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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.


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I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).


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Entries in VNAV (4)


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.


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.


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


Altitude and Speed Intervention Explained 

The flight deck can be an extreme work environment, especially during the high-task descent and approach phase of the flight. 

Altitude and speed intervention were designed to allow pilots to easily and quickly change either the altitude or speed of their aircraft without re-programming the FMC, disengaging VNAV, or spending excessive time 'heads down'.

LEFT:  Altitude Intervention button on MCP.

The intervention buttons are strategically located on the MCP.  When the buttons are selected, the aircraft's altitude or speed can be altered quickly on ‘the fly’

In this article, I will examine the use of Altitude and Speed Intervention and demonstrate the use of these modes.  In a follow-on article, I will discuss alternate methods that can be used to change altitude whilst maintaining Vertical Navigation.  The reason for separating the two articles, is to avoid confusion that can develop between the different modes.

I recommend reading the appropriate section in the Flight Crew Operations Manual (FCOM) , Flight Crew Training Manual (FCTM) and the Cockpit Companion for a more thorough understanding. 

Furthermore, whether intervention modes function in the simulator will depend upon which avionics suite and FMC software version is used.  This article will deal only with ProSim-AR (ProSim737 avionics suite) which uses U10.8 A.  In this article I use the words altitude and flight level (FL) interchangeably.

Important Points:

  • Altitude and Speed Intervention are company options that may or may not be ordered at the time of airframe purchase.
  • Altitude and Speed Intervention will only operate when an a flight plan is active and VNAV is selected.
  • Altitude and Speed Intervention is more often used when a temporary change in altitude and/or speed is required with a return to the original altitude/speed imminent.  

MCP, VNAV & FMA Nomeclature and Displays

Prior to examining altitude and speed intervention, it may be fruitful to quickly discuss common words, that are used when describing the operation of VNAV and the MCP.

(i)       CONDITION means that a mode will become active only when a condition(s) occurs;
(ii)      ARM means that a mode is armed pending engagement;
(iii)     ACTIVE means the mode is engaged/selected;
(iv)     SELECT means to select or engage the mode (turn on); and,
(v)      DESELECT means to deselect or disengage (turn off) the mode.

An often misunderstood facet of the MCP is that the annunciators illuminate to indicate a particular mode is active.  This is not entirely correct, as the presence of an illuminated annunciator light does not always indicate whether a mode is active or not.

Table 1:  FMA displays observed when Altitude and Speed Intervention is engaged (click to enlarge).  Also (downloaded as a .pdf - click FMA displays).

For example, the VNAV annunciator on the MCP will remain illuminated when VNAV is either active or armed.  Furthermore, active modes that are not able to be deselected, do not display an illuminated annunciator.

To determine whether a mode is active or not, the Flight Mode Annunciator (FMA) should be consulted.  The FMA is located above the Primary Flight Display (PFD) and displays various alerts and status messages.  

Refer to Table 1 for a synopsis regarding the various displays that the FMA will generate when intervention is used.



Important Points:

  • A mode change highlight symbol (green rectangle) is displayed around the command name, in the Flight Mode Annunciator (FMA), whenever a mode has been armed and is about to become active.  The green rectangle will remain displayed for a period of 10 seconds.
  • It’s prudent to cross reference between the FMA, MCP and CDU to determine what mode is armed or active at a given time.
  • Altitude and Speed Intervention, when active, will take precedence over VNAV, although VNAV remains armed.


The aircraft is flying at FL150 (15,000 feet) at 275 kias.  The FMC has a flight plan engaged (Company Route) that includes altitude and speed constraints (in the LEGS page of the CDU). 

In level flight, with the autopilot, LNAV and VNAV engaged, the following will be observed:

(i)     LNAV and VNAV will be active;
(ii)    The FMA will display MCP SPD / LNAV / VNAV PTH;  
(iii)   The annunciators on the MCP - LNAV, VNAV & CMD A/B will be illuminated;
(iv)   The speed window located on the MCP will be blank (no speed displayed); and,
(v)    LNAV/VNAV will be displayed in white text on the PFD.

