The RISKS Digest
Volume 11 Issue 78

Monday, 3rd June 1991

Forum on Risks to the Public in Computers and Related Systems

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

Please try the URL privacy information feature enabled by clicking the flashlight icon above. This will reveal two icons after each link the body of the digest. The shield takes you to a breakdown of Terms of Service for the site - however only a small number of sites are covered at the moment. The flashlight take you to an analysis of the various trackers etc. that the linked site delivers. Please let the website maintainer know if you find this useful or not. As a RISKS reader, you will probably not be surprised by what is revealed…


o Lauda Air Crash
Paul Leyland
Carsten Wiethoff
Ralph Moonen
Mark Evans
o Re: AFTI-F16
John Rushby
o Lottery bar codes no risk, spokesman says
Martin Minow
o Re: Viper
Pete Mellor
o Re: The FBI and computer networks
Jim Thomas
o Re: Voting by phone
Bob Rehak
Larry Campbell
Arnie Urken
o Re: The Death of Privacy?
Brett Cloud
o Re: Credit reporting
David A. Curry
Bill Murray
o Info on RISKS (comp.risks)

Lauda Air Crash

Paul Leyland <>
Mon, 3 Jun 91 13:36:39 +0100
_The Times_ (London), Monday June 3rd 1991.

        Lauda says crash engine failed

New evidence that the crash of a Lauda Air Boeing 767-300 in Thailand had been
caused by in-flight reversal of the thrust on one engine has stunned the
aviation industry.  Niki Lauda, owner of Lauda Air, made the claim in Vienna
after returning from Washington, where the flight data and cockpit voice
recorders of the jet were being analysed.  The Austrian transport ministry
supported the assertion.  If the diagnosis were confirmed, the accident would
be unprecedented, Herr Lauda said.  The crash investigators have yet to

The computerised airliner's systems have the capacity for self-analysis and
should have corrected such a basic error.  Reverse thrust is normally locked
out during flight and only used on the ground.  It is thought to be virtually
impossible for reverse thrust to be deployed when the engine is pushing at
maximum, as the jet would have been.  The incident happened about 16 minutes
after takeoff.

Investigators for Boeing, which manufactured the thrust reversers, were allowed
access to the crash site for the first time on Friday.  Herr Lauda said the
flight data recorder was damaged and could not be used to analyse the crash.
He said the cockpit voice recorder indicated an advisory light had come on
seconds before the crash and that a voice was heard saying the light was
glowing intermittently.  Seconds later, First Officer Josef Thurner was heard
saying: "It deployed."  Herr Lauda took that to mean the reverse thrust was
engaged.  The entire incident, from the moment of the first warning light until
the plane broke up, took no more than a minute.  Last night a spokesman for the
Civil Aviation Authority said that no checks were to be ordered immediately on
767s owned by British airlines.

LaudaAir Boeing 767-300 crash in Thailand

Carsten Wiethoff <>
Mon, 3 Jun 91 11:57:10 MET DST
  [...] Private Speculation: Could it be something like a simple overflow/sign
error to cause the engine go from max forward to max backward thrust?

Lauda air plane crash

Ralph 'Hairy' Moonen <>
Mon, 3 Jun 91 08:51 MDT
[...] Now when the media make something out to be a computer error, they
hardly ever are specific. What I want to know is, was it a hardware error, or a
software error. And certainly, wouldn't a mid-air reversal of thrust just rip
off the wing, leaving the plane to plummet down totally out of control? Any
air-experts out there willing to comment on this?
                                                        --Ralph Moonen

Tiland air crash

Mark Evans <>
Mon, 3 Jun 91 10:04:16 +0100
[...] However the reverse thrust system is only used on landing. It would
appear unlikly (not least because of the safety aspects) that reverse thrust in
NOT under the direct control of the flight management system.  Also mentions
mechanical interlocks, these are presumably in addition to the hydrolic systems
actuating the thrust deflectors being turned off.  The principle is quite
simple.  The thrust deflectors are locked in place unless the plane is on the
ground (also landing gear is down) and the control in the cockpit is activated.

