach Point
MCP – Mode Control Panel
ND – Navigation Display
NPA – Non Precision Approach
RoD – Rate of Descent
RNP - Required Navigation Performance
RNAV - Area Navigation
ToD – Top of Descent
V/B – Vertical Bearing
VBI – Vertical Bearing Indicator
V/S – Vertical Speed
VDS – Vertical Deviation Scale and pointer (also called Vertical Track Indicator)
VERT DEV – Vertical Development