LNAV will be controlling the lateral navigation of the aircraft while VNAV will be controlling the speed and altitude of the aircraft.

ATC request a decrease in speed from 275 kias to 240 kias.

Speed Intervention (SPD INTV)

Select (press) the SPD INTV button on the MCP.  The MCP speed window becomes active and displays the current speed of 275 kias.  Dial in into the speed window the new speed requirement of 240 kias. 

LEFT:  Speed Intervention button.

Notice the speed indicator display above the speed tape on the PFD has changed from 275 kias to the new speed of 240 kias.  Also note that the VNAV annunciator light on the MCP remains illuminated - in this case VNAV is active.  The aircraft will reduce its speed to 240 kias.

If you cross check with the cruise altitude in the CDU (CRZ ALT key/TGT SPD), the FMC will still indicate the original cruise speed of 275 kias.  This is because the speed is an intervention speed and, as such, will not have been updated in the FMC.

If you wish to stay at this speed (240 kias), you will need to manually change the cruise speed to 240 kias in the CDU.  However, in this case the reduction in speed is momentary, and ATC advise you to return to your original speed.  

Returning to Original Speed

Press the SPD INTV button (or unselect and reselect VNAV on the MCP).  Doing this, will return the speed to the original speed (275 kias).  It will also change the speed indication on the PFD from 240 kias back to 275 kias.  The MCP speed window will also become blank (no speed displayed). 

Important Point:

  • When SPD INTV is active, the FMA will display MCP SPD.  When SPD INTV is not active (deselected) the FMA will revert to FMC SPD.

Altitude Intervention (ALT INTV)

Altitude Intervention is slightly more sophisticated in comparison to Speed Intervention.  This is because, amongst other factors, the relationship changes when the aircraft is ascending or descending.

LEFT:  ALT INTV button on MCP.

In level flight, with the autopilot, LNAV and VNAV engaged, the following will be observed:

(i)     LNAV and VNAV will be active;
(ii)    The FMA will display FMC SPD / LNAV / VNAV PTH;  
(iii)   The annunciators on the MCP - LNAV, VNAV & CMD A/B will be illuminated;
(iv)   The speed window located on the MCP will be blank (no speed displayed); and,
(v)    LNAV/VNAV will be displayed in white text on the PFD.

ATC request a descent from FL150 to FL120.

DESCENT Using ALT INTV (descent from FL150 to FL120)

Dial into the MCP altitude window the new altitude (FL120). 

Notice the altitude display above the altitude tape on the PFD has changed from FL150 to the new altitude of FL120.   Also note that the VNAV annunciator light on the MCP remains illuminated - in this case VNAV is armed.  ALT INTV takes precedence over VNAV.  

LEFT:  CDU cruise page showing 12000 in scratch pad.  Selecting line select 1 left will update FMC.

Select (press) ALT INTV button on the MCP and the FMA will annunciate FMC SPD / LNAV / VNAV PTH.   The aircraft will descend at 1000 fpm (default) until FL120 is reached.  

If you cross-check the cruise altitude in the CDU (INIT PERF/PERF/CRZ ALT or CRZ key/CRZ ALT), it will display the original cruise altitude of FL150.  The FMC has NOT automatically updated to the lower altitude – this is normal and not a fault.  

By default, Altitude Intervention will always maintain a vertical descent at 1000 fpm. 

If you want to remain at FL120, you will need to manually update the cruise altitude in the FMC (INIT PERF/PERF/CRZ ALT), or (CRZ key/CRZ ALT) and press the EXEC key.  

Important Point:

  • When the CDU page is open on CRZ (CRZ key), it will display in the scratch pad any change to the altitude in the MCP.  This provides a ‘shortcut’ to insert the new flight level should it be desired to make it permanent.  All that is needed is to press the CRZ/CRZ ALT (in the CRZ page) and the FMC cruise altitude will be updated.  The altitude in the LEGS page will also be updated.

