Fly By Wire - Introduction

The utilization of the concept fly by wire had been widely used in the military aircraft. The use in civil aircraft has started to gain popularity.

When we mention "fly by wire", it would mean that the pilot is controlling the aircraft through the wire connections. To explain this concept, we need to understand how the aircraft were controlled before "fly by wire" technology was used.

1. The Changing Methods to move the Flight Control.

a. Direct Mechanical link control column to the control surfaces- The earlier aircraft were small.The forces acting on the flight control surfaces, like ailerons, elevator and rudder was small too.So the pilot can manually move the surface through a mechanical connection. The control surfaces were mechanically connected through bell cranks, levers, pulleys and mechanical cables to the control column or steering wheels.




b. Assisted Control

Images from FAA AMT manual-

The aileron is controlled by the tab. The movement of the control column to left or right, through mechanical linkages and cable, moves the control tab. The aileron will move in the opposite direction to the tab.






c. Powered Control
Typical example from B737
Take note that the control column transmit the signal to aileron power control unit through a mechanical linkages.

d. Fly by wire (by electrical signal)

2. Factors Leading to Why Fly By Wire.
a. There are a number of reasons why the shift from the mechanical control to the electrical control. With the advance in the technology, majority of the aircraft systems are operated using digital technology. The technology is now available to support the fly by wire and it meets the requirement of the safety and redundancy.

b. The second factor is the sheer size of the aircraft, making the mechanical linkages and cable to complex and heavy. Any additional weight will be an additional cost to the fuel bill.

 
3. Advantages of Fly by wire system.
a. There is an overall reduction in weight, the lesser the airframe weight, the bigger will be the payload.
b. It makes the Integration of several federated systems into a single system easier.
c. It provide a superior handling. In fact the handling characteristics can be modified and controlled. This had the added advantages in the pilots training.
d. ease of maintenance, the rigging is much simplifies. The troubleshooting is more focused on the LRUs and BITE checks.

This conclude the introduction of fly by wire. The next postings will focus on the systems installed on A320 and Boeing 777.