cf6,
Firstly I must thank you for starting a very good thread. It has produced many a theory on the AT system. Some on the money, some close and some nowhere near the ballpark so to speak.
Anyhow after much digging around and harassing some of our engineers, I may have had a "Eureka" moment. So here's my two cents worth.....
On approach at 25ft RA the AFDS sends a flare retard signal to the AT and causes the AT servo to drive the throttles to the aft stops (interestingly this happens during a MANUAL or AUTO landing). SPD will be replaced by IDLE on the FMA.
At <5ft RA TOGA is inhibited (but AT is still armed) until >5ft RA or reverse thrust selected (and AT disarmed). If you do not select reverse thrust and use only braking, the AT mode will remain at IDLE and will not resume the SPD mode.
For the SPD mode to engage several conditions must be met. These are....
- AT armed
- AIRCRAFT IN THE AIR>400ft
- Thrust limit mode not takeoff
- Go-around mode not active
- FLCH mode not active
- VNAV mode not active
- G/S mode not active
- Select SPD mode on MCP or
- Default SPD selection, for example, exit of FLCH or VNAV.
Hope this helps.
McHale.