Explained: From Cables to Code: Airbus Flight Laws

‍DALLAS — In the world of aircraft manufacturers, Airbus arguably stand out today as the most important in terms of technology and sales. Their flagship models, such as the Airbus A320, A330, A350, and A380, are highly advanced in terms of avionics and flight systems. 

These aircraft no longer depend solely on mechanical cables and linkages to govern flight. Instead, these aircraft are flown using sophisticated and state-of-the-art digital flight control systems, a system known as the “Fly-By-Wire” (FBW). 

At the core of the process are a set of flight control laws and levels of oversight reasoning implemented in flight computers that guide pilots in flying the aircraft, while also preventing dangerous maneuvers, reducing workload, and optimizing safety. 

Additionally, digital systems enable pilots to focus more on higher-level strategic decisions by automating more mundane tasks. The fly-by-wire computers constantly track airplane operations, adjusting as necessary to safeguard the ship and protect its controllers. In addition to complexity, however, these developments have unquestionably enhanced air safety over the decades.

From Cables to Code: Why Flight Laws Were Introduced

In the past, flight controls, which used yokes and rudder pedals to directly operate control surfaces via cables, rods, and hydraulics, were the norm. Pilots had complete freedom of the flight controls, which also meant full responsibility. They were at risk of overbanking the aircraft, stalling at low altitudes, or overstressing the airframe structure.

Then came the fly-by-wire system, which altered this norm and introduced safety features for the aircraft. In an FBW system, the pilots’ inputs are sent to computers, which in turn send commands to the hydraulic actuators and servo motors attached to the control surfaces. 

This, in turn, is directed by software that steps in to trim, correct turbulence, and also provides “envelope protection,” which prevents stalls, overspeed, or extreme attitudes. Flight controls are now electrically controlled and hydraulically actuated. 

Airbus designed its FBW architecture with multiple flight control laws and modes of operation that degrade steadily, depending on the nature of any malfunction. From Normal Law to Mechanical Backup, which is at the top of the hierarchy of control authority, these laws present a pyramidal structure for safe flight performance, allowing the aircraft to continue flying safely in adverse conditions.

Architecture of Airbus Fly-By-Wire Systems

Airbus flight control systems are designed with redundancy in mind. Although the exact architecture will differ between aircraft variants and systems, the core structure includes:

• Flight Control Computers (FCCs):
  For example, the A320 is equipped with:
  
  • 2 ELACs (Elevator Aileron Computers)
  • 3 SECs (Spoiler Elevator Computers)
  • 2 FACs (Flight Augmentation Computers)
    Meanwhile, the A330 has:
  • 3 PRIMs (Primary Flight Control Computers)
  • 2 SECs (Secondary Flight Control Computers)
• Input Sources:
  
  • Sidestick and rudder pedal signals
  • Air Data Inertial Reference Units (ADIRUs)
  • Flight Management and Guidance Computers (FMGCs)
  • Slat/Flap Control Computers (SFCCs)
  • Landing Gear Control Interface Units (LGCIUs)
  • Accelerometers

Two Flight Control Data Concentrators (FCDCs) report to the Electronic Instrument System (EIS) and the Central Maintenance Computer (CMC).

The Four Airbus Flight Laws

1. Normal Law: Full Envelope Protection

This is the standard mode of operation that provides the highest level of flight envelope protection. It provides, 

• Pitch Attitude Protection: Limits pitch to 30° nose-up and 15° nose-down
• Load Factor Protection: G-forces are limited to within safe structural limits
• High Angle of Attack Protection: Prevents stalls by limiting sidestick input based on AoA (Angle of Attack)
• High-Speed Protection: Prevents overspeed by adding a nose-up command and restricting nose-down inputs
• Bank Angle Protection: Limits the bank angle to 67°, with automatic return to 33° when the sidestick is sensed to be released
• Low Energy Protection: Triggers a loud aural “Speed Speed Speed” warning in the cockpit and provides automatic TOGA thrust if energy levels are dangerously low (between 100–2,000 ft AGL with flaps in config 2+)

In Normal Law, pilots fly the aircraft through the sidestick, whose input translates to the load factor, rather than the pitch rate. The aircraft auto-trims and maintains a 1g flight. This law is in effect from the time of rotation to being 100 feet above ground during landing.

Flare Mode:

At 50 feet above the runway, the aircraft drops ever so slightly nose down, which requires the pilot to counter by pulling back on the sidestick, thereby mimicking the action of traditional aircraft during landing.