Mark Evans, University of Aston, Birmingham, England

Re: AFTI-F16 (RISKS-11.61, 62, 64)

John Rushby <>
Sat 1 Jun 91 16:15:06-PDT
     For RISKS readers interested in digital flight control systems (DFCS), I
highly recommend the papers by Mackall and his colleagues on the AFTI-F16 (and
some other) flight tests; [4] is particularly thorough and informative.

The following extracts from [6] summarize some of the points (fairly, I hope).
It seems that redundancy management became the primary source of unreliability
in the AFTI-F16 DFCS.  *'s indicate footnotes; they, and the references are
given at the end.


     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

     A plausibly simple approach to redundancy management in an N-modularly
redundant system is the "asynchronous" design, in which the channels run fairly
independently of each other: each computer samples sensors independently,
evaluates the control laws independently, and sends its actuator commands to an
averaging or selection component that chooses the value to send to the actuator
concerned.  The triplex-redundant DFCS of the experimental AFTI-F16 was built
this way, and its flight tests reveal some of the shortcomings of the approach
[2, 4].

     First, because the unsynchronized individual computers may sample sensors
at slightly different times, they can obtain readings that differ quite
appreciably from one another.  The gain in the control laws can amplify these
input differences to provide even larger differences in the results submitted
to the output selection algorithm.  During ground qualification of the
AFTI-F16, it was found that these differences sometimes resulted in a channel
being declared failed when no real failure had occurred [3, p. 478]. *1
Accordingly, rather a wide spread of values must be accepted by the threshold
algorithms that determine whether sensor inputs and actuator outputs are to be
considered "good."  For example, the output thresholds of the AFTI-F16 were set
at 15% plus the rate of change of the variable concerned; also the gains in the
control laws were reduced.  This increases the latency for detection of faulty
sensors and channels, and also allows a failing sensor to drag the value of any
averaging functions quite a long way before it is excluded by the input
selection threshold; at that point, the average will change with a thump [4,
Figure 20] that could have adverse effects on the handling of the aircraft.

     The danger of wide sensor selection thresholds is dramatically illustrated
by a problem discovered in the X29A. This aircraft has three sources of air
data: a nose probe and two side probes.  The selection algorithm used the data
from the nose probe provided it was within some threshold of the data from both
side probes.  The threshold was large to accommodate position errors in certain
flight modes.  It was subsequently discovered that if the nose probe failed to
zero at low speed, it would still be within the threshold of correct readings,
causing the aircraft to become unstable and "depart."  This error was found in
simulation, but 162 flights had been at risk before it was detected [5].

     An even more serious shortcoming of asynchronous systems arises when the
control laws contain decision points.  Here, sensor noise and sampling skew may
cause independent channels to take different paths at the decision points and
to produce widely divergent outputs.  This occurred on Flight 44 of the
AFTI-F16 flight tests [4, p. 44].  Each channel declared the others failed; the
analog back-up was not selected because the simultaneous failure of two
channels had not been anticipated and the aircraft was flown home on a single
digital channel.  Notice that all protective redundancy had been lost, and the
aircraft was flown home in a mode for which it had not been designed--yet no
hardware failure had occurred.

     Another illustration is provided by a 3-second "departure" on Flight 36 of
the AFTI-F16 flight tests, during which sideslip exceeded 20deg, normal
acceleration exceeded first -4g, then +7g, angle of attack went to -10deg, then
+20deg, the aircraft rolled 360deg, the vertical tail exceeded design load, all
control surfaces were operating at rate limits, and failure indications were
received from the hydraulics and canard actuators.  The problem was traced to
an error in the control laws, but subsequent analysis showed that the side air
data probe was blanked by the canard at the high angle of attack and sideslip
achieved during the excursion; the wide input threshold passed the incorrect
value through and different channels took different paths through the control
laws.  Analysis showed this would have caused complete failure of the DFCS and
reversion to analog backup for several areas of the flight envelope [4, pp.

