FBW Assign

Alexandra Slabutu DT011/1 A/C Aerodynamics – Declan Barker 06 May 2014

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Bachelor of Engineering Technology

(B.Eng.Tech) Aviation Technology

Aircraft Aerodynamics & Structure and Systems 1

-The development and architecture of FBW system and conventional mechanical flight controls-


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Declaration

This is an original work. All references and assistance are acknowledged.

Date: 6th May, 2014

Candidate Name: Alexandra Slabutu

If an assignment or project or part of an assignment or project has been plagiarized from

any source, this will result in a fail for that assignment or project. (DIT, 2011)


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Contents

1. Introduction _______________________________________________________ 4

2. Conventional mechanical flight controls__________________________________4

3. Hydraulic control____________________________________________________5

4. Fly-by-wire system __________________________________________________5

5. Conclusion ________________________________________________________7

6. References ________________________________________________________7


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1. Introduction

Throughout the years, the development of the architecture of the flight controls

has remarkably changed. Dating back to early aircraft types and first flights, the most

simplistic flight control system designs are mechanical. These are handled by the flight

deck crew through a series of mechanical parts such as pulleys, push-pull rods and cables.

This system is still used in today’s aircrafts, usually in the small light/sport category

aircraft. However, due to rapid technology advancement the aviation industry has also

experienced a rapid growth in the development of aircraft sizes. Larger, faster airplanes

and the rise of flight envelopes, has almost made impossible for pilots to still use the

conventional flight controls in order to control the articulated aerodynamic surfaces.

Therefore the need of electronic powered systems had to be implemented.

Fly-by-wire systems were first introduced in the military fighter jets fallowed by

the commercial jetliners such as Concorde, A320 and B777. This system eliminates the

mechanical linkage from the cockpit controls to the flight control surfaces, with an

electronic interface and in addition boosted by hydraulic or electrical actuators.

In this write up the development of these systems will be analyzed.

2. Conventional mechanical flight controls

The most common and basic mechanical linkage, as stated in the introduction, that

connect the flight deck controls to the control surfaces are the push-pull rods and cable-

pulleys. These direct linkages are possible only if the aircraft size and its flight envelop

allow. That’s why it’s often used and still met in today’s small aircrafts. On this type of

aircrafts the pilot muscular efforts of maneuvering the direct linked flight controls are

easily achieved, because of the low hinge moment produced by the deflection surface.

In the case of push-pull rods, these are connected in a series of rods that link the

cockpit input with the control surface. Polytechnic of Milan (2014) notes that “Bell-crank

levers are used to change the direction of the rod routings…the bell-crank lever is here

necessary to alter the direction of the transmission and to obtain the conventional

coupling between stick movement and elevator deflection”. Therefore the push-pull rod

system has a design requirement to be met, such as stiffness of the linkage, in order for

this to avoid any unwanted deflection during flight and due to fuselage expansion-

contraction. Another requirement would be the exclusion of axial instability during

compression.


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Fig.1 Cables and Pulleys for elevator control. Polytechnic of Milan (2014)

The cable-pulley system operation is very similar to the push-pull rods, except

that the rods are replaced with couples of cables. In many cases the cable-pulley would be

favored as it is more flexible and permits remote areas of the aircraft to be reached by it.

Fig 1 shows an example where the cables are connected to the cabin column that is linked

via a rod to a quadrant.

3. Hydraulic control

The hydraulic system is a powered system that comes in aid for the pilots when

their action is not directly sufficient for maneuvering the controls. The use of this system

has become a high liability for modern aircrafts as it demonstrated to be a much better

solution for actuation in terms of safety, weight per unit power and flexibility. Its

operation occurs as fallowing: the pilot sends a command to a valve which allows

hydraulic fluid flowing, managing one or more actuators. To signal the valve near the

actuators the use of push-pull rods/cables and pulley (mechanically) or electrical

signaling can be executed in the case of modern airplanes.

