The lack of manufacturer written advice, limitations, cautions, or warnings (written or aural) about autopilot response to manual pilot control inputs meant that pilots may be unaware that their actions can lead to significant out of trim situations, and associated aircraft control issues.
Response to the safety issue by Cessna Aircraft Company
The functionality described is true of virtually every autopilot [fitted to FAR 23 certified aircraft]. The type of autopilot behavior being discussed is covered in chapter 4 of the Federal Aviation Administration’s Advanced Avionics Handbook (FAA-H-8083-6, attached). The first page of the chapter discusses autopilot concepts and in the “How to use an autopilot function,” says, “Allow the FD/autopilot to accomplish the modes selected and programmed without interference, or disengage the unit. Do not attempt to “help” the autopilot perform a task. In some instances, this has caused the autopilot to falsely sense adverse conditions and trim to the limit to accomplish its tasking. In more than a few events, this has resulted in a total loss of control and a crash.”
ATSB comment/action in response
Knowledge is imparted in a number of ways, which include reading operating manuals, conducting training, communicating with experienced pilots and learning from one’s own experiences.
Advanced avionics and autopilot systems are now fitted to a significant number of primary training aircraft, such as the Cessna 172S (VH-ZEW). The level of knowledge that individual pilots have in this environment starts at a low base level and builds over time. The ATSB contends that student pilots are not likely to be aware of issues surrounding manual manipulation of the flight controls with the autopilot on. It is therefore important to implement methods that enhance pilots’ awareness of the issue, including aircraft and avionics systems operating manuals having the requisite limitations, cautions and warnings in place.
It is clear from the content of the FAA handbook that they believe a warning about the issue is required. Although the handbook provides very good educational material, it does not take the place of aircraft operating manuals as a reference guide.
The G1000 avionics and GFC700 autopilot systems are fitted to numerous aircraft types. Some flight manuals include limitations, cautions and warnings and some do not. Additionally, there were no audible warnings provided for mistrim to enhance pilot awareness in the G1000 avionics system with the GFC 700 autopilot fitted, despite that being a feature of the superseded autopilot system.
As a result, the ATSB has released the following safety recommendation.