The Risks Digest

The RISKS Digest

Forum on Risks to the Public in Computers and Related Systems

ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator

Volume 10 Issue 29

Saturday 1 September 1990


o What is "safety-critical"?
Nancy Leveson
o Re: Computer Unreliability and Social Vulnerability
David Gillespie
o Risks of selling/buying used computers
Fernando Pereira
o Accidental disclosure of non-published telephone number
Peter Jones
o Info on RISKS (comp.risks)

What is "safety-critical"?

Nancy Leveson <nancy@murphy.ICS.UCI.EDU>
Fri, 31 Aug 90 18:08:40 -0700
RISKS-10.28 was certainly provocative.  I do not want to enter into the basic
argument, but I would like to comment on a couple of statements that appear
incorrect to me.

Peter Mellor writes:
   >Safety-*related*, I would say, but not safety-*critical*. The usual
   >definition of safety-critical is that a system failure results in

System safety engineers in the textbooks I have read do not define
"safety-critical" in terms of catastrophic failure.  They use "hazards"
and the ability to contribute to a hazard.  A safety-critical system or
subsystem is one that can contribute to the system getting into a hazardous
state.  There are very good reasons for this, which I will attempt to explain.

Accidents do not usually have single causes -- most are multi-factorial.
We usually eliminate the simpler accident potentials from our systems which
only leaves multi-failure accidents.   The basic system safety goal
is to eliminate all single-point failures that could lead to unacceptable
consequences and minimize the probability of accidents caused by multi-point
failures.  Using Pete`s definition, there are almost NO systems that are
safety critical including nuclear power plant and air traffic control systems
because we rarely build these systems so that a single failure of a single
component can lead to an accident.  That the failure of the whole nuclear power
plant or air traffic "system" is an accident is tautological -- what we are
really talking about are failures of components or subsystems of these larger
"systems" like the control components or the simulators (in this case).

The term "safety-related" seems unfortunate to me because it is too vague
to be defined.  Some U.S. standards talk about "first-level interfaces"
(actions can directly contribute to a system hazard) or "second-level"
(actions can adversely affect the operation of another component that can
directly contribute to a hazard).  Another way of handling this is to talk
about likelihood of contributing to a hazard -- with the longer chains having
a lower likelihood.  But I would consider them all safety-critical if their
behavior can contribute to a hazard -- it is only the magnitude of the risk
that differs.  I have a feeling that the term "safety-related" is often used
to abdicate or lessen responsibility.

There is a very practical reason for using hazards (states which in
combination with particular environmental conditions have an unacceptable risk
of leading to an accident).  In most complex systems, the designer has no
control over many of the conditions that could lead to an accident.  For
example, air traffic control hazards are stated in terms of minimum separation
between aircraft (or near misses) rather than in terms of eliminating
collisions (which, of course, is the ultimate goal).  The reason is that
whether a collision occurs depends not only on close proximity of the
aircraft but also partly on pilot alertness, luck, weather, and a lot of
other things that are not under the control of the designer of the system.
So what can the designer do to reduce risk?  She can control only the
proximity hazard and that is what she does, i.e., assumes that the environment
is in the worst plausible conditions (bad weather, pilot daydreaming) and
attempts to keep the planes separated by at least 500 feet (or whatever, the
exact requirements depend on the circumstances).

When assessing the risk of the air traffic control system, the likelihood
of the hazard, i.e., violation of minimum separation standards, is assessed,
not the likelihood of an ATC-caused accident.  When one later does a complete
system safety assessment, the risk involved in this hazard along with the
risk of the other contributory factors are combined into the risk of a

Why does this push my hot button?  Well, I cannot tell you how many times
I have been told that X system is not safety-critical because it's failure
will not cause an accident.  For example, a man from MITRE and the FAA argued
with me that the U.S. air traffic control software is not safety critical (and
therefore does not require any special treatment) because there is no possible
failure of the software that could cause an accident.  They argued that the
software merely gives information to the controller who is the one who gives
the pilot directions.  If the software provides the wrong information to
the controller, well there was always the chance that the controller
or the pilot could have determined that it was wrong somehow.  But an ATC
software failure does not necessarily result in a catastrophe so it is not
safety critical (as defined above).  Perhaps this argument appeals to you,
but as a person who flies a lot, I am not thrilled by it.  As I said,
this argument can be applied to EVERY system and thus, by the above
definition, NO systems are safety-critical.

Again, there may be disagreement, but I think the argument has been pushed
to its absurd limit in commercial avionic software.  The argument has been
made by some to the FAA (and it is in DO-178A and will probably be in
D0-178B) that reduction in criticality of software be allowed on the
basis of the use of a protective device.  That is, if you put in a backup
system for autoland software, then the autoland system itself is not safety
critical and need not be as thoroughly tested.  (One could argue similarly
that the backup system is also not safety-critical since it's failure alone
will not cause an accident -- both of them must fail -- and therefore neither
needs to be tested thoroughly.  This argument is fine when you can accurately
access reliability and thus can accurately combine the probabilities.  But
we have no measures for software reliability that provide adequate confidence
at the levels required, as Pete says).  The major reason for some to support
allowing this reduction is that they want to argue that the use of n-version
software reduces the criticality of the function provided by that software
(none of the versions is safety critical because a failure in one alone will
not lead to an accident) and therefore required testing and other quality
assurance procedures for the software can be reduced.

