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 3 Issue 46

Saturday, 30 August 1986


o Human error
Nancy Leveson
Lindsay F. Marshall
o Re: F-16 Tales
Earl Boebert
Phil Ngai
o Correction to note about flight simulators
Martin Minow
o Supermarket grinds to a halt
David Sherman
o Video processing
Guy Schafer
o ATMs
Jacob Palme
o Info on RISKS (comp.risks)

Human error

Fri, 29 Aug 1986 18:48 EDT
    From: Nancy Leveson 

Re: Human Error

"Lindsay F. Marshall" <>
Fri, 29 Aug 86 12:08:04 bst
Someone who has looked at the changing attitudes to "human error" as
against "mechanical failure" is Michael Lesk. He has been studying
reports of railway accidents in the UK to extract from them information
about the attitudes of the reporters and investigators towards the
causes of the accidents. I don't know if he has written this up anywhere
or not, nor do I know if he reads RISKS.  He is well worth talking to
about the subject however and has uncovered some exceedingly
interesting points.
                                  Lindsay F. Marshall

  [Will someone at Bell Labs who reads this please give Mike a nudge?  PGN]

Re: F-16 Tales

Fri, 29 Aug 86 10:51 CDT
Weight on wheels is a basic sensor input that tells the flight program
whether or not the aircraft is airborne.  In advanced systems like the
F-16 its is probably confirmed by air data computer and inertial
platform inputs; in older systems, where the computer does just nav and
weapons delivery, it is the prime indicator.  It is therefore unlikely
in the extreme that this would be overlooked in a design or an ordnance
safety analysis ((weight_on_wheels = TRUE) & (master_arm = TRUE) &
(weapon_release = TRUE) is clearly an undesired state).  I am also
skeptical that the gear would be controlled by the flight computer, but
I am not familiar with the F-16 so cannot comment further.

F-16 software

Phil Ngai <amdcad!phil@decwrl.DEC.COM>
Fri, 29 Aug 86 19:57:30 pdt
It sounds very funny that the software would let you drop a bomb on the wing
while in inverted flight but is it really important to prevent this? Is it
worth the chance of introducing a new bug to fix this very minor problem? Is
it worth the chance of making the code too big to fit in memory? What is the
chance that a pilot would really make this mistake?

     [The probability is clearly NONZERO.  It is very dangerous to start
      making assumptions in programming about being able to leave out an
      exception condition simply because you think it cannot arise.  Such
      assumptions have a nasty habit of interacting with other assumptions
      or propagating.  PGN]

Correction to note about flight simulators

Martin Minow, DECtalk Engineering ML3-1/U47 223-9922 <minow%regent.DEC@decwrl.DEC.COM>
29-Aug-1986 1406
In a private mail exchange, Danny Cohen ("COHEN@B.ISI.EDU") was
kind enough to point out that I had mis-remembered my article
from Smithsonian where I claimed the article stated that a flight
instructor flew as a flight engineer on a commercial flight.

> The plane encountered a wind-shear situation on take off. The
> instructor, from his flight engineer's position, reminded the pilot
> that the correct recovery for wind-shear is opposite to the correct
> recovery for a stall (which has a similar appearance to the pilot)." 

According to Danny (I can't find my copy of this issue), the article does
not talk about anything being "opposite to the correct recovery for the stall."

I'm sorry for the confusion this might have caused anyone.  At least, I did
learn a lot about flying and recovery from dangerous conditions.  Danny did
ask me to clarify my purpose in submitting the article to RISKS — whether
it was to show that computer-based simulators contribute to airline safety,
or to "highlight the risks in using computers for whatever purposes."  To
set the matter straight, it was to show that computer-based simulation is a
factor in increased airline safety, as it lets pilots learn about situations
that are either dangerous or unusual (or both) in real life.

Danny is still looking for pointers to accidents caused by computer-based

     [I don't mean to take a potshot at Martin, who has been a delightful
      contributor.  But PLEASE, all of you, if you see something that you 
      think is appropriate for RISKS, make a note of it at the time rather
      than subsequently half-remember it.  I keep a huge stack of old items
      next to my terminal just in case I have to dig back...  PGN]

Supermarket grinds to a halt

Fri, 29 Aug 86 17:04:53 edt
Last week I went to our local Miracle Food Mart supermarket (in
northern Toronto) at 9 a.m. on a Sunday, when they were just opening.
They discovered that they couldn't get any of the cash registers
to work; something was down in the central system. So they had the
cashiers writing each number down on a pad of paper and totalling
them up by hand, which slowed checkout down to a crawl. After a
while, someone found a desk calculator with a paper tape, which made
things a bit faster.  When I left they had someone at the door warning
customers not to bother coming in because the terminals weren't working.

Obviously, this kind of thing can happen only where cash registers
are no longer cash registers but terminals connected to a central
system, which is becoming more and more the case.  I can't believe
MFM doesn't have some type of backup system, since they're a large
chain. My speculation is that someone wasn't prepared for the system
to be running on Sunday morning; supermarkets must be closed in
Ontario on Sundays, and the ones near us started opening only about
a month ago...

David Sherman, The Law Society of Upper Canada, Toronto   
{ ihnp4!utzoo  seismo!mnetor  utzoo  hcr  decvax!utcsri  } !lsuc!dave

Video processing

Guy Schafer <decwrl!amdcad!amdimage!prls!philabs!linus!axiom!gts@ucbvax.Berkeley.EDU>
Thu, 28 Aug 86 15:38:31 edt
Now that sophisticated hardware for capturing and altering video images
exists for even the modest IBM-PC (AT&T's Truevision products), several
concerns arise:

Because images can be captured in real time (for less than $5000), and
it has been proven that at least one method exists for over-powering
('hi-jacking') a cable video broadcast, some program can be altered and
re-broadcast (with a delay equal to the video processing time).  This
could be especially dangerous if it is done to, say, 2 minutes of a
news broadcast or televised political proceedings.

Video post-processing can also be an effective means to control the behavior
of an individual by tapping directly into her cable coming into her house.
An appropriate stock tip given by a seemingly authentic Ruekeiser (sp?) might
cause a major stockholder to get on her phone to a broker with predictable
(and thus profitable) results.  We always knew a hacker with a PC had quite
a bit of power; if this hacker can alter someone's main source of
information (television broadcasts) he suddenly has quite a bit more.

Also, video tapes which are used as evidence in court can be changed without
simple means of detection.

Actors can be cheated out of royalties--especially in commercials where
post-processing 30 seconds of video could cost less than the royalties of
an often-repeated performance.  The features of the face can be "airbrushed"
or distinguishing marks can be added or removed (by software--e.g. TIPS by
AT&T) and the actor told that someone else got the part.

Any comments?

    >< ...{ decvax!linus | seismo!harvard }!axiom!gts

ATMs (RISKS-3.45)

30 Aug 86 16:57 +0200
<>..Their dispensing machines cannot be cheated in this way, because they have
<>a steel door in front of the machine which does not open until you insert a
<>valid plastic card.
>People who swindle ATM's don't have cash cards?????

If you have a legally obtained cash card, and insert it into the
machine, this act is immediately recorded, so that if the swindle
is detected, they can find out who did it.

If you have an illegally obtained cash card, you probably do not
know the password you have to input on the keyboard. When you
input the wrong password (or do not input any password at all),
the machine swallows the card, and you never get it back again.

At least that is the way the Swedish ATM's work.  

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