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.