The FD is guidance (innerloop commands) to the pilot as a result of the outerloop computations. With only FD engaged AP off the pilot is made part of closing the outer loop (e.g, Flight control movements) to feed the innerloop
This sums it up nicely:
Outer Loop:
Provides the autopilot with navigation (guidance) function. It adds the intelligence to the process (e.g. tracking a radial, holding a speed, climbing a VNAV path) The outer loop tells the inner loop what pitch, roll or yaw to hold for the maneuver, then the inner loop executes this. All the “intelligence” involved is mainly done in this loop.
Examples of outer loop autopilot operating modes:
Roll modes:
HDG (Select & Hold)
Nav Track (Track Hold)
VOR/LOC (Capture & Track)
Lateral Navigation (LNAV)
FMS Lateral Navigation
Pitch (flight path) modes:
Altitude (Select & Hold)
IAS/Mach (Hold)
Level change
Altitude Acquire (Capture)
Vertical speed
Glideslope (Capture & Track)
Vertical Navigation (VNAV)
FMS Vertical Navigation
Flare
FPAH (Flight Path Altitude Hold)
Combines Roll and Pitch modes:
Approach
Go-Around
Control Wheel Steering (CWS)
I still use mine in the Jetprop via the Aspen display almost every flight that involves either manual control in the upper flight levels where the sensitivity of the controls is very high, or the manual portion of an instrument approach.
I also use it to cue up the go around pitch when flying missed.
A much easier to interpret visual cue than relying on raw data. I would not be comfortable flying the Jetprop without it, however it would be overkill in my Tecnam where everything happens much more sedately. I think it comes into its own on higher performance aircraft.
E
