The above notes mainly concern manufacturing and design quality, but quality control has to continue into the operating life of an aircraft struc-ture. This is achieved through an approved maintenance schedule which defines a series of checks and inspections to be carried out after so many flying hours and/or elapsed calendar time. For large aircraft these are listed and defined by the aircraft manufacturers, but to give an indication of how a typical schedule works the following would be reasonable for a light aircraft:
Check A – a pre-flight check at least before the first flight of the day Check 1 – 50 flying hours or two months, whichever occurs sooner Check 2 – 100 flying hours
Check 3 – once a year or 500 hours
Check 4 – once every two years or 1000 hours
The following structure items would be among the details inspected at the various checks:
Check A – skins, moving control surfaces, windscreens and windows, door locks, all visually inspected for any obvious defects
Check 1 – all fixed and moving structures, doors, windows, access panels, fuel tanks – inspect for visible corrosion, operation and damage
Check 2 – internal structures, especially near potential moisture accumulation points – inspect for corrosion
Check 3 – structural joints – inspect for slackness or corrosion Check 4 – all structural parts – inspect for corrosion, slackness,
crack-ing wear or any form of deterioration.
Items found to be worn or damaged beyond the limits defined in the air-craft’s maintenance manual at any inspection would be rectified before the aircraft continued in service.
To avoid taking aircraft out of service too frequently or for long periods it is often the practice to agree a system of running checks with the reg-ulator. Under such a system all the items required to be checked yearly (say) would be listed, and instead of all being checked at one grounding of the aircraft they would be attended to in weekly or monthly batches.
11.7 Review of the key points
Quality standards for the aviation industry are set by the relevant air-worthiness authority, e.g. EASA in Europe and FAA in the USA.
Before any safety critical work is released it is first checked by the person doing the work and then independently checked by another appro-priately qualified and approved person. This applies to drawings, reports, tests, manufactured parts and maintenance. Regulators approve organisa-tions as being competent to carry out and certify particular funcorganisa-tions. They then regularly audit these organisations to verify that standards are being maintained.
An example of a quality assurance process is that employed to ensure
‘traceability’. For example, any metal component can be traced back to the raw material from which it was made.
In design the airworthiness engineer guides the designers with regard to the airworthiness requirements with which compliance must be shown before the product can be certified.
The continued airworthiness of a product is ensured by way of a care-fully defined set of in-service inspections.
11.8 References
Although it is the responsibility of companies to set up their own quality assurance and control systems, advice and help is available from the pub-lications of the Airworthiness Authorities and inspection procedures in particular are well documented. Lists of EASA and FAA publications are available on the appropriate web site.
Stressing
12.1 Introduction
Stressing, or stress analysis, is primarily that process which estimates whether or not the proposed structure is strong enough to carry the loads which will be imposed on it by the operation of the aircraft.
The designer (or, with the complexity of modern aircraft, the design team) is required to design a structure which will allow the aircraft to perform in its role efficiently. There are many constraints which influence the designs, such as maintenance accessibility, production costs, aesthetic appeal and so on, but this chapter is concerned only with the structure and its problems.
The overall loads to be applied to the structure are determined by the airworthiness requirements. These required loads, the specification of the role of the aircraft and the nature of the constructional materials and man-ufacturing processes available, are the raw data with which the designer begins.
It is in the nature of aircraft that the lighter they are, the better they perform in their role. Also, commercially, the cost of carrying superfluous weight in terms of total fuel consumed during the life of the aircraft is very high. The designer’s task then includes achieving maximum lightness and to do this he will employ the most advanced materials and con-structional techniques which are within the production capability of his company. (The last sentence is not intended to suggest that complexity is at any time desirable. Simple solutions are almost always the best for any design problem. One of the great aircraft designers, Ed Heinemann, said
‘Simplicate and add lightness’.)
Going hand in hand with the advance of materials and techniques is more and more detailed and accurate analysis by the stressman. This has a twofold aim: to reduce weight by ensuring that every piece of structure is fully employed and also to eliminate expensive test failures. Airworthi-ness approval only occasionally relies on the stressman’s calculations;
there is always sufficient testing on major structural items to confirm com-pliance with requirements, although tests do not necessarily show up components which are over strength. Also, with larger aircraft and more expensive manufacture, companies are very keen to eliminate any early
failures on test. In spite of the fail-safe type of design philosophy, prema-ture failure of one component can scrap a whole test-piece strucprema-ture.
So, if the primary stressing task is to ensure adequate strength, the sec-ondary task is to assist the designer with the aim of producing structures which are exactly strong enough and no more.
Stressing is now a highly complex subject relying heavily on mathe-matical analysis. In this chapter the techniques used are discussed in their most elementary form, with the idea, as stated in Chapter 1, of giving a foundation vocabulary on which readers can build a more detailed study.
Stressing is a mathematically biased subject but it is not necessary for stressmen to be more than competent mathematicians. The majority of the work done in the examination of structure relies on the use of data and formulae which only require practice and sensible application for their satisfactory use. Writing the data and evolving the formulae do require considerable mathematical skill but the stress office always has a place for the type of engineer who has a ‘feel’ for structures and who can recognise the weak points in two minutes, as well as the man with several degrees who can identify the same weaknesses after two weeks of calculating. Both types of stressman are useful and even essential to a balanced team.