law

In fly-by-wire aircraft, a law is a programmed control mode that translates pilot inputs into specific aircraft responses by managing control surface positions, thrust, and stability automatically. Unlike conventional mechanical linkages, the aircraft's flight control computer runs the law as real-time software that interprets stick and pedal movements, then commands elevators, ailerons, rudders, and engines accordingly. The law sits between the pilot's physical inputs and the actual hardware; it is the decision logic that defines how the airplane behaves.

Most modern fly-by-wire transports operate with multiple laws, selectable either automatically or manually depending on system state. A normal law, for example, enforces speed and altitude limits, prevents overspeed, and provides envelope protection. An alternate law activates if primary sensors fail, relaxing some protections to keep the aircraft controllable. Direct law bypasses most automation entirely, giving the pilot raw control of the surfaces themselves, used only in degraded situations or for training. Each law is a distinct set of algorithms with different response characteristics and safety boundaries.

Implementation and failure modes

Laws are implemented in the aircraft's flight control computer, typically with redundant channels running identical code. The computer samples sensor inputs, runs the control law equations, and outputs commands to hydraulic actuators thousands of times per second. Because laws are software, they can be updated, tested, and certified before deployment. Failures occur when sensors provide corrupted data, when computing channels disagree, or when law logic itself contains a defect. A law reversion, where the system switches to a less capable law unexpectedly, is a significant event requiring investigation and crew notification.

The term law derives from the notion of a physical or mathematical rule governing behavior, similar to Newton's laws. In control theory, a control law is the formal set of equations that produces outputs from inputs. Aviation adapted the term to describe each distinct operating mode as a separate law with its own governing rules.

Understanding which law is active is essential for maintenance troubleshooting. Technicians must verify that law transitions occur correctly, that sensor inputs are valid for the active law, and that control surface responses match expected law behavior. Documentation specifies the conditions under which each law activates, the protections it provides, and the performance envelope it enforces.