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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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Thursday
Sep182014

B737-800 NG Flight Mode Annunciator (FMA)

Automatic Flight System - Background

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 can be explained as follows:

LEFT:  B737-800 NG 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 displayed on the MCP.  While some annunciations are straightforward and only illuminate when a function is on or off, others can be confusing, for example VNAV.

Flight Mode Annunciator (FMA)

All Boeing aircraft are fitted with a Flight Mode Annunciator (FMA) of some type and style.  The B737-800 NG FMA is located on the Captain and First Officer side PFD 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 an operational function.

The FMA is divided into three columns and two rows. The left column relates to the A/T while the center and right hand column display roll and pitch modes respectively.  The upper row indicates modes that are operational while the lower row indicates modes that are armed.  Operational modes are always coloured green while armed modes are coloured white.  Below the two rows are the A/P Status alerts which are in larger font coloured green, and the Control Wheel Steering (CWS) displays which are coloured yellow.

When a change to a mode occurs (either by by a flight crew or by the AFS), a mode change highlight symbol (rectangle) is displayed around each mode annunciation for a period of 10 seconds after each engagement.  Depending upon which flight avionics suite is being used, the time that the rectangle is displayed may vary between 2 and 10 seconds.  According to the Boeing manual the default time should be 10 seconds.

The below table and image (ProSim737 avionics suite) indicates the various mode annunciations that the FMA can display.  The 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.

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Reader Comments (5)

More stuff I did not really know. Thanks for putting the time in to make a post. Your site is great! James

September 19, 2014 | Unregistered CommenterJames Maloney

I do not understand the reason for this site..... What does the user get from all this. I want to know how to make a simulator not all this other junk that is useless information that I can read elsewhere

September 19, 2014 | Unregistered CommenterWITHHELD

This is something that was always hazzy with me - I now find it easier to understand what VNAV is doing! It tells you on the FMA. I had not really noticed this previously. Thanks Gary

September 19, 2014 | Unregistered CommenterGary Williams

Very clear since I learn more and more about the Boeing 737 in general. I fly the BoeingNG 737-600 from Ifly to learn and understand the systems.
Specially VNAV puzzles me for sure. Since it seems like automation as LNAV, but it sure does work very diffently. Still not sure despite the explenation above what the exact relation is between the FMC and VNAV. And in flight it work in a difficult way, and acquires much intervention from the FMC.
I assume it is how Boeing designed it, since in real avation altitude demands can chage often. Nevertheless, LNAV seems constantly connected to the MCP programming, while VNAV is not. Still puzzles me..................

December 7, 2015 | Unregistered CommenterFrank

Hi Frank

You are not alone - VNAV can be confusing.

For VNAV to operate you must have a route defined in the CDU. Additionally a few other conditions need to be met such as the correct altitude set in the MCP altitude window and in the CDU.

The relationship between VNAV and the CDU is one of information dispersal. The CDU provides VNAV with the constraints that VNAV requires to be able to establish either VNAV speed or VNAV path. The constraints come from the data that the pilot enters into the CDU or by predefined constraints established in the route (for example, an RNAV approach).

LNAV follows the course (direction) as defined by the route that can be viewed in the LEGS page.

I will posting another post of LNAV, VNAV in a few weeks.

Cheers, F2A

December 7, 2015 | Registered CommenterFLAPS 2 APPROACH

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