Longitudinal Autopilot

The job of the longitudinal autopilot is to use the throttle, T, and elevator deflection, \delta E, to control the airspeed, V, and altitude, z (or sometimes h).

But there's some redundancy here, since the throttle and the elevator deflection can influence both the airspeed and altitude. We resolve this redundancy by dividing flight into four "zones":

We can model this as a state machine, where transitions between states happen when the vehicle crosses between zone boundaries.

Note that this diagram uses \delta t instead of T for thrust. The \delta t ^ * that you see in the second box is a feed-forward value for the thrust based on trim calculations (which you learned how to do in a previous lesson).

These four zones require four separate control loops (though there isn't an exact 1 to 1 mapping between zones and control loops). You can see these loops in the diagram below.