   >With a safety-critical airborne system, we are certifying that it has
   >a certain maximum probability of crashing the aircraft.

Actually, you are more likely to be certifying that it will not get into
(or has a maximum probability of getting into) a hazardous state from which
the pilot or a backup system cannot recover or has an unacceptably low
probability of recovering.  The distinction is subtle, but important as I
argued above.  Few airborn systems have the capacity to crash the aircraft
by themselves (although we are heading in this direction and some do
exist -- which violates basic system safety design principles).

   >RTCA/DO-178A ('Software Considerations in Airborne Systems and Equipment
   >Certification') does *not* define any reliability figures. It is merely a
   >set of guidelines defining quality assurance practices for software.
   >Regulatory approval depends upon the developer supplying certain documents
   >(e.g., software test plan, procedures and results) to show that the
   >guidelines have been followed.

There is currently an effort to produce a DO-178B.  This will go farther than
178A does.  For one thing, it is likely to include the use of formal methods.
Second, it will almost certainly require hazard-analysis procedures.  If anyone
is interested in attending these meetings and participating, they are open
to the public and to all nationalities.

>From Pete Mellor's description of the Forrester and Morrison article:
    >This is likely to change following the publication of the draft defence
    >standard 00-55 by the MoD in the UK in May 1989. The DoD in the US are
    >not doing anything similar, though the International Civil Aviation
    >Organisation is planning to go to formal methods.

Depends on what you mean by similar.  The DoD, in Mil-Std-882B (System Safety)
has required formal analysis of software safety since the early 80s.  MoD draft
defense standard 00-56 requires less than the equivalent DoD standard.  There
has been a DoD standard called "Software Nuclear Safety" that has been in force
for nuclear systems for about 4-5 years.  And there are other standards
requiring software safety analysis (e.g., for Air Force missile systems) that
were supplanted by 882B.  882B is likely to be replaced by 882C soon -- and the
accompanying handbooks, etc. do mention the use of formal methods to accomplish
the software safety analysis tasks.

Re: Computer Unreliability and Social Vulnerability

David Gillespie <>
Fri, 31 Aug 90 21:41:05 -0700
I think one point that a lot of people have been glossing over is that in a
very real sense, computers themselves are *not* the danger in large,
safety-critical systems.  The danger is in the complexity of the system itself.

Forester and Morrison propose to ban computers from life-critical systems,
citing air-traffic control systems as one example.  But it seems to me that
air-traffic control is an inherently complicated problem; if we switched to
networks of relays, humans with spotting glasses, or anything else, ATC would
still be complex and would still be prone to errors beyond our abilities to
prevent.  Perhaps long ago ATC was done reliably with no automation, but there
were a lot fewer planes in the air back then.  If it was safe with humans, it
probably would have been safe with computers, too.  As our ATC systems became
more ambitious, we switched to computers precisely because humans are even less
reliable in overwhelmingly large systems than computers are.

The same can be said of large banking software, early-warning systems, and so
on.  There's no question that computers leave much to be desired, or that we
desperately need to find better ways to write reliable software, but as long as
the problems we try to solve are huge and intricate, the bugs in our solutions
will be many and intricate.  Rather than banning computers, we should be
learning how to use them without biting off more than we can chew.
                                     -- Dave

Risks of selling/buying used computers

Sat, 1 Sep 90 14:27:27 EDT
A 8/31/90 Associated Press newswire story by AP writer Rob Wells, entitled
``Sealed Indictments Accidentally Sold as Computer Surplus,'' describes how
confidential files from a federal prosecutor in Lexington, KY, were left on the
disks of a broken computer sold for $45 to a dealer in government surplus
equipment. The files included information about informants, protected
witnesses, sealed federal indictments and personal data on employees at the
prosecutor's office.  The Justice Department is suing the equipment dealer to
return the computer and storage media and reveal who bought the equipment and
who had access to the files.

The government claims that a technician for the computer's manufacturer tried
to erase the files, but since the machine was broken he could not do it with
the normal commands, and had instead to rely on what the AP calls a ``magnetic
probe'' (a degausser?) which apparently wasn't strong enough to to the job.

The dealer complains that the FBI treated him rudely and tried to search his
premises without a warrant; the Justice Department argues that the dealer
hasn't been helpful and could get at the files. The dealer denies he has done

Fernando Pereira, 2D-447, AT&T Bell Laboratories, 600 Mountain Ave,
Murray Hill, NJ 07974            

  [One of the earlier classics was related in RISKS-3.48, 2 Sep 86:
  "Air Force puts secrets up for sale", in which thousands of tapes
  containing Secret information on launch times, aircraft tests,
  vehicles, etc., were auctioned off without having been deGaussed.  PGN]

Accidental disclosure of non-published telephone number

Peter Jones <AINT@UQAM.bitnet>
Fri, 31 Aug 90 16:27:12 EDT
The following appeared on a recent mailout from the Bell Canada telephone
company. It's probably about time to repeat this warning, even if it has
already appeared on the list:

  "If you have a non-published telephone number, please keep in mind that if
  someone else uses your phone to make a collect call or to charge a call to
  another number, your number will appear on the statement of the person billed
  for the call. This is necessary because the customer being billed must be
  able to verify whether or not he or she is responsible for the charge."

Peter Jones   UUCP: ...psuvax1!uqam.bitnet!maint   (514)-987-3542

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