LB
The Safety Case is a structured argument, supported by a body of evidence, that provides a compelling, comprehensive and valid case that a system is safe for a given application in a given environment.
I've quoted Def Stan 00-56, but for example Boscombe Down and Westland use "Is the study of an aircraft or item of aircraft equipment to identify and show acceptability (or otherwise) of the potential hazards associated with it. The Safety Case provides a reasoned argument supported by evidence,establishing why the Design Authority is satisfied that the aircraft is safe to use and fit for its intended purpose". It is used to assist A&AEE in making (airworthiness) recommendations for a particular standard of aircraft".
Personally, I like the latter definition, mainly because it specifically mentions the build standard. Also, because it makes clear the Design Authority's responsibility, and talks of both aircraft and aircraft equipment (which MoD makes provision for in separate ways). In other words, the Def Stan was too simplistic, but served its purpose in the days when the target audience were taught this stuff at birth.
However, the first version is still ok. The "application and environment" bit means that a fundamental pre-requisite is a Statement of Operating Intent and Usage. (The Chinook Mk2 didn't have one, which is one of the things that buried MoD). All aircrew are required to have read and to have access to the SOIU. It forms the basis of the Aircraft Specification which the Build Standard(s) reflect. A version of the Safety Case exists against each Build Standard (in practice, a series of call-ups are used, in the same way drawings are controlled). The Release to Service is based on the Build Standard presented for trials.
It follows that SOIU, Build Standard, Safety Case and RTS must be maintained. This glider case is all about a breakdown of the process that does that maintaining.
There is seldom a "Safety Case" in the form of a single document - especially not on aircraft systems. What you have is a hierarchy of hundreds of not thousands of Safety case reports, which the Aircraft DA collates, forming a Whole Aircraft Safety Case. Trying to keep it simple here, but in this case MB would produce a Safety Case for the seat, which BAeS would subsume into the higher level Aircraft case. It is this process that determines, for example, why the ejection seat might be safe in a Hawk, but not in another aircraft. (Where the application bit comes in). Another example - this is one of the things that buried MoD on Nimrod XV230. The AAR modification/system was safe in one application, but not in the application to which it was put in Sept 2006. Similarly, C130 XV179.
Hope that helps.
Edit - sorry, Engines got in first.