The Autopilot Flight Director System (AFDS) is located on the Main Instrument Panel (MIP). There are two identical units; one situated the Captain-side and the other on the First officer-side.
The AFDS is one of several components belonging to the Automatic Flight System (AFS) and is also referred to as the autoflight annunciator and autopilot/autothrottle indicator. The FMC annunciator is often referred to as the FMC alerting indicator.
The purpose of the unit is two-fold; to provide the flight crew with a visual warning of disengagement of the Autopilot and Autothrottle, to an alert on the FMC, and to enable the resetting of and testing of the unit (light test).
The unit has two annunciation colours, red and amber in either a flashing or steady state which correspond to either an alerting or advisory messages. Red precedes Amber in the level of importance. The A/P and A/T annunicators have dual colour capability while the FMC annunciator displays only amber.
This unit was removed from an United Airlines Boeing 737. On inspection, it was observed that the toggle was slightly bent. The bent toggle may have been the reason why the part was scrapped; it failed certification. The toggle was easily straightened.
Conditions for Operation
There are four operating conditions:
1: Autopilot (A/P) Disengage Light
The annunciator will flash RED if either the autopilot or autothrottle is disengaged. The former will also trigger the A/P disengaged tone (whoop, whoop, whoop). To extinguish the flashing light and reset the unit, the flight crew must push either of the two annunciators (A/P P/RST or A/T P/RST) or press the yoke disengage switch twice.
The annunciator will illuminate a steady RED in any of the following conditions:
The stabilizer is out of trim below 800 feet RA on a duel channel approach
The ALT ACQ mode is inhibited during an autopilot go-around (is stabilizer not trimmed correctly)
The disengage light test switch is held in position 2, or
The automatic ground system fails.
The annunciator will illuminate flashing AMBER when the autopilot automatically reverts to CWS pitch or roll mode while the in Command (CMD). To extinguish the light, press either the A/P P/RST annunciator or press another mode of the MCP.
The annunciator will illuminate steady AMBER when the light test switch is held in position 1, or when a downgrade in autoland capability occurs.
2: Autothrottle (A/T) Disengage Light
The annunciator will illuminate flashing RED if the autothrottle (A/T) is disengaged.
The annunciator will illuminate steady RED if the light test switch is held in position 2.
The annunciator will illuminate flashing AMBER to indicate an autothrottle airspeed error exists under either of the following conditions:
The annunciator will illuminate steady AMBER if the light test switch is held in position 1.
3: Light Test Switch
The AFDS is not connected to the main light test toggle; therefore, it’s equipped with its own light test switch. The central spring-loaded toggle is used to determine if the unit is operational.
If the toggle is pushed toward TEST 1, it will illuminate the autopilot, autothrottle and FMC alert annunciator in a steady AMBER colour. The FMC alert is delayed a few seconds.
If the toggle is pushed toward TEST 2, it will illuminate the autopilot and autothrottle annunciator in a steady RED colour and the FMC alert annunciator will illuminate steady AMBER. The FMC alert is delayed a few seconds (see last photograph this page).
4: FMC Alert Light
The FMC P/RST will illuminate steady AMBER when an alerting message exists on the CDU, the fail light on the CDU is illuminated, or the test switch is in position 1 or 2. To extinguish the annunciation the flight crew can either clear the message from the CDU scratchpad or push the FMC annunciator.
FCOM - Simple yet Confusing
The above information has been interpreted from official documentation from Boeing and whilst straightforward to understand, can appear confusing because of to the repetitious nature of the information and the similar functionality of the unit.