Fly-By-Wire Flight Control system

If you are curious to understand what exactly happens when the pilot gives input on the control stick, yoke or the rudder, this article is for you.

Boeing 737-800 Flight deck

Traditionally, aircraft had steel wires running from the control column to the elevators, rudders, and ailerons. Elevators control the pitch, ailerons control the roll and rudders control the yaw. Pitch, roll, and yaw are movements of the aircraft on 3 different axes and it’s the combination of these controls that make a successful flight.

What happens when the pilot moves the yoke?

de Havilland DH82A Tiger Moth elevator and rudder control cables

When the pilot pulls the control stick back the aircraft climbs and vice versa. On the older jet aircraft, the control columns were connected to a series of high tension cables and pulleys, so the pilot was manually controlling the control surfaces. This is called mechanical flight control. A Cessna 172 is the best example and such trainer aircraft still use the mechanical flight control systems even today. But back in the day even large jets had mechanical flight control systems that were later transitioned into Hydro-Mechanical flight controls that used hydraulic pumps to assist the pilot.

What is Fly-By-Wire?

Fly by wire is a modern flight control system that replaced the manual flight controls with an electronic interface. This means the pilot's input is converted into electronic signals transmitted by wires and thus got its name. It also uses a computer to determine how to move each control surface to get the desired performance from the aircraft. Improved fully fly-by-wire systems interpret the pilot's control input as the desired outcome and calculates the control surface activities required to deliver that outcome; this results in different combinations of the rudder, elevator, aileron, flaps, and engine controls in different situations using a closed loop.

The need for Fly-by-wire

The traditional mechanical flight control systems were heavy and risky because the high tension lines that controlled the surfaces were subjected to constant wear and tear making it prone to snapping from fatigue stress. If the tension lines snapped the pilots would certainly crash because there were no backup systems. Most importantly this type of control system relied on the muscular power of the pilot and he had to physically fight the control sticks during non-normal situations like wind-shear or stalls. This made it unsafe in large jet aircraft. The Hydro-mechanical control system gave a solution to the physical strain the pilots had with the traditional mechanical controls but also added a considerable amount of weight to the aircraft making it less efficient.

The first few airliners to have fly-by-wire

The first airliner flown with fly-by-wire was the Avro Canada CF-105 Arrow in 1958, the next feat achieved with FBW (Fly-By-Wire) was the Concorde in 1968, the Concorde’s FBW also included solid-state components and system redundancy was designed to be integrated with computerized navigation and automatic search and tracking radar was flyable from ground control with data uplink and downlink, and provided artificial feel (feedback) to the pilot. The A320 was the first airliner to have a full glass cockpit and a modern FBW system based on control laws.

The basic operation of Fly-By-Wire

Closed feedback loop control

The pilot commands the stick to make the aircraft perform a certain maneuver by moving the sidestick or control column, the flight control computer then calculates what control surface movements will cause the plane to perform that action and issues those commands to the electronic controllers for each surface. The controllers at each surface receive these commands and then move actuators attached to the control surface until it has moved to where the flight control computer commanded it to. The controllers measure the position of the flight control surface with sensors such as LVDTs.

Automatic Stability Systems

FBW systems allow the computer to perform tasks without input from the pilot. The aircraft is equipped with many sensors and gyroscopes that sense small movements in pitch, yaw, and roll mainly due to weather and corrects them to maintain stability. This makes the flight smoother since it’s impossible for pilots to manually make such minor movements in such a quick time.

Safety in FBW

If a traditional mechanical or hydro-mechanical control systems fail than it renders the aircraft uncontrollable and would result in a fatal crash since the pilots have no control on the control surfaces. But FBW incorporate the meaning of either redundant computer there would be 3 or even 4 independent channels in case one or even 2 fail the Another channel takes over to provide the electrical signals.

Internal configuration of Airbus A320 sidestick

Some aircraft have Hydro-mechanical control systems as a backup in case FBW fails for example the Boeing 777. The above picture is the internal configuration of the sidestick of the Airbus A320. In conclusion, Fly-By-wire is a technological advancement in aviation that simply has no cons, makes the aircraft fly smoother and incorporates a higher level of safety.

Thanks for reading!

Fly-By-Wire Flight Control system