passengers are mainly concerned about whether the journey will be safe and whether the aircraft will take off and land on the scheduled times. The second concern is, by airlines, expressed by the dispatch reliability, which is defined as the probability that a scheduled departure takes place within a specified time after the scheduled departure time. Many airlines use a 15‐minutes margin between actual and scheduled departure time for a flight to be considered as having departed on time. The achieved dispatch reliability indicator for a (past) period is reported as the percentage of all departures that departed on time.
For technical items, the airlines are mainly using the reliability metrics listed in Section 1.4.1
1.5 Approaches to Reliability Analysis
Three main branches of reliability can be distinguished:
Hardware reliability
Software reliability
Human reliability
The present book is concerned with hardware items (existing or in design) that may or may not have embedded software. Within hardware reliability, two different approaches may be used: the physical approach and/or the systems approach.
1.5.1 The Physical Approach to Reliability
In the physical approach, the strength of a technical item is modeled as a random variable
where
Figure 1.5 Load and the strength distributions at a specified time
The load may vary with time and be modeled as a time‐dependent variable
and the survivor probability
The physical approach is mainly used for reliability analyses of structural elements, such as beams and bridges. The approach is therefore often called structural reliability analysis (Melchers 1999). A structural element, such as a leg on an offshore platform, may be exposed to loads from waves, current, and wind. The loads may come from different directions, and the load must therefore be modeled as a vector
Figure 1.6 Possible realization of the load and the strength of an item.
1.5.2 Systems Approach to Reliability
By the systems approach, all our information about the operational loads and the strength of an item is incorporated in its probability distribution function
Quantitative results are based on information about the reliability of the components. Such information comes from statistical data on past experience with the same or similar components, laboratory testing, or from expert judgments. This approach has similarities to actuarial assessments, and the systems approach to reliability is, therefore, sometimes referred to as an actuarial approach. This book is concerned with the systems approach to reliability.
System Models
In reliability studies of technical systems, we always have to work with models of the systems. These models may be graphical (networks of different types) or mathematical. A mathematical model is necessary in order to be able to bring in data and use mathematical and statistical methods to estimate reliability parameters. For such models, two conflicting interests always apply:
1 The model should be sufficiently simple to be handled by available mathematical and statistical methods.
2 The
