Initial Airworthiness

Frontmatter

Chapter 1. What Is Airworthiness?

Abstract

This book is aimed at advanced students and professionals, who are expected to have to make decisions regarding the fitness for service of new aviation products, whether whole aircraft or subsystems. This chapter compares initial and continued airworthiness and what goes into determining airworthiness, focusing primarily on civilian aeroplanes, with a brief foray into military codes and specifications. The ICAO process and main civilian design codes (both ICAO and sub-ICAO) for aeroplanes are described, with how to decide which applies to what aircraft. The differences between the civil and military initial airworthiness approaches are briefly described.

Guy Gratton

Chapter 2. The Atmosphere

Abstract

The atmosphere is the medium through which all terrestrial aircraft travel. Thus, it is vital that any discussion of airworthiness begins with a discussion of the properties of the atmosphere, and how, in calculation and testing, we characterise the medium. The core components of the atmosphere, its different layers and how they are characterised are described. The International Standard Atmosphere (ISA)—its standard definitions and reference latitudes, and how ISA influences and informs airworthiness practice are also described. An integral part of airworthiness is human survivability in the airborne environment and as the final part of this chapter, how atmospheric conditions (pressure, oxygen content and temperature) affect human survivability are discussed.

Guy Gratton

Chapter 3. The Pitot-Static System

Abstract

The measurement of airspeed is a vital component in the determination of airworthiness. There are many ways in which to measure airspeed, as well as many different definitions. The most widespread method however, is that of a Pitot-static system, which applies Bernoulli’s equation of incompressible flow to determine airspeed using pressure measurements. This chapter will discuss the various definitions of airspeeds, the different types of Pitot-static systems, how the measurements are used in the calculation of airspeed, and the methods commonly in use to calibrate Pitot-static systems.

Guy Gratton

Chapter 4. The Flight Envelope

Abstract

The flight envelope is the region of velocity-normal acceleration space that defines the conditions under which an aircraft may be safely flown without significant risk of structural failure. It is bracketed by the gust envelope, which defines the safe ranges of airspeed and normal acceleration for manoeuvering and for operating in turbulent conditions, and some other operating limitations. This chapter describes how to calculate these conditions for any given aircraft, and what the relevant civil airworthiness standards deem to be maximum acceptable values.

Guy Gratton

Chapter 5. First Principles of Structural Approval

Abstract

The investigation and formal approval of aircraft structures is a necessary step in ensuring initial and continued airworthiness. While component can be predicted using analytical tools, ultimately, the structures must be tested under flight conditions. Substantial over-engineering is unacceptable, due to weight constraints. This chapter therefore describes the main principles of structural approval of aircraft structures, by combination of analysis and test.

Guy Gratton

Chapter 6. Approving an Aircraft’s Main Flight Structure

Abstract

This chapter describes the methods by which an aeroplane’s main flight structure is proven to be fit for purpose, through a combination of analysis and test, explaining that the use of numerical (finite element) methods is usually unsuitable as an approval tool. Special cases where approval by test only are described. Materials fatigue is also considered, describing the philosophy behind achieving fatigue resistant structures.

Guy Gratton

Chapter 7. Undercarriage Structural Approvals

Abstract

The undercarriage (or landing gear) of an aircraft is the most important structure in ensuring safe landing (and take off) of an aircraft. As such, it is subject to intense stresses at take-off, landing, and indeed, in-flight, and in order to perform correctly, it must be able to survive these stresses repeatedly. This chapter discusses in detail the experimental procedures used to determine the response of an aircraft undercarriage to the various stresses it must endure, in order to determine the initial (and continued) airworthiness of any given design of undercarriage. Analytical methods are also briefly discussed, but these tend to be simulations of the experimental tests that this chapter covers in detail.

Guy Gratton

Chapter 8. Control Surfaces and Circuits

Abstract

Control systems and circuits are the mechanisms through which aircraft are controlled. In order for a system to be considered airworthy, control inceptors generally follow a standardized layout and must follow defined structural standards. Circuits must not be prone to jamming under “normal” operational conditions, and should, generally, be “intuitive” to a pilot. This chapter discusses the recommended standards and layouts, together with common problems and pitfalls.

Guy Gratton

Chapter 9. Powerplant Airworthiness

Abstract

The powerplant is the part of an aircraft that converts fuel into thrust. As such, it is also the part of the aircraft that has the potential most hazardous in event of a failure, and is generally isolated from the main aircraft body by a firewall. Therefore, airworthiness codes are designed to minimise this risk, and ensure that in the event of failure, there is no unacceptable damage to the rest of the aircraft. For the purposes of this book, the powerplant will be one of three main possibilities: a gas turbine engine, a propeller driven by a gas turbine, or a piston engine driving one or more propellers. This chapter concerns itself with discussion of airworthiness codes, principles and best practices to ensure airworthiness of everything forward of the firewall.

Guy Gratton

Chapter 10. Crashworthiness and Escape

Abstract

While no aeronautical engineer expects or wants their design to crash, accidents do happen, and in the case of aircraft, have obvious potential to be catastrophic. How an aircraft responds after a crash, in terms of access to emergency exits, resistance to fire in passenger and crew compartments, and continued structural integrity to facilitate easy escape, determines the survivability of an aircraft accident. Thus, designers and airworthiness engineers must reasonably ensure that aircraft can be exited and escaped from after a crash. This is a question of attention to detail and consideration of the massive amount of historical data that exists. This chapter deals with the primary causes of non-survivability after a crash and how they can be mitigated, as well as the means—both airborne and post impact, of aircraft escape.

