On Saturday, Dec. 11, 2021, I had the pleasure of trying out the ThrustSense Autothrottle system for the first time. IS&S is the abbreviation for Innovative Solutions and Support, the company that designed, tested and certified this system that adjusts power lever position to maintain a torque value or an airspeed value that the pilot desires.
Textron Aviation has made this system standard equipment on all King Air 260s and 360s, the only civilian King Air models currently being manufactured. I expect that with this factory “approval” we will be seeing many more ThrustSense systems installed in the retrofit marketplace.
At KAG IV – the fourth King Air Gathering – held at Fredericksburg, Texas, in 2019, I rode in the right seat while the system was being demonstrated. I was favorably impressed but felt that I knew very little about the system based on this one brief flight.
As most of you know, I have been involved in numerous King Air training videos that may be found on the King Air Academy (KAA) channel of YouTube. Kevin Carson, manager of KAA, does the hard work of filming and editing while I get to have fun flying and teaching about various King Air particulars. Based on these videos, IS&S Director of Sales Larry Riddle approached Kevin with the idea of making a video of us using the ThrustSense system. Kevin and I thought it was a great idea since it would expose the system to a wider audience, would serve as one more endorsement for the King Air Academy, and be a useful training aid for all future ThrustSense users.
Larry offered a time at which the IS&S demonstrator and flight test King Air B200GT (N313BM, serial no.
BY-60) would be available to arrive at Deer Valley Airport (KDVT) in Phoenix where KAA is located. He also emailed us the Pilot’s Guide and Quick Check Reference for the system so that I could be somewhat prepared before starting the flight phase. Eric Smedberg, the IS&S test and demonstration pilot, would be the PIC in the right seat, I would fly the airplane and experiment with the IS&S system from the left seat, while Kevin would be filming from the seat behind the co-pilot. We were happy to discover that, since this airplane is their demonstrator, Kevin’s seat was now installed in a forward-facing position and the cockpit/cabin divider had been removed. Both changes made his filming considerably easier.
I created a flight test plan for what I wanted to accomplish in the airplane. The plan was to fly from KDVT to Winslow (KINW), then to Flagstaff (KFLG) and finally return to KDVT. As it turned out, we decided to refuel at KINW and canceled our leg to KFLG. As is so typical of Arizona, the weather was perfect and we made the entire flight in visual conditions without an IFR flight plan. (You can examine our tracks on the FlightAware site.)
The control and display panel for the ThrustSense system is referred to as the ISU: Integrated Standby Unit. It replaces the normal ESIS (Electronic Standby Instrument System) display that comes standard with the Collins Pro Line 21 and Pro Line Fusion systems. That’s where the “Standby” part of the name comes from. Like the original ESIS, there is a switch for power on the pilot’s left subpanel and it has a backup internal battery. The ISU is mounted just above the radio tuning unit near the center of the instrument panel. It includes both an AI and HSI display as well as a power button, menu button and rotary select knob.
In my opinion, one of the best features of the ThrustSense system is that the potential for PLM (Power Lever Migration) is eliminated. Yay! The actuators that move the power levers provide their own friction and it never needs to be adjusted or changed by the pilot. Because of this, I had a concern that when I was using the power levers manually, not using the autothrottle system, they would be too stiff for my taste. I found that not to be the case: they moved quite easily and very similar to the non-modified levers with medium friction. As I have written in the past, when one or both power levers spring back as the pilot reaches for the landing gear handle after takeoff, the result is usually rather funny: The pilot sees what has happened, pushes the levers back to where they belong, tightens the friction knobs properly and continues the flight. But woe be to the airplane and its occupants if the power lever movement is not observed and the pilot assumes an engine failure. The retardation of the power lever turns off autofeather and the windmilling propeller, usually combined with reduced power on the right engine as well as the left, will see the airplane reaching stall speed quite rapidly when the pilot maintains the +10° pitch he was taught to use in a takeoff engine-loss situation.
With the engines not yet started but with the airplane’s battery and the ISU turned on, the system performs two internal tests. The first of these takes about 30 seconds and there is nothing for the crew to observe. But when the 30 seconds elapse, the second self-test causes the system to move both power levers from idle to full forward and then back to idle. You can see why this test cannot be done with the engines running, eh?!
Once the engines are started, the autothrottle system basically is out of the picture during the normal taxi and run-up procedures. The power levers are moved forward as needed and into and out of the Beta range as normal. To arm the system for takeoff power application, the pilot hits the power button on the ISU and the ISU indicates that the system is armed. Once lined up on the runway, hitting the GA button puts the system in the takeoff mode and the power levers evenly and smoothly advance. “Look, Ma! The levers have an invisible hand moving them!”
The unit receives the inputs that tell it OAT and airport elevation and the torque is adjusted to ensure that the “Minimum Takeoff Power” requirement is met for those conditions. The -52 PT6s on the B200GT are so flat-rated that there is almost no takeoff situation in which the full 2,230 ft-lbs of torque is not used.
One minute after liftoff, the system changes from Takeoff to Climb mode, ensuring that the climb ITT limit is always observed. I found it easy to get somewhat complacent in watching the torque and ITT engine instruments since the ThrustSense system always took care of them perfectly for me.
I intentionally departed a bit northwest of KDVT to keep us under the Class B airspace for a longer period of time. As the autopilot leveled us at 4,500 feet (Class B started at 5,000 in this area) I switched the system into speed mode by pushing the knob on the ISU and dialed in a speed of 195 knots, mindful of the 200-knot “below Class B” restriction.
