B737-800 NG Flight Mode Annunciator (FMA)

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oem Flight Mode annunciator (737-800)

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).  



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.

figure 1: common mode annunciations that the FMA can display.  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). the pitch mode FOR column and CWS display are not populated. 

ERRATUM: ILS, SINGLE CH and IDLE HAVE NOT BEEN INCLUDED WHEN THEY SHOULD HAVE

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.

Video

Boeing 737 ILS CAT IIIa Autoland PFD demonstrating FMA.