Guy Gratton

Chapter 11. An Introduction to Flying Qualities Evaluation

Abstract

The flying qualities of an aircraft can be split into two components—handling and performance. Handling qualities determine how responsive the aircraft is to perturbations from regular flying conditions, and how easily, and safely, a pilot can recover from such a perturbation. This chapter will concentrate mainly on handling qualities, and in particular the civil codes that define minimum handling standards and how these relate to preferable handling qualities.

Guy Gratton

Chapter 12. Longitudinal Stability and Control

Abstract

Longitudinal stability and control links control in speed, altitude and normal acceleration. This chapter relates to the stability seen by the pilot, as opposed to the aerodynamic characteristics of the aircraft. This Chapter will discuss static and dynamic stability, and minimum airworthiness codes that define acceptable ranges and responses. Definitions of stability and control terms are discussed, as are the usual tests for determining whether an aircraft’s response is airworthy.

Guy Gratton

Chapter 13. Lateral and Directional Stability and Control

Abstract

While the theoretical basis of lateral and directional stability and control is highly mathematical and often taught in universities, the practical application is less so. This chapter, like the previous one, again focuses on the “apparent” lateral and directional stability, i.e. on what the pilot sees and feels. This is the methodology used in most flight test schools and companies when assessing lateral and directional stability from an airworthiness perspective. Thus, an understanding of the methodology is vital for anyone intending to work in the field.

Guy Gratton

Chapter 14. Aeroplane Asymmetry

Abstract

Asymmetry in aircraft can arise either by design or due to an accident. However an asymmetry is introduced, being able to compensate for the handling effects of asymmetric thrust or steering is vital to safe handling of the aircraft. In this chapter, the various methods of introducing asymmetry, and their effects on aircraft handling, are described with particular reference to how civil aviation codes deal with asymmetry and the major behavioural issues that can arise from aircraft asymmetry.

Guy Gratton

Chapter 15. Departures from Controlled Flight

Abstract

All aircraft are potentially prone to departures from controlled flight: most are recoverable, but are nonetheless likely to be undesirable. The stall mode is considered in depth as this must be evaluated on all aeroplanes, and the spin mode is considered with regard to different depths of evaluation depending upon whether an aircraft will be certified for deliberate spinning or not, and whether it is to be certified for as spin resistant. Two further modes, poorly catered for in airworthiness standards, are also described—the spiral dive, and the tumble.

Guy Gratton

Chapter 16. Systems Assessment

Abstract

While an individual aircraft can be considered as a unified system, it is, in terms of airworthiness, necessary to consider most individual aircraft as a sum of several different systems. Each of those systems must be able to work as part of the whole, and normal operation of one system must not hinder normal operation of any other. Furthermore, if one primary system is incapacitated, a secondary, independent system must be able to replicate its function to allow safe operation of the aircraft until repairs can be made. This Chapter deals with the classification of systems, how risk to those systems is assessed, mitigated and what constitutes acceptable risk. The conditions under which those system risks are assessed is also discussed.

Guy Gratton

Chapter 17. Environmental Impact

Abstract

Aircraft emissions fall into four categories—noise, surface pollutants, greenhouse gases and end of life waste. Noise is of greatest concern to communities about airports, and has been subject to steadily greater regulation since 1972. Surface pollutants include NO_X and particulates, but are hard to separate from the many other pollution sources at airports. Greenhouse gas emissions are of great concern, and are steadily being reduced per passenger mile, but reductions need to be increased because of industry growth. The industry is being increasingly successful in recycling aircraft at end of life, with up to 90% now routinely being recycled, although new materials are creating recycling challenges.

Guy Gratton

Chapter 18. Facilitating Continued Airworthiness

Abstract

Initial airworthiness determines whether an aircraft or new part is fit for entry into use. However, for an aircraft to provide continued, safe, effective operation continued airworthiness is key. The way in which a part or aircraft deemed airworthy initially maintains its airworthiness is by regular inspection, maintenance, replacement when required, and a periodic overview of all standards. This chapter describes how initial airworthiness facilitates continued airworthiness and the general methods and procedures used to ensure that once an aircraft or part enters service as an airworthy component, it is maintained in an airworthy condition.

Guy Gratton

Chapter 19. Professional Ethics Within Airworthiness Practice

Abstract

This chapter describes the importance of ethical principles underlying professional practice, and presents and analyses several codes of practice. It then shows that a moral code is unsufficient, but must be supplemented by a decision making process—presenting Davis’ 7 stage process as an available method. Case studies are used to illustrate this.

Guy Gratton

Chapter 20. Running a Certification Programme

Abstract

Certification programmes must integrate with broader programmes, and good integration is necessary to overall success. Certification programmes typically run in eight phases: design stage integration, library building, analysis, ground test, developmental flight test, operational flight test, product approval, operational test and evaluation, and finally post-introduction initial airworthiness activity. These do not run sequentially—there is significant overlap and interdependence. These activities are similar in civil and military regimes, and for OEMs and after market providers, save for the availability and sources of information.

Guy Gratton

Backmatter