<s>
A	O
safety-critical	B-Application
system	I-Application
(	O
SCS	O
)	O
or	O
life-critical	B-Application
system	I-Application
is	O
a	O
system	O
whose	O
failure	O
or	O
malfunction	O
may	O
result	O
in	O
one	O
(	O
or	O
more	O
)	O
of	O
the	O
following	O
outcomes	O
:	O
</s>
<s>
A	O
safety-related	B-Application
system	I-Application
(	O
or	O
sometimes	O
safety-involved	B-Application
system	I-Application
)	O
comprises	O
everything	O
(	O
hardware	O
,	O
software	O
,	O
and	O
human	O
aspects	O
)	O
needed	O
to	O
perform	O
one	O
or	O
more	O
safety	O
functions	O
,	O
in	O
which	O
failure	O
would	O
cause	O
a	O
significant	O
increase	O
in	O
the	O
safety	O
risk	O
for	O
the	O
people	O
or	O
environment	O
involved	O
.	O
</s>
<s>
Safety-related	B-Application
systems	I-Application
are	O
those	O
that	O
do	O
not	O
have	O
full	O
responsibility	O
for	O
controlling	O
hazards	O
such	O
as	O
loss	O
of	O
life	O
,	O
severe	O
injury	O
or	O
severe	O
environmental	O
damage	O
.	O
</s>
<s>
The	O
malfunction	O
of	O
a	O
safety-involved	B-Application
system	I-Application
would	O
only	O
be	O
that	O
hazardous	O
in	O
conjunction	O
with	O
the	O
failure	O
of	O
other	O
systems	O
or	O
human	O
error	O
.	O
</s>
<s>
Some	O
safety	O
organizations	O
provide	O
guidance	O
on	O
safety-related	B-Application
systems	I-Application
,	O
for	O
example	O
the	O
Health	O
and	O
Safety	O
Executive	O
(	O
HSE	O
)	O
in	O
the	O
United	O
Kingdom	O
.	O
</s>
<s>
A	O
safety-critical	B-Application
system	I-Application
is	O
designed	O
to	O
lose	O
less	O
than	O
one	O
life	O
per	O
billion	O
(	O
109	O
)	O
hours	O
of	O
operation	O
.	O
</s>
<s>
Safety-critical	B-Application
systems	I-Application
are	O
increasingly	O
computer-based	O
.	O
</s>
<s>
Several	O
reliability	O
regimes	O
for	O
safety-critical	B-Application
systems	I-Application
exist	O
:	O
</s>
<s>
Fail-safe	B-General_Concept
systems	I-General_Concept
become	O
safe	O
when	O
they	O
cannot	O
operate	O
.	O
</s>
<s>
Famously	O
,	O
nuclear	O
weapon	O
systems	O
that	O
launch-on-command	O
are	O
fail-safe	B-General_Concept
,	O
because	O
if	O
the	O
communications	O
systems	O
fail	O
,	O
launch	O
cannot	O
be	O
commanded	O
.	O
</s>
<s>
Railway	O
signaling	O
is	O
designed	O
to	O
be	O
fail-safe	B-General_Concept
.	O
</s>
<s>
Fail-secure	B-General_Concept
systems	O
maintain	O
maximum	O
security	O
when	O
they	O
cannot	O
operate	O
.	O
</s>
<s>
For	O
example	O
,	O
while	O
fail-safe	B-General_Concept
electronic	O
doors	O
unlock	O
during	O
power	O
failures	O
,	O
fail-secure	B-General_Concept
ones	O
will	O
lock	O
,	O
keeping	O
an	O
area	O
secure	O
.	O
</s>
<s>
Fault-tolerant	B-General_Concept
systems	I-General_Concept
avoid	O
service	O
failure	O
when	O
faults	O
are	O
introduced	O
to	O
the	O
system	O
.	O
</s>
<s>
Software	B-General_Concept
engineering	I-General_Concept
for	O
safety-critical	B-Application
systems	I-Application
is	O
particularly	O
difficult	O
.	O
</s>
<s>
There	O
are	O
three	O
aspects	O
which	O
can	O
be	O
applied	O
to	O
aid	O
the	O
engineering	O
software	O
for	O
life-critical	B-Application
systems	I-Application
.	O
</s>
<s>
The	O
avionics	O
industry	O
has	O
succeeded	O
in	O
producing	O
standard	O
methods	O
for	O
producing	O
life-critical	B-Application
avionics	O
software	O
.	O
</s>
<s>
Similar	O
standards	O
exist	O
for	O
industry	O
,	O
in	O
general	O
,	O
(	O
IEC	O
61508	O
)	O
and	O
automotive	O
(	O
ISO	O
26262	O
)	O
,	O
medical	O
(	O
IEC	B-Application
62304	I-Application
)	O
and	O
nuclear	O
(	O
IEC	O
61513	O
)	O
industries	O
specifically	O
.	O
</s>
<s>
Another	O
approach	O
is	O
to	O
certify	O
a	O
production	O
system	O
,	O
a	O
compiler	B-Language
,	O
and	O
then	O
generate	O
the	O
system	O
's	O
code	O
from	O
specifications	O
.	O
</s>
<s>
Another	O
approach	O
uses	O
formal	B-Architecture
methods	I-Architecture
to	O
generate	O
proofs	O
that	O
the	O
code	O
meets	O
requirements	O
.	O
</s>
<s>
All	O
of	O
these	O
approaches	O
improve	O
the	O
software	O
quality	O
in	O
safety-critical	B-Application
systems	I-Application
by	O
testing	O
or	O
eliminating	O
manual	O
steps	O
in	O
the	O
development	O
process	O
,	O
because	O
people	O
make	O
mistakes	O
,	O
and	O
these	O
mistakes	O
are	O
the	O
most	O
common	O
cause	O
of	O
potential	O
life-threatening	O
errors	O
.	O
</s>