Now about setting the speed: Golly, what a slow learner I was! The Pilot’s Guide that had been sent to me and which I had studied quite thoroughly told me that the knob on the ISU would be used to set speeds just as it is used to set desired torques: The two modes available. However, in the time after the manual had been written and before this demonstration flight, a software change had been made. Now, instead of setting airspeed using the ISU’s knob, it was done using the speed knob up on the Pro Line 21 autopilot control panel near the glareshield. I think this makes a lot of sense and is a better, more intuitive, way to go. But, damn, I must have reached for the wrong knob 90 percent of the time! Eric was very understanding and patient with me, but I am sure he was less than impressed with my fumbling.
Once I finally had the 195 knots dialed in (using the correct knob!) the power levers moved back and set the proper value that kept us legal under Class B. When clear of the Class B overhang, I then put the autopilot into Pitch mode, dialed +8° nose up, and returned the autothrottle system to Climb mode. I could also have used IAS or FLC (Indicated Airspeed or Flight Level Change) mode, but I find that a constant pitch attitude mimics the POH’s climb speed schedule from Sea Level to the limit … FL350 in the case of this B200GT! Depending on aircraft weight and engine power, this attitude, for all King Airs, is between 7 and 10 degrees.
Those who have read my books, viewed some videos, or took training from my old Flight Review company or KAA, know the importance of “magic numbers.” It is quite surprising how certain torque values will yield certain indicated airspeeds for different configurations, regardless of altitude or OAT. The only thing that has a noticeable effect on the torque value is aircraft weight. We leveled off at 13,500 feet and I dialed in 1,000 ft-lbs of torque, the number that usually yields 160 KIAS, clean configuration. As the autopilot held altitude, we came out about 10 knots fast but then realized that there were only four of us onboard and about 1,000 pounds of fuel so we were quite light.
Next, I extended approach flaps and landing gear. As expected, we slowed to 120 KIAS. Then, simulating a precision approach, I dialed in a 600 fpm descent, which told the ThrustSense system to hold 120 knots. As expected, the resulting torque came out to near 700 ft-lbs, the magic number for this situation. I found there was very little “searching” and that the power levers made small corrections smoothly and accurately, just as a human pilot would.
We cleaned up gear and flaps, dialed in a torque of about 2,000 ft-lbs, went back to the +8° pitch, and climbed up to near 15,000 feet. Next, keeping that high torque value, I aggressively selected a pitch attitude of about -5° and we started into a rapid dive. Sure enough, as expected, when we got close to VMO (Maximum Operating Velocity), the power levers moved aft and our speed stabilized a few knots below redline.
I have not yet mentioned propeller speed. As is typical for members of the King Air 200-series, takeoff is at 2,000 RPM, climb is at 1,900 and cruise is at 1,700. (Climb and cruise are usually 100 RPM less with the Raisbeck props.) I was pleased to discover that when transitioning from takeoff to climb, I could manually bring the prop levers back while automatically the power levers came aft a touch to prevent torque from exceeding the limit. Without the autothrottle system, we always have to ensure that we have a bit of a torque “cushion” below redline by reducing the power levers a tad, before pulling the prop levers back. That’s not necessary any longer with ThrustSense.
Eric programmed the FMS for the VOR or GPS RWY 11 at Winslow and we tracked to the IZSAH IAF to begin. I “played” with the autothrottle system, dialing in various torque and airspeed values as the autopilot flew the arc that took us from IZSAH to DMJCH where we turned inbound. The INW vortac is the FAF for this approach so as the autopilot leveled us at the FAF altitude of 6,200 feet I dialed in a speed of 120, with approach flaps and gear down. I then used the autopilot to control the descent from the FAF to MDA while the autothrottles held the speed. Even though it was severe clear, we pretended that we were still in IMC on the approach and at the MAP I depressed the GA button on the left power lever. There’s that invisible hand again! Both power levers came up smoothly to takeoff power as I pitched up into the flight director’s command bars, advanced props to full forward and retracted flaps and gear. (GA mode disconnects the autopilot, as you know.)
We decided to remain in the left traffic pattern for this runway so I hit the A/T (autothrottle) button on the right power lever which disconnects the ThrustSense system and I proceeded to manually fly and make the landing. We went in for fuel and a break.
On the way back to KDVT – after filming the power levers move through the self-test before start and making a circle around the famous Arizona meteor crater for Eric to have his first look – I checked the underspeed protection. I dialed in a torque – 300 ft-lbs – that was too low for us to hold altitude at a safe airspeed. As expected, as the airspeed neared stall speed, the power levers moved forward and held us 3 knots above “Stall Speed plus 10”… a nice comfort margin. I decided not to “play with” the VMCA power mitigation safety built into the ThrustSense system. Why? Because in every VMCA demonstration I have ever conducted in a King Air, the stall is always encountered before VMCA. If we have stall protection, we have VMCA protection also. Yes, if we execute a rejected landing from 50 feet above the runway with gear down and full flaps AND we have an engine flameout at the same time, then a loss of directional control could be encountered before stall. The fact that the ThrustSense system will reduce the good engine’s power so as to allow directional control to be maintained is a good thing … although we may touch down on or near the runway at least we will be right-side up!
Back at Deer Valley we requested and received permission for the RNAV (GPS) RWY 7R. We began from over BANYO intersection following the transition to the AZNUP IAF. I dialed in the various speeds that I wanted and did not turn off autothrottle until after selecting full flaps at about 500 feet AGL. It worked well.
I came away from this demonstration and practice with a high level of confidence in and respect for the ThrustSense system. Of course, pilots can manage their own power lever movement as they have been doing for eons. But this invisible hand certainly does reduce pilot workload noticeably. Engine limitations protection, overspeed and underspeed airspeed protection, VMCA mitigation, and having no worry about Power Lever Migration … what very nice icing on the cake this system provides.
Like me, I believe that if you try it, you will like it … a lot!