AIRCRAFT STRUCTURE SYSTEM

Aircraft Structure System

Aircraft Structure System

Introduction

Modern flight control systems use fly-by-wire technology for the automatic stabilisation (or stability enhancement) of aircraft. In fly-by-wire systems there is no longer a mechanical link between the pilot's controls and the control surfaces of the aircraft. Instead, only an electrical link exists, with a flight control computer system processing the pilot's commands and the data of the aircraft motion sensors in order to actuate the control surfaces of the aircraft. Such flight control systems are obviously safety-critical, i.e. mishaps in such systems could cause injury or death to humans, and appropriate safety measures have to be taken. It should be pointed out that the automotive industry has already started thinking about drive-by-wire and brake-by-wire systems. So this contribution may very well also apply to automobiles in the near future.

There is some literature on flight control systems and on control law design ((Collinson, 1996, p189), but relatively few authors deal with the software aspects of actual implementations, for example Frisberg (1998). The present contribution also takes a software engineering point of view and reports on the experience of an actual implementation of flight control laws. Most of the presentation is based on (simplified) examples, as the whole subject cannot be covered in a single paper.

Programming language issues

The two major forms of software redundancy on flight control system are N Self-Checking Programming and N-version programming.

N Self-Checking Programming: A self- checking program results from adding redundancy to a program so that it can check its own dynamic behaviour during execution. A self-checking software component consists of either a variant and an acceptance test or two variants and comparison algorithm.

N-version Programming: In an N-version software system, each module is made with up to N different implementations. Each variant accomplishes the same task, but hopefully in a different way. Each version then submits its answer to voter or decider which determines the correct answer. An important distinction in N-version software is the fact that the system could include multiple types of hardware using multiple versions of software. The goal is to increase the diversity in order to avoid common mode failures. Design diversity in a very expensive approach, as the same software has to be developed several times, by several teams Flight control system requires fault tolerance software (diversity) to complete fault tolerance hardware. Both redundancy and diversity increase hardware costs, weight, and power requirements for all redundant components. (Frisberg, 1998, 19)

System Architecture And Redundancy

Flight control system requires fault tolerance software

(diversity) to complete fault tolerance hardware.

The analysis of Airbus and Being FCS shows that the design

and implementation of such a safe system of operation

through the combined use of redundancy and diversification

to minimize the probability of failure common mode between

units and redundant to make independent software design

faults can be optimized by proper adjustments of the

redundancy. It also shows that a level of redundancy is very

important. This "over-redundancy" is justified by the desire

for a demonstration of safety, which is guided by both

regulations and ...