Proficient Pilot

Fly by wire

September 1, 2003

Retired TWA captain Barry Schiff has logged more than 26,000 flight hours.

Armand Jacob, an experimental test pilot at the Airbus factory in Toulouse, France, gestured for me to climb into the left seat of an Airbus A318 for what was to be my first experience with a fly-by-wire airplane.

Fly by wire means that the sidestick controller (in this case) is not connected directly to the flight controls with cables. Instead, the controller is loosely analogous to a rheostat that electronically signals the pilot's hand movements to computers that command the control surfaces to move in such a way as to make the airplane do what the pilot wants.

Because of feel units that provide an artificial feel to the sidestick, there is no way to detect that you are flying by wire instead of by cable. The airplane responds well, has no control-system lag, and feels just like a conventional airliner.

The advantage of fly by wire in an Airbus is that the computers increase performance efficiency and are programmed to prevent extreme attitudes.

The computers allow a respectable roll rate of up to 15 degrees per second and prevent banking in excess of 67 degrees. It is difficult to imagine an airline pilot ever needing more than this. Release the sidestick and the airplane automatically rolls out of a steep turn and maintains a 33-degree bank angle (or any lesser bank angle established by the pilot). Pitch attitude is maintained during normal turns (33 degrees of roll or less); the pilot does not have to add elevator backpressure.

Haul back on the sidestick and the nose rises sharply, but you cannot exceed plus-2.5 or minus-1 Gs no matter how hard you try. Nor can you violate the barber pole (a turbine airplane's redline). As speed approaches V MO (or M MO), the computers raise the nose automatically to prevent trespassing beyond this limit despite pilot efforts to the contrary. Similarly, you cannot stall the airplane. Although the airplane can be flown on the verge of a stall, the computers do not allow the wings to reach their critical angle of attack.

At one point Jacob told me to imagine that we were breaking out of a cloud and into visual conditions only to discover that we were about to fly headlong into a mountain. "Pull the controller all the way back and as far to either side as quickly as you can," he admonished. "Go, go, go!" I pulled the controller into the right, aft "corner," and the Airbus responded with a 2.5 G pull-up while simultaneously rolling into a 67-degree banked turn. The engines spooled up to maximum, and had the spoilers been deployed, they would have retracted automatically. The airspeed bled to where I was certain that we were going to stall, then stabilized slightly above the speed at which the coefficient of lift would be at a maximum (the stalling angle of attack).

In a conventional airplane, such an escape maneuver is performed manually while flying by feel and with just enough backpressure to keep the airplane on the verge of a stall (as indicated by the onset of the stickshaker stall-warning indicator). I do not believe that there is any way in which such a maneuver performed manually could be as efficient and provide as much obstacle clearance as when flying by wire.

Similarly, fly by wire provides improved safety margins when attempting to escape from a serious wind shear encountered shortly after liftoff.

It is natural to wonder what would happen if the airplane were to suffer a total electrical power loss (extremely unlikely). The fly-by-wire system, of course, would be kaput; conventional roll and pitch control would be lost. This is why elevator trim and rudder are manually controlled with cables. Test pilots are required to demonstrate landing the Airbus using only an elevator-trim wheel and the rudder.

During normal flight, it is unnecessary to use elevator trim, which is why there is no electric trim switch. The autoflight system detects when the elevator is deflected with respect to the trimmable horizontal stabilizer and automatically adjusts the stabilizer. Voila! A constant-trim system.

Another feature of the Airbus is that either pilot can depress his takeover switch to prevent the other from having any control whatsoever, a handy feature in case of pilot incapacitation. A sidestick priority light (an illuminated arrow on the glare shield) points to the pilot in control.

It took time to adjust to the autothrust system. When power increases automatically (such as when maintaining a constant airspeed while raising the nose), the engines spool up, but the thrust levers don't move. Fore and aft throttle movement would provide a more realistic sense of what the engines are doing.

The sidestick controllers on Airbus aircraft are operationally similar to those found on some newer general aviation airplanes, such as Cirrus models. A major advantage is that they allow pilots to have an unobstructed view of the instrument panel. One quickly gets used to sidesticks, but I would have preferred conventional control sticks (old habits die hard).

Undoubtedly, fly by wire will eventually debut in light airplanes. Some pilots will resist it because fly by wire precludes them from exercising total control and limits their freedom to bend the airplane and break their necks. The flip side is that fly by wire has the potential to prevent many accidents caused by control loss and induced structural failure.


Visit the author's Web site ( www.barryschiff.com).