MODELLING THE EFFECTS OF ENGINEERING MAINTENANCE ERRORS IN AIR NAVIGATIONAL AIDS
Case of Instrument Landing Systems in Kenya
BY
OMUSONGA ROBERT JERE
ABSTRACT
Instrument Landing System (ILS) is a ground-based instrument approach system that provides precision guidance to aircrafts approaching and landing on a runway using combinations of radio signals. The problem of monitoring the ILS signal has been investigated for a number of years. Both experimental and theoretical studies have yielded information about system performance, but little has been done to model effects of maintenance engineering errors in the ILS localizer. The composite signal from the ILS localizer operates in a manner to maintain stable equilibrium by ensuring that its courseline information is in tandem with the runway centerline. However, errors due to drifts in electronic component values have the tendency to destabilize this equilibrium. The bearing of drift errors on parameters of signal quantities and their subsequent effects on lateral courseline information has significant impact on approaching and landing aircrafts. The purpose of this study is to conduct a test experiment based on linear continuous-time state equations and develop models for analyzing ILS localizer maintenance engineering errors. The second purpose is to design a MATLAB tool to be used in predicting the effects of these errors on aircrafts approaching Kenyan airports. It will be ascertained that this study shall contribute in enhancing the design process of instrument flight approach procedures for air traffic control services and thus reducing the number of aircraft accidents that occur within airport zone boundary.
INTRODUCTION
1.1 Background
As the worldwide air transportation traffic volume grows rapidly, safety in aviation becomes a burning problem over many countries today. Aviation accidents may result in human injury or even death. It influences the reputation and the economy of the air transportation industry of a country. According to Mineata (1997), when the accident rate is applied to the traffic forecast for 2015, the result would be the crashing of an airliner somewhere in the world almost every week.Braithwaite and Faulkner (1998) stated that in order to achieve safety and reduce accident rate, risk must be quantified and balanced with appropriate safety measures.
Accident statistics based on International Civil Aviation Authority (1997-2006) show that 51% of air accidents occur during final approach, landing and take-offs of an aircraft (Kebabjian, 2008). It is indicated that final approach, landing and take-offs are the periods when the flight maximizes usage of Air Navigational Aids (Navaids). When an aircraft is about to make an approach and landing on an airport during bad weather conditions, there is need to radiate navigational information to carter for lost visibility. Navaids are used for this purpose. One of the Navaids systems used is the Instrument Landing System (ILS).
An ILS is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals. With reference to Greenwell (2000), ILS consists of two independent sub-systems, one providing lateral guidance (localizer) and the other providing vertical guidance (glide path) to aircrafts approaching a runway. A modulation depth comparison of two radio signal beams radiated strategically from the localizer (LOC) and received by the ILS receiver in the aircraft provides course-line information (runway centre-line) while a similar comparison from the glide path (GP) provides the slope information (inclination angle). Air navigational aids must keep a certain degree of accuracy set by International Civil Aviation Organization (ICAO). Accuracy standards are enforced by flight inspection organizations which periodically check critical parameters using properly equipped aircrafts to calibrate and certify ILS precision.
ICAO annex 10 (2000) presents some of the engineering errors that occur in ILS localizer as a result of momentary drifts in critical parameters due to geographical factors, human manipulations and design constraints. The three types of errors are; parallax errors, multipath propagation errors and Maintenance Engineering Errors (MEEs). Maintenance engineering errors come as a result of difference in the signal parameters received on the runway centerline due to drift in value of circuit components. MEEs can also be caused by maladjustment of signal levels that lead to deviation of ILS courseline from runway centerline. The worst case of these errors is the provision of false ILS information that misguides the aircraft to miss the runway centerline and crash outside or within the airport. This proposal is focused to study the effects of MEEs on ILS signal using modeling and simulation. Huschem (1994) experiment found that ILS signal could be suitably modeled along linear continuous-time state model equations. These findings were later enhanced by the research conducted by Tromboni (2010).
1.2 Problem Statement
The signal from the ILS localizer operates to maintain stable equilibrium by ensuring that its courseline information is in tandem with the runway centerline. The drifts in electronic component values have the tendency to destabilize this equilibrium and thus creating a window for aircraft accidents. Air navigational aids and aircrafts operate in real-time domain that involves human life and expensive equipment. This constraint makes the study of effects of maintenance engineering errors risky and probably unattainable in real time. The purpose of this study is to conduct a test experiment using linear time-invariant state equations to develop models for analyzing ILS localizer maintenance engineering errors. Another purpose is to design a MATLAB tool to be used in predicting effects of these errors on aircrafts approaching Kenyan airports.
1.3 Objectives
The broad objective of this study is to analyze and predict the effects of maintenance engineering errors in air navigational aids using modeling and simulation.
The specific objectives;
1) With reference to state model equations for linear continuous-time systems and ILS localizer signal equations, develop canonical state variable model for localizer systems in Kenya.
2) Perform experiments to determine matrix constants for the ILS localizer state variable model.
3) Using ILS localizer state variable model design a MATLAB program to simulate maintenance engineering errors and predict the magnitude of their effects on landing aircrafts.
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