In many FS9 panels, the Flight Director (FD) is the master control for speed (IAS or Mach). Simply turning off the outopilot has no effect on the FD
bill
Let me add a few things here.
In real life, the FD is NOT associated with any autothrottle system, nor is it designed to be. The only way the FD can be used to control speed is by obeying the pitch commands, and that is only valid in climbs and descents. You will see this in many smaller aircrafts and RJ's that does not come with thrust management.
...But this is just half the thruth, see the AP will work even if the FD is turned off, weird huh. Because what you turn off is the visible command bars, not the actual autoflight function. What really drives the AP is the FCC -flight control computer. The FCCs - usually there are more than one - positions the FD command bars and send control commands to the pitch and roll hydraulic servos (which operate the actual flight controls)
Saying that the FD is the master control for speed is not something I agree with, you can "shut off" the FD and still have speed maintained by either the A/T or the AP (by pitch commands)
The AP and FD are the so called AFDS (autopilot flight director system)...and the AFDS simply consists of two or more FCC's. The Auto Flight System (AFS) is the AFDS and the A/T...confusing eh