The faster reaction time of computers allows control surfaces to be made smaller, reducing overall weight. System reliability and maintainability is improved.

Electronic control gives control augmentation and envelope protection which would be more difficult to provide in a mechanical system, such as:

bank angle protection;

turn compensation;

stall and over-speed protection;

pitch control and stability augmentation;

thrust asymmetry compensation.

A gradient control actuator is attached to the control-column-feel units, proportionally increasing column force during increase in airspeed. Additional actuators back-drive the control column, wheel and pedals, in response to autopilot commands.

The PFCS has three operating modes: normal, secondary, and direct:

In normal mode, the PFCs supply all commands through the ACEs, with all enhancement, envelope protection and ride-quality features.

In automatic secondary mode, the PFCs still supply the ACEs, but envelope-protection functions, may not be active because of failures, eg: a loss of air data.

In direct mode, the ACEs do not take commands from the PFCs, but decode commands directly from the cockpit controls, in closed loop to the control-surface actuators. This mode is also automatic upon total failure of PFCs, internal ACE failures, or loss of databuses. It can also be selected manually.

The yaw control's gust suppression reduces aircraft tail-wag by applying rudder to oppose fin movement. A wheel/rudder cross-tie reduces side-slip during banked turns by adding small amounts of rudder.

Thrust-asymmetry compensation, (TAC) automatically applies up to 10¡ rudder when thrust asymmetry exceeds 10% of rated thrust, to nearly cancel the yawing moment. TAC operates at all airspeeds above 80kt and on the ground during take-off, except during thrust reverse.

The outboard ailerons and spoilers 5 and 10 are locked and faired above an airspeed dependent upon speed and altitude, roughly corresponding to flaps-up speed.

Source: Flight International