by Michael J. “Mick” Kaufman
During EAA AirVenture Oshkosh 2024, there was an aircraft accident not related to the convention that claimed the lives of seven individuals that were members of a gospel singing group. Prior to the accident, the pilot reported to ATC that he was having autopilot issues with the Pilatus P-12. A friend of mine was visiting me during AirVenture and had flown his Pilatus PC-12 to Eagle River, Wisconsin from California, which created some discussion as to possible causes.
Several months ago, I wrote an article for my column entitled “Putting Your Autopilot On Probation.” During a recent flight test for one of my instrument students, the pilot examiner posed an autopilot question that was not answered correctly, which led to some research that proved interesting. We will probably never know the real cause of the Pilatus accident as there was not much left at the crash site to investigate. I have always stated in my column that I will not play “Monday Morning Aviation Quarterback,” as to what may have happened, but rather discuss some things we should be aware of that may save lives.
First, we need to read the supplement from the manufacturer as to the limitations of the autopilot, and follow these limitations until real time experiences show these limitations not to be correct. It can be confusing at times as there may be add-on modules installed by the avionics shop to get certain devices to talk with each other. There are also limitations placed on autopilot use. For example, the Garmin G500 autopilot is not authorized for use below 200 feet on an approach or engaged below 800 feet on a departure. There is also a description of the autopilot test sequence that is automatically performed on start-up. On my autopilot, the S-Tec 50, the autopilot test must be initiated by the pilot and should be performed on the ground prior to takeoff if the pilot intends to use it in flight. During the test, the autopilot locks the controls twice for about 10 seconds each, making it difficult, if not impossible, to move the controls during the test sequence. With a satisfactory test, a green light appears, meaning the autopilot is ready to use.
In reviewing the autopilot on the PC-12, there are a lot of safety features built in which could cause the autopilot to disconnect. In reviewing other situations and accidents in this airplane, this is not the first.
With so many different autopilots installed in the general aviation airplanes we fly, we could say that most autopilots, especially the legacy ones, have three motors controlling the flight controls called “servos.” There is a roll servo, a pitch servo and a trim servo, and for about 5K more, you can have a rudder servo refered to as a yaw damper. With this configuration, the pitch servo will work hard to hold the altitude it was programed to hold. Should the aircraft encounter vertical turbulence, the pilot will see the yoke moving in and out to hold the altitude, just like the pilot would do while hand flying. Should the forces on the yoke while flying on autopilot exceed a preprogramed limit, the pitch servo will ask the autopilot to engage the trim servo. In teaching instrument flying to new students, a test question is “Trim is used to set? (altitude), (attitude), (airspeed) or (groundspeed)?”
Assume we are flying along on autopilot in altitude-hold mode, and we encounter a severe updraft. The airplane will pitch down using the pitch servo and upon reaching its preprogramed limit, it will ask the trim servo for help. With the airspeed increasing rapidly and the limits on the autopilot exceeded, the autopilot will disconnect giving the occupants a wild ride. While flying in the mountains, I have encountered this situation often. It can be noticed by an unusually high airspeed indication, and it can last for several minutes. If uncorrected, it can exceed “Vne” speed, over “never exceed” speed.
The pilot can help correct this situation by reducing power to an idle, and in certain situations, disconnect altitude hold, let the aircraft climb, and advise ATC. I have never been a big fan of electric trim, so when updating my autopilot, I elected not to have it installed. With my airplane autopilot configuration, I have a light that illuminates telling me to trim up or down manually. Should I encounter a severe updraft and do not correct for the high airspeed by reducing power, the aircraft will climb once the limits are exceeded if I do not trim manually. Remember, we trim for airspeed, not altitude.
There are so many items to consider when looking at this tragic PC-12 accident when flying our personal airplane in instrument conditions using an autopilot. First, how do we disconnect the autopilot and/or the electric trim?
Most autopilots have a red disconnect button on the yoke (mine does not). Using the electric trim on the yoke disconnects most of our autopilots – the PC-12 does not. The PC-12 autopilot has a higher level of sophistication, such as G-sensors and dual angle of attack indicators, where either stalling the aircraft or high levels of turbulence will cause a disconnect.
There have been six accidents/incidents in the PC-12 with similar situations – turbulence, autopilot, and IMC weather conditions. I am slow to call pilot error as I have learned that there are situations where under certain conditions, no pilot could handle them.
For example, if the autopilot should kick off and the electric trim servo would continue to run until it reached its mechanical stop and continue to fight the pilot… just a possible speculation. If we should encounter turbulence to the degree where we are concerned, slow to “Va” maneuvering speed. (Hence, Velocity (V) of Acceleration (a), Va.)
It seems that so many of my articles in Midwest Flyer Magazine have centered around autopilots and avionics, but I am a geek for technology. If I was your instructor for your instrument rating or additional training if you were already instrument rated, you will learn flying by the numbers; pitch, power and airspeed; prioritizing tasks; and checklists and flow patterns before we touch on sophistication and avionics.
Learn everything you can about your airplane. Get an instructor to help you as I have seen too many avionics installations not done properly, as well as manuals and supplements that are not correct.
Until next time, please fly safe and often!
EDITOR’S NOTE: Michael J. “Mick” Kaufman is a Certified Instrument Flight Instructor (CFII) and the program manager of flight operations with the “Bonanza/Baron Pilot Training” organization. He conducts pilot clinics and specialized instruction throughout the U.S. in many makes and models of aircraft, which are equipped with a variety of avionics. Mick is based in Richland Center (93C) and Eagle River, Wisconsin (KEGV). He was named “FAA’s Safety Team Representative of the Year” for Wisconsin in 2008. Readers are encouraged to email questions to captmick@me.com, or call 817-988-0174.
DISCLAIMER: The information contained in this column is the expressed opinion of the author only, and readers are advised to seek the advice of their personal flight instructor and others, and refer to the Federal Aviation Regulations, FAA Aeronautical Information Manual, and instructional materials before attempting any procedures discussed herein.
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