Returning to Original Flight Level

To return to the original flight level (FL150), dial into the MCP the previous flight level (FL150) and press ALT INTV.  The aircraft will ascend to FL150.  

Important Points:

  • The FMC will NOT be automatically updated to the lower altitude.  If desired, this will need to be done manually.
  • When returning to the original flight level, VNAV will not engage unless the original flight level (FL150) is manually dialled into the altitude window of the MCP.
  • ALT INTV takes precedence over VNAV.  The VNAV annunciator on the MCP will remain illuminated.  VNAV will be in armed mode.
  • To determine if VNAV is the active mode (or not) the FMA display must be consulted – not the annunciator light on the MCP.

ASCENT Using ALT INTV (ascent from FL120 to FL150)

The ALT INTV button operates a little differently when you ascend.   For a start, it automatically replaces (updates) the flight level (CRZ ALT) in the FMC.  It will also update the altitude in the LEGS page in the CDU. 

The FMA will annunciate  N1 / LNAV / VNAV SPD during the climb phase of the flight, changing to FMC SPD / LNAV / VNAV PTH when the new flight level is reached.  When climbing using ALT INTV, the thrust mode uses N1.

Important Point:

  • When a flight level of a higher altitude is dialled into the altitude window, and ALT INTV selected, the FMC will automatically be updated to the new flight level.

Considerations When Using ALT INTV

When using ALT INTV, several variables that relate to the altitude constraint (s) will change, depending upon whether you are in VNAV climb, cruise or descent.  Rather than rephrase what already has been written, I have scanned the appropriate page (below) from the Cockpit Companion written by Bill Bulfer.

Using ALT INTV and SPD INTV During a VNAV Approach Phase

ALT INTV is a very handy tool, if during an VNAV approach, the flight crew fail to change the altitude in the MCP to the next lowest altitude constraint.  

To demonstrate, the aircraft is flying a published STAR that will join an VNAV approach.  VNAV and LNAV are active and the flight plan has several altitude and speed constraints.  To meet these constraints, the crew must update the MCP altitude to the next lowest altitude (displayed in the LEGS page of the CDU) prior to the aircraft crossing the constraint.

If the crew fails to update the MCP, then the aircraft with transition from descending flight (VNAV PTH) to level flight (VNAV ALT).   In this situation a crew could engage LVL CHG or V/S,  however, doing so would deselect VNAV.  

A simpler solution is to change the altitude in the MCP window to the next lowest altitude constraint (or MDA) and press ALT INTV.  This will cause VNAV to descend the aircraft, at a variable descent rate, to meet the required constraint.   By using ALT INTV, the aircraft will remain in VNAV.

Additionally, SPD INTV is a straightforward way to control the speed of the aircraft during the approach while maintaining VNAV.  Company policy at some airlines insist that Speed Intervention be used approximately 2 nautical miles short of the Final Approach Fix (FAF).

Reliability of ALT INTV in Descent Mode - ProSim-AR

ProSim-AR (Version 1.49) exhibits difficulty in holding a lower altitude level when ALT INTV is used.

The Boeing system is designed in a way that once the V-Path is intercepted; the Flight Director cross hairs maintain the new altitude by pitch.  In ProSim-AR this pitch is often difficult to hold and a resultant pitching of the aircraft (up and down) occurs as the system attempts to hold the lower altitude.  When using LVL CHG or V/S this does not occur.  Note that this behaviour does not occur when using INTV ALT to ascend.

It is not certain if this behaviour is common only to my system or is more widespread; but a way to solve the issue is to either:

(i)   Use an alternate descent mode; or,
(ii)  Manually change the altitude values in the FMC (INDEX/PERF/CRZ ALT), or (CRZ key/CRZ ALT) and press EXEC.