     Several other difficulties and failure indications on the AFTI-F16 were
traced to the same source: asynchronous operation allowing different channels
to take different paths at certain selection points. The repair was to
introduce voting at some of these "software switches."  In one particular case,
repeated channel failure indications in flight were traced to a roll-axis
"software switch."  It was decided to vote the switch (which, of course,
required ad hoc synchronization) and extensive simulation and testing were
performed on the changes necessary to achieve this.  On the next flight, the
problem was found still to be there.  Analysis showed that although the switch
value was voted, it was the unvoted value that was used [4, p. 38].

     The AFTI-F16 flight tests revealed numerous other problems of a similar
nature.  Summarizing, Mackall [4, pp. 40-41] writes:

"The criticality and number of anomalies discovered in flight and ground tests
owing to design oversights are more significant than those anomalies caused by
actual hardware failures or software errors.

"...qualification of such a complex system as this, to some given level of
reliability, is difficult ...[because] the number of test conditions becomes so
large that conventional testing methods would require a decade for completion.
The fault-tolerant design can also affect overall system reliability by being
made too complex and by adding characteristics which are random in nature,
creating an untestable design.

"As the operational requirements of avionics systems increase, complexity
increases.  Reducing complexity appears to be more of an art than a science and
requires an experience base not yet available.  If the complexity is required,
a method to make system designs more understandable, more visible, is needed.

"The asynchronous design of the [AFTI-F16] DFCS introduced a random,
unpredictable characteristic into the system.  The system became untestable in
that testing for each of the possible time relationships between the computers
was impossible.  This random time relationship was a major contributor to the
flight test anomalies.  Adversely affecting testability and having only
postulated benefits,*3 asynchronous operation of the DFCS demonstrated the need
to avoid random, unpredictable, and uncompensated design characteristics."


1: Also, in the flight tests of the X31 the control system "went into a
reversionary mode four times in the first nine flights, usually due to
disagreement between the two air data sources" [1].

2: However, the greater the benefit provided by DFCS, the less plausible it
becomes to provide adequate back-up systems employing different technologies.
For example, the DFCS of an experimental version of the F16 fighter (the
"Advanced Fighter Technology Integration" or AFTI-F16) provides control in
flight regimes beyond the capability of the simpler analog back-up system.
Extending the capability of the back-up system to the full flight envelope of
the DFCS would add considerably to its complexity--and it is the very
simplicity of that analog system that is its chief source of credibility as a
back-up system [2].

3: The decision to use an asynchronous design for the AFTI-F16 DFCS was because
"the contractor believed synchronization would introduce a single-point failure
caused by electromagnetic interference (EMI) and lightning effects" [4, p. 7]
--which may well have been correct given the technology of the early 1980s.


[1] Michael A. Dornheim.  X-31 flight tests to explore combat agility to 70
deg.  AOA.  Aviation Week and Space Technology, pages 38-41, March 11, 1991.

[2] Stephen D. Ishmael, Victoria A. Regenie, and Dale A. Mackall.  Design
implications from AFTI/F16 flight test.  NASA Technical Memorandum 86026, NASA
Ames Research Center, Dryden Flight Research Facility, Edwards, CA, 1984.

[3] Dale A. Mackall. AFTI/F-16 digital flight control system experience. In
Gary P. Beasley, editor, NASA Aircraft Controls Research 1983, pages 469-487.
NASA Conference Publication 2296, 1984.  Proceedings of workshop held at NASA
Langley Research Center, October 25-27, 1983.

[4] Dale A. Mackall.  Development and flight test experiences with a
flight-crucial digital control system.  NASA Technical Paper 2857, NASA Ames
Research Center, Dryden Flight Research Facility, Edwards, CA, 1988.

