Re: Ariane 5 - not an exception?

[Ken Garlington <garlingtonke@lmtas.lmco.com>]

Robert I. Eachus wrote:
> 
>    First, I think of mission critical as a different category than
> safety critical.  In safety critical systems, fail safe is often an
> option where in mission critical systems you need to fail operational.
> And yes, systems can be safety AND mission critical.  Those are the
> expensive ones.

Actually, safety-critical systems can either be fail-safe or fail-op, just
like mission critical systems. A nuclear reactor might be able to be fail safe,
but a flight control system might have to be fail op.

You can get a lot of definitions of safety critical if you work at it.
Here's AFISC SSH 1-1's definition:

"Those software operations that, if not performed, performed out-of-sequence,
or performed incorrectly could result in improper control functions (or lack
of cotnrol functions required for proper system operation) which could
directly or indirectly cause or allow a hazardous condition to exist."

"Hazardous" usually gets defined as loss of life, serious injurity, or major
property loss.

If the absense of the software function doesn't lead to a hazardous condition,
then the system can be fail-safe. If the software function must be present to
avoid a hazardous condition, then it usually has to be fail-op. However, there's
not exactly a hard and fast rule here. It depends on the system.

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