By manually changing the cruise altitude to the lower altitude in the CDU, the aircraft will be stable (no pitching), because the FMC logic now commands the altitude, rather than the logic that commands ALT INTV.

The developers at ProSim-AR are continually tweaking these variables.  In future software releases (post version 1:49) this issue may well be rectified.

Final Call

During a flight there are many of reasons why you may need to alter speed and/or altitude; be it to divert around a localized weather or to abide by an Air Traffic Control.  Whatever the reason, often the changes are short-lived and a return to the original constraint imminent.

The use of Altitude and Speed Intervention, in addition to being a time saver, minimises  time spent in the ‘heads down’ position during the high-task descent and approach phase of a flight.

In this article, I have explained the intervention functionality of the Boeing 737 and provided workarounds should VNAV not operate as anticipated. 

This article has been re-written (for clarity) and updated from a post published in 2013.

Acronyms and Glossary

Annunciator - A push button to engage a particular mode – often has a light that illuminates
ALT INTV - Altitude Intervention

CDU – Control Display Unit (display screen and keyboard to input data into the FMC)
Flight Level – Altitude that the aircraft will fly at (set in FMC)

FMA – Flight Mode Annunciator

FMC – Flight Management Computer  (part of the Flight Management System)
FMS – Flight Management System
LNAV – Lateral Navigation

MCP – Mode Control Panel 

PFD – Primary Flight Display

SPD INTV - Speed Intervention

VNAV – Vertical Navigation


VNAV 'Gotchas' - Avoiding Unwanted Level-Offs

One aspect of using VNAV during published instrument departures, arrivals, and approaches is that it can cause unnecessary level-offs. 

These level-offs can cause engines to spool needlessly, increase fuel cost and stagger a Continuous Descent Final Approach (CDFA) such as when executing  an RNAV approach. 

LEFT:  RAAF B737 Wedgetail transitioning a STAR into YSSY (Sydney Australia).  It is not only domestic airliners that must meet altitude constraints; military aircraft also  must meet the same requirements when landing at a non-military airport (click to enlarge).  Image is copyright xairforces.net.  For those interested in flying the Wedgetail, there is a model available for ProSim-AR users on their forum page.

To avoid this, and ensure that minimum altitude constraints are met, two techniques can be used.

METHOD 1Constraints Are Not Closely Spaced.

This technique is normally used when waypoints with altitude constraints are not closely spaced (in other words, there is a moderate distance between altitude constraints).

During climbs, the maximum or hard altitude constraints should be set in the Mode Control Panel (MCP).

Minimum crossing altitudes need not be set in the MCP as the FMC message function will alert the crew if these constraints cannot be satisfied.

During descent, the MCP altitude is set to the next constraint or clearance altitude, whichever will be reached first.

Immediately prior to reaching the constraint, when compliance with the constraint is assured, and when cleared to the next constraint, the MCP altitude is reset to the next constraint/altitude level.

METHOD 2:  Constraints Are Closely Spaced. 

Where constraints are closely spaced to the extent that crew workload is adversely affected, and unwanted level-offs maybe a concern, the following is approved:

For departures, set the highest of the closely-spaced constraints.

For arrivals, initially set the lowest of the closely-spaced altitude constraints or the Final Approach Fix (FAF) altitude, whichever is higher.

IMPORTANT: When using either technique, the FMS generated path should be checked against each altitude constraint displayed in the CDU to ensure that the path complies with all constraints.  Furthermore, the selection of a pitch mode other than VNAV PTH or VNAV SPD should be avoided, as this will result in the potential violation of altitude constraints.

To enlarge more on VNAV is beyond the scope of this post.  A future post will address this topic in more detail.

Crew Controls Automation - Not Vice Versa

However, the system is only as good as the knowledge of the person pushing the buttons.  It is very important that a flight crew control the automation rather than the automation control the flight crew. 