[5] Dale A. Mackall and James G. Allen.  A knowledge-based system
design/information tool for aircraft flight control systems.  In AIAA Computers
in Aerospace Conference VII, pages 110-125, Monterey, CA, October 1989.
Collection of Technical Papers, Part 1.

[6] John Rushby.  Formal specification and verification of a fault-masking and
transient-recovery model for digital flight-control systems.  Technical Report
SRI-CSL-91-3, Computer Science Laboratory, SRI International, Menlo Park, CA,
January 1991.  Also forthcoming NASA Contractor Report.

Lottery bar codes no risk, spokesman says

Martin Minow 01-Jun-1991 1222 <>
Sat, 1 Jun 91 09:33:46 PDT
>From the Boston Globe, Friday May 31, 1991 "Ask the Globe":

  Q. The state lottery's instant tickets now have bar codes with which
  sales agents can verify both their validity and the winning amount.
  What is to prevent agents or lottery officials from "reading" the
  codes on unsold tickets and taking the big prizes for themselves?

  A. Lottery spokesman David Ellis tells us that, once an instant
  ticket is "read" by a bar-code reader, it is invalidated. If a previously
  read bar code is reread, the computer "flags" the ticket as "previously
  cashed" or "cashed-stolen." The bar-code reader, Ellis adds, is "the first
  step in a long and complex series of protections" built into the game
  by Richard Finocchio, the lottery's computer manager. "The security system,"
  he concludes with pride, "is the envy of lotteries around the world."

In sending this to Risks, I am *not* suggesting that confidence in their
security system is misplaced: I can think of several ways to implement
this that, given acceptable security at the Lottery central computer, would
prevent cheating.  I can also think of several ways that would appear
safe, but aren't.  However, both the question and answer raise interesting
questions of trust (and, as dear Pres. Reagan would say, verification).

Full disclosure: I play the lottery twice a year — when they send me
their advertising freebie.  In four years, I've won exactly one dollar.

Martin Minow

Re: Viper (Randell, RISKS-11.73)

Pete Mellor <>
Fri, 31 May 91 17:14:40 PDT
> The only civilian customer for the technology has been the Australian National
> Railway Commission, which at the end of 1988 adopted Viper as the core of a
> new signalling system. The Australians have now gone so far with developing
> the system that they would have difficulty switching to another technology.

When I last heard of them (see RISKS DIGEST 6.48, 7.18, 7.36, 8.41, 9.1), the
Australian National Railway Commission was suing its commercial supplier
(Charter Technologies??) for precisely the same reason that Charter is now
suing MoD.

Peter Mellor, Centre for Software Reliability, City University, Northampton Sq.,London EC1V 0HB +44(0)71-253-4399 Ext. 4162/3/1 (JANET)

Re: The FBI and computer networks (Bellovin, RISKS-11.77)

Fri, 31 May 91 14:36 CDT writes:

> I spoke a bit loosely when I used the phrase ``probable cause''; that's a
> technical term, and does not (quite) apply.  Nevertheless, I stand by the
> substance of what I said.  The FBI is not allowed to engage in systematic
> monitoring of speech without reasonable suspicion of criminal activity.

We can cavil over nuances of specific terms, but smb is essentially correct.
The FBI (and other gov't agencies) are generally, either by statute or policy,
prohibited from routine systematic monitoring of speech outside of specific
investigatory needs.  This includes routine monitoring by informants or
undercover operatives. Although the practice may be otherwise (see "The FBI's
Misguided Probe of CISPES" by Gary M. Stern of the Center for National
Security Studies), current policies are still (on paper, at least) guided by
the Attorney General's Guidelines on FBI Undercover Operations" (Jan. 5, 1981)
that restricts the use of undercover investigation, which is the most
Constitutionally dangerous from of monitoring.

