In an F.B.W. ( Fly by Wire ) equipped plane, there are three Flight Control Laws, providing greater or lesser pilot assistance. Jean-Michel Roy, a test pilot for Airbus agreed to decipher these various modes for us in the development simulator.
Jean-Michel Roy, Experimental Test Pilot - Airbus :
"The Flight Control Laws are simply the relationship between the movement of the pilot sidestick controller ( joystick / control yoke in smaller 'planes ) and the aircraft's reaction. So, it's how I fly the 'plane. There are several flight control laws that depend on the integrity ( state) of the aircraft viz-a-vis its environment. When everything is functioning as it should, we are in Normal Law, when the plane enters an overstressed situation, then we enter Alternate Law, and when you have more severe failures, the Direct Law mode comes into action."
The application of these various control laws depends on the number of flight control computer failures. Right up to a computer failure, the airplane is under what is called Normal Law, i.e. in a secure protected flight. An easily recognisable mode visible on the cockpit's control panel.
Jean-Michel Roy, Experimental Test Pilot - Airbus :
"We have small green bars here, inclined at 67 degrees, indicating that the 'plane cannot exceed more than a 67 degree inclination. It's not only blocked, but also transposed to different axes. There are certain limits in the longitudinal control protection, which we cannot exceed, whether positively or negatively; we cannot exceed certain inclinations of the 'plane, which may lead to its loss of control, nor can we go below a certain speed.''
If now, two of the five onboard computers are found to be out of service, the aircraft will then change to Alternate Law by itself. This control mode is indicated to the pilot by replacing the small green bars with these amber crosses. This means that the various protections, that were installed, have now disappeared and no longer function. In the case of triple failures of the computers, the aircraft is then in Direct Law. The pilot must therefore fly his aircraft manually, without the assistance of any automated systems.
Jean-Michel Roy, Experimental Test Pilot - Airbus :
"It's not catastrophic, it's simply a situation where a portion of the assistance, together with all the necessary information, that was provided to the pilot, has disappeared, as you can clearly see here, but that forms part of his basic and ongoing training."
This mode, although extremely rare for commercial aircraft pilots, is a pre-requisite for test pilots during new programmes. When a prototype makes its first test flights, the pilots, in effect, fly it under Direct Law, since it is incumbent upon them, and the design offices, to develop the protection systems on the aircraft.
The goal during the first flights is to determine the natural limits of the aircraft in order to regulate these Flight Control Laws and therefore define the Normal Law, together with the other artificial flight control protective systems that will be installed later on all the following production aircraft.
