Answer to Question 1
When developing safety-critical systems, a key assumption must be that safety will not automatically result from following an organizations standard development methodology. Safety-critical software must go through a much more rigorous and time-consuming development process than other kinds of software. All tasksincluding requirement definition, systems analysis, design, coding, fault analysis, testing, implementation, and change controlrequire additional steps, more thorough documentation, and vigilant checking and rechecking. As a result, safety-critical software takes much longer to complete and is much more expensive to develop.
Answer to Question 2
Reliability and safety are two different system characteristics. Reliability has to do with the capability of the system to continue to perform; safety has to do with the ability of the system to perform in a safe manner. Thus, a system could be reliable but not safe. For example, an antiaircraft missile control system may continue to operate under a wide range of operating conditions so that it is considerably reliable. If, however, the control system directs the missile to change direction and to fly back into its launching device, it is certainly unsafe.
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