The pilot workload reduction, particularly in a busy ATC environment, is dramatic. You don't realize how much attention it takes to adjust power to maintain a desired airspeed even though the autopilot is flying pitch and roll to stay on the desired course and altitude. But with an ATS, the workload drops to near zero with the autopilot engaged, and I estimate it is cut by at least half when hand-flying. There is no way to appreciate how much an ATS reduces pilot workload from the mundane chores and allows attention to the rapidly changing situation around the airplane until you fly with an ATS that performs as smoothly and precisely as AutoPower.
During climb, AutoPower transitions from airspeed to Mach hold automatically. When level, it changes to a cruise mode where the gains in the system are tailored to smooth operation, so it doesn't change power for very little variation in speed. The system holds Mach within one or two one-hundredths but does it very smoothly, unlike some earlier systems that would saw away on the throttles chasing every little variation. Gulfstream has been able to measure an improvement in efficiency because of the smoothness and precision of AutoPower and says G150 pilots will see increased range thanks to the system.
The interaction between AutoPower and the PlaneView autopilot system is much more complicated than, say, cruise control in a car. As any pilot who has advanced beyond the most basic training knows, pitch sometimes controls airspeed, but sometimes it is power that sets speed, and most often both pitch and power inputs are needed. The pitch-power relationship is probably most complex during descent.
When you start down, AutoPower transitions into descent mode automatically, and depending on what profile you have asked the autopilot to fly, it behaves differently. A common way to descend is in airspeed hold mode (flight level change), and in that case the autopilot will adjust pitch to hold the desired speed while AutoPower brings the throttles back to near idle. But, if you tell the autopilot to descend at a selected vertical speed, then the ATS must adjust power in concert with the autopilot to hold both target airspeed and selected vertical speed. If you ask for too much of both, the ATS and autopilot work together to retard the throttles and pitch the nose up to prevent exceeding the airplane's speed limits.
Back in the terminal area, AutoPower shifts to speed mode and holds the selected airspeed within a knot or two with smooth, but aggressive, movements of the throttles. Unlike we human pilots, the ATS computer is never distracted for even a millisecond, so its focus on airspeed control is uninterrupted. Because it spots a change in the airspeed trend instantly, it can make a quicker, and thus smaller, throttle movement than would be necessary for the human pilot who needs to spot the change, process the required throttle action and then move the levers. AutoPower makes almost continuous changes, particularly in turbulence, but they are so small that they are usually imperceptible unless you are watching the levers move.
It was an unusual weather evening in Savannah, with the wind at 1,500 feet blowing 32 or more knots, while the wind was only six knots at the surface. It wasn't really turbulent, but the air was, as you can imagine, stirred up. It was a good test of the system as we maneuvered for the approach with flaps and gear coming out, and AutoPower transitioned to each new configuration target airspeed perfectly. VREF target approach speed was 117 knots, so we selected 123 for the approach given the possibility of at least a little wind shear with the difference in wind velocity close to the ground.