Altitude and Speed Intervention Explained

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Altitude Intervention (ALT INTV) button

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'.

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.

In this article I use the words Cruise Altitude (CRZ ALT) and Flight Level (FL) interchangeably.  Also to avoid confusion the Control Display Unit (CDU) is the keypad used to interface with the Flight Mode Computer (FMC) that forms part of the Flight Management System (FMS).

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 at the time of writing uses U10.8 A. 

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 a route has been programmed in the CDU, and is active.  VNAV must be selected for either intervention mode to function.

  • 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 Nomenclature 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.

Table 1:  FMA displays observed when Altitude and Speed Intervention is engaged

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.

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 (download button at bottom of article) 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 will remain armed.

Scenario

The aircraft is flying at FL150 (15,000 feet) at 275 kias.  The FMS has an active route (Company Route) that includes altitude and speed constraints (in the LEGS page of the CDU). 

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

(i)     LNAV and VNAV will be active;

(ii)    The FMA will display MCP SPD / LNAV / VNAV PTH or VNAV ALT;  

(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 vertical altitude of the aircraft.

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

Speed Intervention (SPD INTV) button

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 into the speed window on the MCP the new speed requirement of 240 kias. 

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 speed of the aircraft will be reduced to 240 kias.

If you cross check with the Cruise Altitude in the CDU (CRZ ALT key/TGT SPD), the CDU 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 become blank (no speed displayed) to indicate the VNAV is the controlling mode. 

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 dependent on whether the aircraft is ascending or descending, and whether there are active restrictions (constraints) programmed for waypoints (U10.8.A).

In level flight, with 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 altitude window on the MCP the new altitude (FL120). 

CDU cruise page showing 12000 in scratch pad.  Selecting line select 1 left (LS1L) will update the CDU to the new Flight Level

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.  

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 descent speed) 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 the Flight Level to the lower altitude – this is normal and not a fault.  

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

Important Points:

  • 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.

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

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 automatically update the Flight Level 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 dialled into the altitude window of the MCP.  For VNAV to be active, the Cruise Altitude in the CDU and the altitude set in the MCP must be identical.

  • ALT INTV takes precedence over VNAV.  The VNAV annunciator on the MCP will remain illuminated and  VNAV will be in armed mode (when ALT INTV is selected).

  • To determine if VNAV is the active mode (or not) the FMA display must be consulted – not the annunciator light on the MCP.

  • U10.8A bring some important changes from earlier U releases.  If there are no altitude restrictions, pressing ALT INTV will automatically update the altitude in the CDU to the lower selected altitude.  However, if an altitude restriction is present the lower altitude will not be updated.

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 CDU.  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 Points:

  • When a Flight Level of a higher altitude is dialled into the altitude window and ALT INTV selected, the new Flight Level will be updated in the CDU.

  • U10.8A bring some important changes from earlier U releases.  If the selected MCP altitude is BELOW any altitude restriction, then that restriction will be DELETED.  Also, altitude restrictions will be DELETED if they are between the current altitude and the selected MCP altitude (when ALT INTV is pressed).

  • If ascent and descent do not function correctly. In the first instance consult the FMS software for the U version in service.

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 fail to update the MCP to the next lowest altitude constraint, then the aircraft will 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 command 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 from 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 that once the V-Path is intercepted, the Flight Director (FD) 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 CDU (INDEX/PERF/CRZ ALT), or (CRZ key/CRZ ALT) and press EXEC.

Procedure (ii) manually changes the Cruise Altitude (CRZ ALT) to the lower altitude in the CDU.  This causes the command logic to switch from the logic that commands Altitude Intervention to the logic that commands altitude in thr FMC.  The aircraft will not pitch and will be stable.

The developers at ProSim-AR are continually tweaking these variables.  In future software releases (post version 221.b12) this issue may well be rectified.

Final Call

There are many of reasons an aircraft will need to alter altitude and/or speed; be it to divert around a localized weather event, or to abide by an Air Traffic Control directive.  Whatever the reason, often the changes are short-lived and a return to the original altitude/speed constraint imminent.

In these situations Altitude and Speed Intervention enable the aircraft to easily and quickly transition between Flight Level changes whilst VNAV is active.   Furthermore, the use if this functionality can minimise the 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 Altitude and Speed Intervention functionality of the Boeing 737.  I also have documented "work-arounds" should VNAV not function as anticipated. 

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