- The AT is a complementary system to the AP and may be used with or without the AP.
- The 737 is certificated for autoland using dual APs; the AT, if engaged, will retard at 27ft.
- The 737 is also certificated for single AP approach to a minimum approach altitude (80 ft … cert authorities/737 versions may differ). The AP has to be disengaged for landing, but the AT may remain engaged and will retard at 27ft. Presumably this option enables a single AP MEL auto approach.
- If a pilot elects to fly a manual approach with AT, it may remain engaged and will retard at 27 ft.
Why have so many AT options … ? Because over the years pilots thought that it would be a good idea – it reduced workload, made life easy, wouldn’t it be nice if … etc, etc, and Boeing attempted to satisfy the widest possible market and retain a good MEL – which is also operator driven.
It appears that it is the method of interfacing the inputs and components of the FGS to provide these options, which has left a loophole in operations in the event of malfunction (but not necessarily for a failure).
You can keep some of the people happy all of the time, but … … … and that’s the point where opportunity for mistakes, misinformation, and false knowledge arises.
Speed reversion is an alternative pitch mode providing limited protection from abnormal low speed. It is normally engaged automatically from another mode. It is not available from GS.
GS mode is a AP ‘path on elevator’ mode, where the AT controls the selected speed datum. A speed reversion would require AP ‘speed on elevator’ mode, where a climb or descent is determined by the thrust setting. Reversion from GS would require a high thrust setting to ensure a climb. This logic is already built into the TOGA mode which provides a pitch up and an AP ‘speed on elevator’ climb, commanding the appropriate thrust setting on the AT.
Speed reversion is not an Airbus 'alpha floor' type of function, which is a higher order safety protection and operates independently of the AP/AT.