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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.

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Wednesday
Apr262017

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.

Scenario

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

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