If VNAV begins to do something that is unplanned or unexpected, do not spend precious time ‘thinking about the reasons why’ – disconnect VNAV and use a more traditional method or hand floy the aircraft.  Then, determine why VNAV did what it did.  The most common comment heard in today's modern cockpits is ‘What is it doing now…

Final Call

VNAV is an easy concept to understand, but it can be confusing due to innumerable variables associated with vertical navigation.  VNAV is probably one of the more complicated systems that virtual and real pilots alike have to understand.  When using VNAV it is paramount to maintain vigilance on what it is doing at any one time, especially during descent and final approach.     Furthermore, it is good airmanship to always have a redundancy plan in place – a ‘what if’ should VNAV fail to do what was anticipated.

This is but one post that explains VNAV.  The below articles deal with VNAV:

Cognitive Engineering Analysis of the Use of VNAV

Acronyms and Glossary

CDU - Control Display Unit (aka FMC)
FAF – Final Approach Fix
FMC - Flight Management Computer
FMS - Flight Management System.  Supply of data to the FMC and CDU
Gotcha - An annoying or unfavorable feature of a product or item that has not been fully disclosed or is not obvious.
LNAV – Lateral Navigation
MCP – Mode Control Panel
NPA - Non Precision Approach
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed


RNAV, RNP, LNAV and VNAV Operations - Overview 

New flyers to the Boeing 737NG often become confused understanding the various terminology used with modern on-board navigational systems.

Although the concepts are easy to understand, the inter-relationship between systems can become blurred when the various types of approaches and departures are incorporated into the navigational system.

LEFT:  Collins Mode Control Panel (MCP) showing illuminated LNAV annunciation (click to enlarge).

This post will not provide an in-depth review of these systems; such a review would be lengthy, confusing and counterproductive to a new virtual flyer.  Rather, this post will be a ‘grass-roots’ introduction to the concept of RNAV, RNP, LNAV and VNAV.  I will also touch on the concept of Performance Based Navigation (PBN).

In the Beginning there was RNAV  

RNAV is is an acronym for Area Navigation (aRea NAVigation). 

Prior to complex computers, pilots were required to use established on-the-ground navigational aids and would fly directly over the navaid.  Such a navaid may be a VOR, NDB or similar device.  Flying over the various navaids was to ensure that the flight was on the correct route.  Often this entailed a zigzag course as navaids could not be perfectly aligned with each other in a straight line - airport to airport. 

When computers entered the aviation world it became possible for the computer to 'create' an imaginary navigation aid based on a direction and distance from a ground-based navaid.  Therefore, a straight line could be virtually drawn from your origin to destination and several waypoints could be generated along this line.   The waypoints were calculated by the computer based on ground VORs and positioned in such a way to ensure more or less straight-line navigation.

In essence, RNAV can be loosely defined as any 'straight line' navigation method similar to GPS that allows the aircraft to fly on any desired path within the coverage of referenced NAVAIDS.

Required Navigation Performance (RNP) and Performance Based Navigation (PBN)

Simply explained, Required Navigation Performance (RNP) is a term that encompasses the practical application of advanced RNAV concepts using Global Navigation Satellite Systems (GNSS).

However, there is a slight difference between RNP and RNAV although the principles of both systems are very similar. 

RNAV airspace generally mandates a certain level of equipment and assumes you have a 95% chance of keeping to a stated level of navigation accuracy.  On the other hand, RNP is performance based and requires a level of on-board performance monitoring and alerting.  This concept is called Performance Based Navigation (PBN).

RNAV and RNP both state a 0.95 probability of staying within 1 nm of course.  But RNP (through PBN) will let you know when the probability of you staying within 2 nm of that position goes below 0.99999.  In essence, RNP and PBN enable an aircraft to fly through airspace with a higher degree of positional accuracy for a consistently greater period of time. 

To achieve this level of accuracy a selection of navigation sensors and equipment is used to meet the performance requirements.  A further enhancement of this concept is the use of RNP/ANP (Required Navigation Performance and Actual Navigation Performance.  Advanced RNAV concepts use this comparative analysis to determine the level or error between the required navigation (the expected path of the aircraft) and the actual navigation (what path the aircraft is flying.)  This information is then displayed to the flight crew.


