Flight Controls, Fly-by-Wire and Autopilot Actuation
Primary and secondary controls
Primary controls are elevator, aileron and rudder (pitch, roll, yaw); secondary are flaps, slats, spoilers and trim. Control surfaces are actuated by cables and pulleys, push-pull rods, or hydraulically powered actuators. Reversible systems let aerodynamic forces feed back to the pilot; irreversible systems use artificial feel to simulate control force because the actuator isolates the pilot from the surface loads. Feel is provided by springs, q-pot (dynamic pressure) bellows, or electronic feel computers.
Fly-by-wire
In fly-by-wire, pilot inputs are converted to electrical signals processed by flight control computers that command hydraulic actuators. Normal law provides protection: pitch attitude, bank angle, high-speed and high-AOA (alpha) protection, and stall protection by limiting angle of attack. Alternate and direct laws degrade these protections. The trap is assuming protections always exist: in direct law, the aircraft behaves like a conventional aircraft with no envelope protection. Sidesticks are not mechanically linked; a priority button resolves conflicting inputs.
Autopilot and autothrust
An autopilot has aileron, elevator and rudder servos; it may be coupled to the flight director. Autopilot modes include heading, altitude hold, vertical speed, and approach (localiser and glideslope). Autothrust controls thrust for speed or Mach. Disengagement can be automatic (e.g. excessive trim, power loss) or manual; know the aural and visual warnings.
Worked example
If a q-feel system uses dynamic pressure to increase control force, then at high speed and low altitude (high q) the force per unit deflection is greatest. A question asking where control forces are highest is answered by highest q, not highest altitude.
Traps
Confusing trim (removes steady force) with feel (creates force), and thinking spoilers are primary roll control when ailerons fail - spoilers assist roll but are not full replacements on all types.