Thesis on "Human Factors in Aviation Safety"

Thesis 10 pages (3180 words) Sources: 5

[EXCERPT] . . . .

Human Factors in Aviation Safety

The purpose of this project is to study fly-by-wire technology on commercial aircraft. Fly-by-wire is a system that utilizes computer-configured controls, where a computer system is interposed between the pilot and the control actuators or surfaces. This modifies the manual inputs of the pilot in accordance with control parameters. We will study this system in two parts: part one consists of a description of the technology and application of the system; part two will study the human factors involved with fly-by-wire systems. From our work on this paper we will become more familiar with the technology itself, its application in modern commercial aircraft, and the human factors considerations of a working fly-by-wire system.

Technology Application

On May 25th, 1972, Gary Krier took off from Edwards Air-Force Base, California in an F-8 that bore the tail number "NASA 802." Unique to this flight was that every command Krier gave to the aircraft went first from a joystick and through a digital computer before being relayed to the hydraulic systems that operated the control surfaces: flaps, elevators, rudder, thrust and so-on. This aircraft was the first experiment in digital fly-by-wire but it was already hip deep in the idea: without the computer, Krier would have had extreme difficulty controlling the aircraft because the designers had sacrificed stability for speed and maneuverability. So began a one-way migration away from direct human control of things and towards computer mediated control. It began with warplanes, and may yet end with people and their cars (Wenham).

Conventional aircraft control systems rely on mechanical and hydraulic links between the aircraft's controls and the flight surfaces on the wings and tail. The controls and flight surfaces are directly connected. Mechanical links are also used for the engine control

The words "Fly-by-Wire" (FBW) imply an electrically-signaled only control system. However, the term is generally used in the sense of computer-configured controls, where a computer system is interposed between the operator and the final control actuators or surfaces. This modifies the manual inputs of the pilot in accordance with control parameters. These are carefully developed and validated in order to produce maximum operational effect without compromising safety (Aircraft flight control systems).

Fly-by-wire is a means of aircraft control that uses electronic circuits to send inputs from the pilot to the motors that move the various flight controls on the aircraft. There are no direct hydraulic or mechanical linkages between the pilot and the flight controls (Fly-by-wire).

The principle used is that of error control in which the position of a control surface (the output signal) is continually sensed and 'fed back' to its flight control computer (FCC). When a command input (the input signal) is made by the pilot or autopilot, the difference between the current control surface position and the apparently desired control surface position indicated by the command is analyzed by the computer and an appropriate corrective signal is sent electrically to the control surface (Fly-by-wire).

Digital Fly-by-Wire Flight Control System fly-by-wire system is built to interpret the pilot's intention and translate it into action, where the translation process will consider environmental factors first. On old aircraft the act of pulling back on the control column would raise the elevator flaps in direct proportion to how far the pilot was pulling, but on a fly-by-wire system they usually raise in direct proportion, but the computer could make subtle changes to account for turbulence. The ratio between the control column that's in the pilot's hands and the flaps on the wing is not 1:1, it's not a direct influence (Wenham).

First Fly-by-Wire on A320

In February, 1987, the first fly-by-wire A320 -- which was also the first commercial aircraft with fly-by-wire -- rolled off the line at Toulouse. The A320's fly-by-wire technology was not only a way of improving flight controls and reducing weight. It enabled Airbus to take safety to a new level by introducing flight envelope protection. Pilots flying the A320 were free to operate it as normal, but the flight envelope protection prevented the aircraft from performing maneuvers outside its performance limits (Corporate information/history: Fly-by-wire).

Fly-by-wire also firmly established the concept of commonality which is so central to the appeal to customers of Airbus aircraft. No matter how one aircraft varies in size or weight from another, fly-by-wire commonality allows the pilot to fly them in the same way because the computer "drives" the aircraft's flight controls. This leads to considerable reductions in the time and costs involved in training pilots and crew to operate them (Corporate information/history: Fly-by-wire).

At Boeing the first aircraft to deliver with a full three-axis fly-by-wire system was the 777, which entered service in 1995.

How the Airbus Fly-by-Wire Works

Since there are innumerable versions of fly-by-wire on commercial aircraft, I will look at how it works on Airbus aircraft. Most systems will have many similarities with the Airbus system, but there would be differences as well.

In the Airbus system there are three primary flight control computers. They are responsible for calculations concerned with aircraft control and with sending signals to the actuators associated with the control surfaces and engines.

There are also two secondary flight control computers. These serve as backup systems for the primary flight control computers, and control the switch automatically to the backup from the primary if the primary becomes unavailable. There is only one computer required for flight control, therefore quintuple redundancy is supported by this system. All operational computers operate in parallel so there is no switching delay.

Two data concentrator computers gather information from the flight control system and pass this to warning and display systems, flight data recorders, and maintenance systems (Sommerfield).

Safeguards for the systems include that the primary and secondary flight control computers use different processors. The primary and secondary flight control computers are designed and supplied by different companies. The processor chips for the different computers are supplied by different manufacturers. All of this reduces the probability of common errors in the hardware causing system failure (Sommerfield).

The design is such that the command unit and the monitor unit are separate channels within a single computer. Each channel has separate hardware and different software, and if the results of the channels disagree (as checked by the comparator) or are not produced at the same time then an error is assumed and control switches to another machine. The software for the different channels in each computer has been developed by different teams using different programming languages. The software for the primary and secondary flight control computers has been developed by different teams. For the secondary computers, different languages are again used for the different channels in each machine (Sommerfield).

The FCS may be reconfigured dynamically to cope with a loss of system resources. Dynamic reconfiguration involves switching to alternative control software while maintaining system availability. Three operational modes are supported:

Normal - control plus reduction of workload

Alternate - minimal computer-mediated control

Direct - no computer-mediation of pilot commands

At least two failures must occur before normal operation is lost.

There is also diversity of controls built into the system. The linkages between the flight control computers and the flight surfaces are arranged so that each surface is controlled by multiple independent actuators. Each actuator is controlled by different computers so loss of a single actuator or computer will not mean loss of control of that surface. and, the hydraulic system is 3-way replicated and these take different routes through the plane (Sommerfield).

Needless to say, fault tolerance is an integral part of the system. Fly-by-wire systems must be fault tolerant as there is no 'fail-safe' state when the aircraft is in operation. In the Airbus, this is achieved by replicating sensors, computers and actuators and providing 'graceful degradation' in the event of a system failure. In a degraded state, essential facilities remain available allowing the pilot to fly and land the plane (Sommerfield).

Problems with the Airbus Fly-by-Wire FCS

There have been few Airbus accidents that may be related to problems with the FCS. One accident (Warsaw runway overrun) has been clearly identified as a problem with the specification and not with the system itself. There is no evidence of any failures of the FCS hardware or software. However, the pilots may misinterpret how the system operates and hence make errors that it can't cope with. (Sommerfield)

Differences Between Airbus and Boeing Systems

The striking difference between the Boeing and Airbus designs for a fly-by-wire system show a contrast in thinking by the two biggest commercial aircraft manufacturers. At Boeing, in the 777, for instance, if there is an emergency situation that requires a steep turn or climb or both that is outside the normal parameters of the FCS, the pilot can override the system.

In an Airbus aircraft, if such a situation occurs, the pilot cannot override the system. The flight control protection parameters of the system will not permit the pilot to fly outside the normal flight profiles. The Airbus aircraft is… READ MORE

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