During normal hydraulic operating conditions, Polytechnic of Milan (2014)

states that the pilot’s efforts into managing the flight controls are very little, only what is

“necessary to contrast the mechanical frictions of the linkage and the movement of the

control valve: the pilot is then no more aware of the load condition being imposed to the

aircraft”.

4. Fly-by-Wire System

The fly-by-wire system was first developed around the 70s’ for the military

aviation as an analogue technique and later on in the 80s’ moved to civil aviation as

digital. Concorde was the first civil aircraft provided with the analogue fly-by-wire.

However Airbus took the lead with A320, for having this system, followed by A319,

A321, A330, A340, B777, and A380 and by the newest generation aircrafts B787 and

A350.


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On a standard aircraft, the pilot commands are send out to the actuators by a series of

mechanical parts. Beside these, flight control computers (FCC) role in the system

architecture is to process signals. The pilot demand is first of all converted into electrical

signals in the cabin and then diverted to different independent data computers. Another

role of these is to sample data related to flight conditions, servo-valves and actuators

positions; the cockpit input is then processed and sent to the actuator, which is tailored to

the actual flight status. Usually the flight data used by the system varies with different

aircrafts; nevertheless Polytechnic of Milan (2014) states that the following data are

processed and sampled:

Pitch, roll, yaw rate and linear accelerations;

Angle of attack and sideslip;

Airspeed/Mach number, pressure altitude and radio altimeter indications;

Stick and pedal demands;

Other cabin demands such as landing gear condition, thrust lever position, etc.

The entire architecture has high redundancy to reinstall the level of reliability of

hydraulic or mechanical systems in the shape of numerous independent and parallel lanes

to create and send the signals, and independent data computers that process them. The

benefits of the fly-by-wire system vary tremendously across the different types of aircraft

and whether is military or civilian aviation. Some of the most significant benefits of this

system are as follows:

Weight and maintenance reduction;

Flight envelope protection;

Increase of stability and handling qualities across the full flight envelope;

Turbulence suppression and consequent decrease of fatigue loads;

Increase of passenger comfort;

Easier interfacing to auto-pilot and other automatic flight control systems;

Drag reduction by an optimized trim setting;

Reduction of airlines’ pilot training costs.

There are four flight modes that are executed by the flight control on a normal

day-to-day operation: ground, take-off, flight and flare, these modes are available only on

civilian aircrafts. It is important to notice that during the transition between the modes,

the pilot is not disturbed in its ability to manage the airplane, as the process is very

smooth.

The development of fly-by-wire control software is stricly done, taking in account

extensive testing and flight control laws in order to reduce the probability of error.

Therefore the risk of aircraft failure due to flight control loss is somewhere around

0.000000002% Polytechnic of Milan (2014).


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5. Conclusion

The development from the conventional flight controls architecture to the modern

electrical fly-by-wire system it’s been explained in this assignment. We’ve seen the transition

from the mechanical direct flight controls such as the push-pull rods, cable-pulleys and hydraulic

actuators to FBW system across the years. I have also identified how the fly-by-wire system was

first introduced to the military aircrafts and then gradually moved to civilian aircrafts such as

Concord up to the new generation B787 and A350 XWB. This innovative system has made

possible for both pilots and passengers, to benefit of more comfortable, safe flights.

It is imperative to state that due to technology advancement the development of these

systems will always vary.

6. References:

FAA.(2014). Chapter 5: Flight Controls. Retrieved May 5th, 2014 from http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2005.pdf

Polytechnic of Milan. (2014). Chapter 6: Flight Control System. Retrieved May 5th, 2014

from http://www.aero.polimi.it/~l050263/bacheca/Dispense_EN/06w-FligCont.pdf

Pascal, T., Isabelle, L., and Jean, S., (2014). Airbus Fly-by-wire: A process toward total

dependability. Retrieved May 5th, 2014 from http://www.skybrary.aero/bookshelf/books/2313.pdf

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2005.pdf

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2005.pdf

http://www.aero.polimi.it/~l050263/bacheca/Dispense_EN/06w-FligCont.pdf

http://www.skybrary.aero/bookshelf/books/2313.pdf

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FBW Assign