There is surely nothing wrong with law enforcement agents, whether in a private
or official capacity, participating in cybercommunication. The line is crossed
when participation moves to surveillance, data acquisition, and record keeping
as occurs in political surveillance.  The Secret Service may have overstepped
this line when it set up it's "sting board" (Dark Side) in 1988 in a way that
seems, uh, "empirically disfactual" with the SS Director's account of it.

Jim Thomas

Re: Voting by phone

Fri, 31 May 91 14:53 CDT
I don't see how one can keep their anonymity in this vote by phone scheme.
Passing laws against release of voter info or entrusting the security of the
voter info to people makes me very nervous.  Having people in the loop causes a
security problem by definition.  Would the people in charge of the 'voter' data
center be willing to give up their right to privacy so we can monitior their
activities so they can be stopped immediately if they try to divulge any voter
information.  Will the system be TEMPEST secured so no electronic eavesdropping
is possible.  Will the 'voter' data center be guarded by elite commandos sworn
to defend it to the death.

As I see it, if the data is stored somewhere, sooner or later the good guys or
the bad guys will get hold of it some way or another either legally or
illegally.  Once the genie is out of the proverbial bottle, it will be too late
to put him/her back.

Imperfect as it is, I like the way the system is now.  If people were really
serious about voting, they would take the time to vote.  Having a vote by phone
system will only allow for more abuse and fraud as some earlier respondents
have suggested.
                        Bob Rehak, DBA At Large, BITNET: A20RFR1@NIU

Re: Voting-by-phone (RISKS-11.75)

Larry Campbell <>
Thu, 30 May 91 23:42:33 EDT
This (voting by phone) is a classic example of an attempt to solve a social
problem by the misguided application of technology.

What, really, is the problem for which this is supposed to be solution?

Invalids?  Travellers?  We have absentee ballots that work quite well, thank

Low turnout?  Just make election day a national holiday, as it is in most
civilized countries (but not the US).

Electronic voting?  Who needs it?

Larry Campbell             The Boston Software Works, Inc., 120 Fulton Street    Boston, Massachusetts 02109 (USA)

voting by phone

Sat, 1 Jun 1991 06:59 EST
There is a tendency to assume that computer-based voting systems such as voting
by phone are more risky.  I say "assume" because all election systems are risky
and depend on certain procedures being carried out with integrity. But anyone
familiar with the manipulation of paper or mechanical voting systems would want
clarification of the statement that voting by phone relies "excessively" on the
honesty of the operators of the system. In fact, allowing the individual voter
to check his/her vote avoids passive reliance on the integrity of election

In this connection, verification of one's vote is possible under paper voting
systems and others, but computer-based systems make it easier for the citizen
to initiate and carryout verification.  I say this on the basis of 5 years of
development work with experimental voting systems in a computer-networking

Grave-yard voting is a problem, but it is not clear that the problem
would be worse with computer-based (phone) voting. It might be less
frequent and more difficult to employ such maniupulation depending
on the method of verification used.

Re time-stamping: the latest schemes of time-stamping and cryptographic coding
based on distributed (not centralized processing) do make it practically
impossible to break the system. Put another way, the amount of information that
needs to be collected to break the system is so great, that watch-guard
programs would detect would-be criminals.

Finally, using "none-of-the-above" is an improvement on plurality voting, which
truncates information about voter preference orderings and is therefore
undesirable from a theoretical point of view.  This may not seem directly
related to voting by phone, but Roy mentioned it as a possibility and it seems
reasonable to extend the idea to consider other methods of voting that do not
distort the voting process.  Moreover, voting methods are related in the sense
that it is not trivial to use them.  I have designed interfaces for
computer-based voting with different methods of voting and found what works and
what doesn't work.  Interfaces for voting by phone must be designed based on
extensive experimentation and we may find that it is infeasible to use voting
methods that have desirable social consequences.