LNAV and VNAV are parts of the Flight Guidance System, and are acronyms for Lateral Navigation and Vertical Navigation'.  Both these functions form part of the automation package that the B737NG is fitted with.

LNAV is the route you fly over the ground. The plane may be using VORs, GPS, DME, or any combination of the above. It's all transparent to the pilot, as the route specified in the clearance and flight plan is loaded into the Flight Management System (FMS), of which the Flight Management Computer (FMC) is the interface.

The route shows up as a magenta line on the Navigation Display (ND), and as long as the LNAV mode on the Mode Control Panel (MCP) is engaged and the autopilot activated, the aircraft will follow that line across the ground. LNAV however, does not tell the plane what altitude to fly, VNAV does this.

VNAV is where the specified altitudes at particular waypoints are entered into the FMS, and the computer determines the best way to accomplish what you want.  The inputs from VNAV are followed whenever the autopilot is engaged (assuming VNAV is also engaged).  

The flight crew can, if necessary alter the VNAV constraints by changing the descent speed and the altitude that the aircraft will cross a particular waypoint, and the computer will re-calculate where to bring the throttles to idle thrust and begin the descent, to allow the aircraft to cross the waypoint, usually in the most economical manner.

VNAV will also function in climb and take into account airspeed restrictions at various altitudes and will fly the aircraft at the desired power setting and angle (angle of attack) to achieve the speed (and efficiency) desired.

There is not a fast rule to whether a flight crew will fly with LNAV and VNAV engaged or not; however, with LNAV and VNAV engaged and the autopilot not engaged, LNAV and VNAV will send their signals to the Flight Director (F/D) allowing the crew to follow the F/D cue display and hand fly the aircraft the way the autopilot would if it were engaged.

Reliance on MCP Annunciators

LNAV and VNAV have dedicated annunciators located on the Mode Control Panel (MCP).  These annunciators illuminate to indicate whether  a particular mode is engaged. 

LEFT:  Flight Mode Annunciator (FMA) showing LNAV and VNAV Path Mode engaged.  The Flight Director provides a visual cue to the attitude of the aircraft while the speed is controlled by the the FMC.  CMD indicates that the autopilot is engaged (ProSim737 avionics suite).

However, reliance on the MCP annunciators to inform you of a mode’s status is not recommended.  Rather, the Flight Mode Annunciator (FMA) which forms part of the upper area of the Primary Flight Display (PFD) should be used to determine which modes are engaged.  Using the FMA will eliminate any confusion to whether VNAV (or any other function) is engaged or not.

This post explains the Flight Mode Annunciators (FMA) in more detail.


In summary, RNAV is a method of area navigation that was derived from the use of VOR, NDBs and other navaids.  RNP through it use of GNSS systems has enabled Area Navigation to evolve to include LNAV and VNAV which are sub-systems of the Flight Guidance System -  LNAV is the course across the ground, and VNAV is the flight path vertically. 

Historically, navigation has been achieved successfully by other methods, however, the computer can almost always do things better, smoother and a little easier – this translates to less workload on a flight crew.  

In my next post, we will discuss RNAV approaches and how they relate to what has been discussed above.


The information for this article came from an online reference for real-world pilots.

Acronyms and Glossary

Annunciator – Often called a korry, it is a light that illuminates when a specific condition is met
DME – Distance Measuring Equipment
FMA - Flight Mode Annunciator
FMC – Flight Management Computer
FMS – Flight Management System
GPS – Global Positioning System
GNSS - Global Navigation Satellite System
LNAV – Lateral Navigation
MCP – Mode Control Panel
ND – Navigation Display
NPA - Non Precision Approach
PBN - Performance-based Navigation
RNAV – Area Navigation
RNP - Required Navigation Performance
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed
VOR – VHF Omni Directional Radio Range