After the primary flight controls—elevator, rudder and ailerons on conventional airplanes—the most frequently used secondary control is pitch trim. That’s because a typical flight in any airplane involves a range of speeds, and pitch trim is used to minimize control forces throughout the operating envelope. Even if the pilot can supply that force, it’s fatiguing. Along the way, a popular feature is powered pitch trim, typically using electric-powered servos and installed as part of a multi-axis autopilot/flight director system.
Of course, a powered trim system brings with it additional failure modes, and its presence means the system is easier to misconfigure, adding to maintenance and inspection requirements, and the risk of catastrophic failure, including what some would call a worst-case scenario: the powered pitch trim system fails in a manner that commands the nose down and it can’t be disabled. This is not unlike what happened to two Boeing 737 MAX jetliners in late 2018 and early 2019, resulting in a worldwide grounding, although their failure mode was more complex than the system aboard typical personal airplanes. While the systems are designed to be capable of being overpowered by a pilot, that’s not always the possible. Disabling them and reverting to manual trim—where someone in the cockpit uses a large wheel, crank or lever to minimize control pressures—is the usual cure.