2. Alternate Law: Partial Protections

During specific system failures, the flight control system reverts straight to the Alternate Law. This law has two sub-modes:

• Alternate Law 1 (ALT1):
  
  • Pitch: Alternate Law
  • Roll: Normal Law
  • Protections retained: Load Factor, Bank Angle
  • Protections lost: Pitch Attitude, Stall, Low Energy, α-Floor
  • Speed stability replaces low-speed protection
  • Audible “STALL” warning introduced
• Alternate Law 2 (ALT2):
  
  • Pitch: Alternate Law
  • Roll: Direct Law
  • Yaw: Alternate Law
  • Protections lost: Pitch Attitude, Bank Angle, High AoA, High Speed, Stall
  • Load Factor protection remains

Triggers for Alternate Law:

• Engine flameouts
• Loss of air data or inertial reference
• Multiple surface actuator failures
• Spoiler or aileron faults
• Dual hydraulic or sensor failures

The autopilot is often unavailable in these cases, and the pilot's workload increases.

3. Direct Law: No Protections, Manual Trim

In this configuration, all protections are unavailable. The sidestick now directly controls the control surfaces as in a typical aircraft (Mechanically). Auto trim is not available, which means the pilots must apply manual trim via the horizontal stabilizer.

Triggers for Direct Law include:

• Dual elevator faults
• Loss of all inertial reference units
• Loss of all three PRIMs
• Engine dual flameout with control computer loss

The autopilot disconnects automatically, and the flight is then controlled solely by the pilot's skill.

4. Mechanical Backup: Last Resort Control

If even Direct Law is unavailable, Airbus FBW aircraft allow basic mechanical control:

• Pitch control via the mechanical stabilizer trim wheel
• Lateral control via rudder pedals, mechanically linked to the rudder

This is not intended for complete flight control, but rather to maintain attitude while systems are reset after a total power failure.

Airbus flight control laws (Credits: Philippe Goupil/Science Direct)

Boeing vs. Airbus: Two Philosophies, One Goal: Safe Fly-by-Wire

While Airbus is known for strict envelope protection, which prevents pilots from overflying pitch, bank, or load factors, Boeing’s FBW logic, which first appeared on the Boeing 777 series and later in the 787/8/9/10 and 747-8 takes a “command, don’t constrain” approach.
Boeing’s Normal Mode also has features that include the use of tactile “soft stops”, visual alerts, and aural warnings that come on as the crew approaches the flight envelope. 

In contrast to Airbus' Normal Law, the pilot can, in an emergency, exceed those soft stops and put the aircraft beyond its design limits. Control ‘feel’ and pitch response are designed to simulate what is felt in a cable-driven plane, which in turn presents the familiar ‘pitch rate’ and ‘speed stable’ qualities that veteran pilots recognize instantly. 

During a failure, the Boeing system reverts to Secondary Mode, similar to Airbus Alternate Law, where the autopilot and most protections are deemed unavailable; however, the PFCs still operate and compute surface commands. Further loss of computers or data puts the aircraft in Direct Mode, where the pilot's inputs are directed to the actuators, mimicking the Airbus Direct Law function. 

In the event of a complete electrical blackout onboard due to electrical failure, the pilots are left with cable-linked stabilizers and roll spoilers, as well as Mechanical Backup, similar to the Airbus Mechanical Backup, which provides rudder and trim wheel control.

Airbus places a strong emphasis on hard limits that cannot be overridden under any circumstances. At the same time, Boeing prefers soft limits, which serve as a guide for pilots and are not required to follow—two different approaches to achieving the same result: protecting the aircraft, crew, and passengers.

Why It All Matters

Airbus flight control systems are not random at all; they are often referred to as smart cascades. Even with multiple failures, pilots can still control their aircraft. These systems are there to reduce the risk of overstress, stall, or pilot-induced oscillations. 

For pilots, such systems result in less workload during turbulent weather, wind shear, or high-risk maneuvers. For engineers, this means using lighter structures to avoid over-engineering, which makes a system less susceptible to failure. And most importantly for passengers, a smooth and safer flying experience.  

Summary: Airbus Flight Law Philosophy

Airbus has proposed a concept of progressive system degradation. Instead of a sudden and complete loss of flight controls, these flight protectors are gradually removed, allowing the flight crew to regain full control of the aircraft using the mechanical backup. Under Normal Law, the aircraft operates within the flight envelope protection. 

In contrast, under Alternate and Direct Law, continued control is maintained even when multiple systems fail. The mechanical backup allows pilots to control the aircraft manually, rather than relying on electrical systems. Due to this system, Airbus remains amongst the safest and most automated airliners in the world.