Every time you move the sidestick, you're not directly moving control surfaces like in conventional aircraft. Instead, you're sending electronic signals to a sophisticated computer network that interprets your intentions and translates them into precise control surface movements. But why this complexity? The fly-by-wire system in the A320/A321 solves fundamental challenges that have plagued aviation for decades: preventing pilot-induced accidents, maintaining optimal performance across all flight conditions, and providing consistent handling characteristics regardless of weight, balance, or configuration.

How the System Works

The Electronic Interface

Your primary flight controls consist of two independent sidesticks (not mechanically linked), rudder pedals with mechanical backup, and a speed brake lever. When you move the sidestick, you're not fighting aerodynamic forces—you're providing input to seven flight control computers that constantly monitor your intentions and aircraft state.

The Computer Network

Seven computers manage every aspect of flight control:

• 2 ELACs (Elevator Aileron Computers): Handle normal pitch and roll control, with ELAC2 as primary and ELAC1 as backup

• 3 SECs (Spoiler Elevator Computers): Control spoilers and provide backup elevator/stabilizer control

• 2 FACs (Flight Augmentation Computers): Manage electrical rudder control, yaw damping, and turn coordination

This redundancy means that you retain control even with multiple computer failures through various degraded modes.

Control Surface Actuation

All surfaces are hydraulically powered but electrically controlled. Each critical surface has multiple hydraulic actuators with different operating modes:

• Active mode: Computer controls surface position

• Damping mode: Surface follows aerodynamic forces passively

• Centering mode: Surface held hydraulically in neutral position

Managing Power Distribution

Pitch Control Hierarchy

The Trimmable Horizontal Stabilizer (THS) and elevators follow a clear priority system. Normally, ELAC2 controls both elevators (left on green hydraulic, right on yellow) and THS motor No. 1. If ELAC2 fails, ELAC1 takes over using blue hydraulic jacks and THS motor No. 2. If both ELACs fail, SEC2 or SEC1 provides backup control with THS motor No. 2 or No. 3.

Roll Control Distribution

Roll control uses ailerons plus four spoilers per wing. ELAC1 normally controls ailerons, while the three SECs each manage specific spoilers (SEC3 controls spoiler 2, SEC1 controls spoilers 3 and 4, SEC2 controls spoiler 5). This distribution ensures that losing one computer doesn't eliminate all roll control.

Yaw Control Integration

The rudder operates mechanically with electrical assistance. FACs provide yaw damping, turn coordination, and rudder trim, but mechanical control remains available if all electrical systems fail. This dual nature means you always have basic yaw control, even in the worst-case scenarios.

Normal Law: Your Primary Operating Mode

Load Factor Demand System

In normal law, your sidestick doesn't directly control surface deflection—it commands load factor. Pull back, and the system maintains whatever G-force corresponds to your input, regardless of airspeed. This means consistent handling, whether you're light and slow or heavy and fast. A neutral sidestick maintains 1 g flight, automatically trimming for speed and configuration changes.

Automatic Protections

Normal law prevents you from exceeding aircraft limitations:

• Load factor protection: Limits to +2.5 g/- 1 g (clean) or +2 g/0 g (configured)

• Pitch attitude protection: Prevents excessive nose-up (30 ° clean, 25 ° configured) or nose-down (15 °) attitudes

• High angle-of-attack protection: Prevents stalls by limiting AOA to αMAX, even with full aft stick

• High-speed protection: Automatically recovers from overspeed conditions

• Bank angle protection: Maintains 33 ° bank when the stick is released, limits maximum bank to 67 °

Turn Coordination

The system automatically coordinates turns using rudder input—you don't need to use rudder pedals for normal turns. The FACs calculate required rudder deflection and apply it automatically, while also providing yaw damping to prevent Dutch roll oscillations.

When Things Go Wrong

Alternate Law

When certain failures occur, the system degrades to alternate law, maintaining load factor demand but losing some protections. You'll see amber X symbols replace green = symbols on your Primary Flight Display (PFD), indicating lost protections. Key differences include:

• No angle-of-attack protection (replaced by low-speed stability)

• No pitch attitude protection (replaced by pitch attitude limitation)

• Reduced bank angle protection

Direct Law

In direct law, your stick inputs directly control surface deflection, like a conventional aircraft. You'll see "USE MAN PITCH TRIM" on your PFD because automatic trim is lost. This mode requires traditional flying skills: you must manually trim for speed changes and actively prevent stalls.

Mechanical Backup

If all computers fail, you retain mechanical control of the THS (via pitch trim wheels) and rudder (via pedals). While challenging, this provides basic aircraft control for emergency situations.

Speed Brake and Ground Spoiler Operations

Speed Brake Logic

Speed brakes use spoilers 2, 3, and 4, with automatic inhibition during critical phases. They're blocked when, e.g.:

• Flaps are in FULL configuration

• Thrust levers are above MCT

• Alpha floor protection is active

• Angle-of-attack protection is active

• Elevator faults are detected

Ground Spoiler Automation

Ground spoilers automatically deploy during landing or rejected takeoff when certain conditions are met. They utilize all spoiler surfaces and feature an aileron anti-droop function when the flaps are extended. The system incorporates "phased lift dumping," which partially extends the spoilers when, for example, one main gear touches down, facilitating full deployment.

What This Means for You

Operational Advantages

The fly-by-wire system provides consistent handling across all weights and configurations, prevents common pilot-induced accidents, and automatically optimizes performance. Thus, you can focus on flying the aircraft rather than fighting its aerodynamic quirks.

Situational Awareness

Understanding the system's logic helps you anticipate its behavior. When you see amber X symbols on your PFD, you know protections are lost and must fly more conservatively. When "USE MAN PITCH TRIM" appears, you know to actively manage pitch trim like a conventional aircraft.

Emergency Preparedness

Knowing the degradation sequence (Normal → Alternate → Direct → Mechanical) helps you understand what capabilities remain after failures. Even in the worst case, you retain basic control through mechanical systems.

The F/CTL system transforms your A320/A321 from a control surface collection into an integrated flight management system. By understanding its logic and limitations, you can use it as the sophisticated tool it was designed to be—one that enhances safety while maintaining the fundamental joy of flight.

Sidestick to control surfaces: seven computers translate your intent into safe, consistent flight control.