Arnie Urken

Re: The Death of Privacy? (Robert Allen, RISKS-11.71)

Brett Cloud <>
Sat, 1 Jun 91 15:37:15 GMT
  I agree with the spirit of this posting.  Since we can not have complete
privacy, we have at least complete access for everybody.  I can readily see the
day come when liberty will have as part of its definition, the freedom to
access information.

  People who abuse the system, could have their access taken away.  Much
along the same lines as a robber being sent to prison.  Also, a person on
parole could be given varying degrees of access as their behavior warrented.

  PC's are becoming so cheap and powerful, that I can not see an effective
monopoly being maintained for long by (m)any central power(s).  The people
will take back their own power.

  While granted, I'd prefer to have more privacy, I think it would be better to
KNOW that information about me is availible as a matter of course, rather than
THINK the information is private--when a privilaged subset actually has access.

  If we are going to have as much known about us as appears to be the case,
then the only way to level the playing grounds is by letting everone have, as a
matter of course, the SAME access, unless and until they abuse the trust
implied by the system.

  If people know that their actions can be/are looked at, maybe they'll think
twice about what they say or do.  Granted that this is a poor sub- stitute for
a proper set of values or a good conscience(sp?), but I think few would dispute
that many people in power could use a constant reminder that they are
answerable to the people.

Re: Credit reporting (Paul Schmidt, RISKS-11.77)

"David A. Curry" <>
Fri, 31 May 91 14:52:44 -0700
If you belong to TRW's Credentials Service, this is one of the "features":
any time anyone accesses your credit report, they send you a nice little
blurb the next day such as (from when I applied for my Gold Card):

  [... assorted introductory stuff ...]

  The following companies have requested and received copies of your TRW
  credit files.

    Date   Company Name and Address           TRW Number  Type of File
  -------- --------------------------------   ----------  ------------
  06-07-90 AMERICAN EXPRESS                    3459491    Credit
       4315 South 2700 West
       Salt Lake City, UT

  [... stuff about where to call with questions ...]

TRW Credentials costs $35/year.  For this, you get credit card protection,
unlimited copies of your credit report, and the little notes above.  They also
let you enter in all your financial data, so that it can be retrieved
electronically with your "access number" (like a PIN) by banks and such when
you apply for loans - so you don't have to fill out the forms (in theory
anyway, I never tried it).

Before anyone re-starts the discussion about the rip-off/non-rip-off nature of
this service, it should be pointed out that:

- Any time you are denied credit, you are entitiled by law to a *free* copy
  of your credit report from the credit bureau(s) which supplied the
  information to the lender.  You can then dispute any incorrect info.

- You can get a copy of your credit report at any time from any credit
  bureau for (usually) $8 and some assorted pieces of I.D.

- You only get the little blurbs when someone accesses your TRW file.  TRW
  is not the only credit bureau in the country, although it's certainly one
  of the largest.  I believe Equifax and EDS offer similar services, though.

So, unless you want the credit card protection and the little blurbs when
your file gets looked at, the service is a rip-off.  Me, I like the little
blurbs, find the service fairly well-run, check my credit report once or
twice a year, and don't mind paying the $35.

--Dave Curry

Credit Reporting

Sat, 1 Jun 91 09:27 EDT
    [similar comment...]

Isn't the market great?  On the other hand, require them to do by law what they
are already prepared to do for a fee?  Guaranteed to cost you more.

In an information society it is possible for the credit worthy to obtain the
credit in minutes that was, in an earlier time, available only to the most well
established in days or weeks.  In part because of this, we all enjoy higher
levels of commerce and a standard of living that would not have come about with
out it.  When we look at the effects of all of this on privacy, we should not
forget the reason that we put the system in place.  Yes, WE; the collective we.
We did it; it was not done to us.  In remedying the difficulties, we must
careful not to overlook or destroy the benefits.

Please report problems with the web pages to the maintainer