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1.6.3.3 These developments led to the general principle that an inverse relationship should exist between the probability of loss of function(s) or malfunction(s) leading to a serious failure condition and the degree of hazard to the aeroplane and its occupants arising therefrom. Airworthiness codes were amended to recognize this principle, two examples being the introduction of paragraphs 25.1309 in the United States Federal Aviation Regulations, Part 25 and the European Aviation Safety Agency, Certification Specifications (CS)-25. To satisfy these requirements, it is necessary to complete a safety analysis of all system and powerplant installations to determine the effect on the aeroplane of a failure condition or malfunction.
1.6.3.4 In assessing the acceptability of a design, it was recognized that rational probability values would have to be established and these were set on the following basis:
a)  historical evidence indicates that the risk of a serious accident due to operational and airframe-related causes is approximately one per million hours of flight. Of this, 10 per cent can be attributed to failure conditions caused by aeroplane system problems. On this basis, it was considered that serious accidents caused by systems should not be allowed a higher probability than this in new designs. Therefore the probability of a serious accident from all such failure conditions should not be greater than one in ten million flight hours, i.e., a probability of less than 1 × 10-7.
b)  to be satisfied that this target can be achieved, it is necessary to analyze numerically all the systems on the aeroplane. For this reason, it is arbitrarily assumed that there are about 100 potential failure conditions which would prevent continued safe flight and landing. The target risk of 1 × 10-7 was apportioned equally amongst these conditions, resulting in a risk allocation of not greater than 1 × 10-9 to each one. Thus, the upper risk for an individual failure condition which would prevent continued safe flight and landing is set at 1 × 10-9 for each hour of flight.
1.6.3.5 Various analytical techniques were developed to assist designers in completing the necessary safety analysis to satisfy the requirements:
a)  Quantitative, by the application of mathematical methods. Such analysis is often used for hazardous or catastrophic failure conditions of systems that are complex, that have insufficient service experience to help substantiate their safety, or that have attributes that differ significantly from conventional systems.
b)  Qualitative, by assessment in a subjective, non-numerical manner. Examples of typical types of qualitative analysis are:
1)  a review of the integrity of the installation and the design, based on experienced judgement; and
2)  a systematic review of each component failure and an evaluation of its effect on the systems of the aircraft. An advantage to this approach is the identification of potential hidden effects of these failures.
1.6.3.6 All hidden (or latent) failures need to be discovered and rectified in a timely manner. The methods for discovering hidden failures may include:
a)  failure monitoring and warning systems;
b)  scheduled maintenance tasks (operational or functional checks of the sub-systems or components); and
c)  special kind of checks (CMRs).
1.6.3.7 Historically, the MRB was the only body responsible for the determination of necessary maintenance tasks to prevent functional system failures, to find out and to eliminate hidden (or latent) failures of redundant systems or components. These tasks being proposed by an industry steering committee (ISC) then form the initial maintenance programme (or the MRB report) for the aircraft type. This document is subject for the approval of the MRB. The MRB report previously was the sole base for continuing airworthiness of the aircraft type. Later, a requirement in U.S and European standards concerning the “latent failures” led to the procedures for certification maintenance coordination committee (CMCC) activities in the area of defining the scheduled tasks for timely elimination of the latent failures. In fact, these are the same activities as those of the MRB, but there is an option for special kinds of flight crew or maintenance personnel tasks. These tasks cover the type design features that cannot be treated effectively by other means (design change, etc.).
　
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