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 24

Thursday, 24 July 1986


o Comet and Electra
Jerry Saltzer
Marv Zelkowitz
Don Chiasson
Bard Bloom
o No gasoline because the computer is down?
Jim Barnes
o HBO Hacker Captain Midnight Caught
via Geoff Goodfellow
o Info on RISKS (comp.risks)

Re: Comet and Electra

Jerome H. Saltzer <Saltzer@ATHENA.MIT.EDU>
Tue, 22 Jul 86 23:26:59 EDT
> - I also heard that the structural defect in the Electra I wing design had
> not been caught by the stress analysis program because of an undetected
> overflow on a critical run.  Can anyone provide documentation for this?  (I
> think this story was on the grapevine at the NATO Software Engineering
> Conferences in 68-69.)

In case it helps anyone recall where that one might be documented:  the
version of the story that came through here had it that some piece of
simulation input data was typed with the wrong minus sign.  (The commonly
available version of the 026 key punch had a minus sign and a hyphen as
distinct characters.  And the input format conversion routines in those days
were both unforgiving and silent about errors.)

Re: Comet and Electra

Marvin Zelkowitz <>
Wed, 23 Jul 86 09:57:25 edt
Horning's recent comment reminds me of two related items:

- On the Electra I wing design defect: My version of the story goes
that the undetected overflow error was finally detected when these
"correct" programs were used as benchmarks for a new computer (a
Burroughs I think), which gave radically different answers. I do not have 
any proof of this, but it might give some additional help in tracking it

- On overflow detection: In the late 60s, a certain vendor's FORTRAN
did not detect overflow. At a users' group meeting, the vendor offered
to add overflow detection at an execution penalty of one instruction
per arithmetic operation (e.g., branch-on-overflow). This was voted down.
The only conclusion is that users would rather be fast than right. 
The issue for RISKS is "Are these people the ones 'still in control'?"

--Marv Zelkowitz

Re: Comet and Electra

Wed 23 Jul 86 09:17:42-ADT
> From: horning@src.DEC.COM (Jim Horning)

> - A numerical analyst once explained to me why all modern airliner windows
> have rounded corners: Anyone concerned with solving partial differential
> equations knows that square corners lead to singularities.  He said that the
> Comet crashes were traced to metal fatigue at the (square) corners of its
> windows.  (He concluded that airplane designers should study Numerical
> Analysis.)  

Most engineers know that any sharp corner on a stressed member will cause
an increase of actual stress over the nominal calculated stress, and the
ratio of these is called the stress concentration factor, K.  The value of
K is sort of inversely proportional to the radius of curvature of the
discontinuity.  High K is the reason cracks propagate so well. The
temporary fix for a crack is to drill a hole at the end of the crack which
increases the radius of the "corner" and decreases K.  It is standard
design practice to avoid sharp corners.  Stress concentration is usually
discussed in design textbooks without going into the differential
equations: there are lots of tables.

This brings up a problem encountered in computer applications: the
difficulty of a programmer learning the standard practices of a field in
which he is working.  Engineers know about stress concentration, but
programmers and mathematicians may not.

> - I also heard that the structural defect in the Electra I wing design had
> not been caught [...].  Can anyone provide documentation for this? 

I can't give a direct answer to this, but I know that a mid 60's computer
which was heavily used in scientific and engineering applications had very
poor accuracy in its trig package.  Is this perhaps the same topic?  (Or was
the Electra designed in the 50's??)  Note: I can identify the manufacturer
and machine, but feel that if I did so, I would be potentially libelous.
                Don Chiasson

Re: Comet and Electra

Wed 23 Jul 86 11:44:00-EDT
  [Structural defect in the Electra I wing design, again.  See Jerry, above.]

I don't know about this, but I was trying to move some software in Fortran
from an IBM to VAX for McDonnell-Douglas one summer.  The program on the VAX
kept dying, with a message to the effect of "I can't take a sine of a number
this large".  The program was trying to take sines of large (order of 10^20)
numbers in 16-digit arithmetic.  The first thing that the sine routine does
is reduce its argument modulo pi, which loses *all* of the precision of the
20-digit number.  The VAX's software generated an error about this.  The IBM
did not; and the programmers hadn't realized that it might be a problem (I
guess).  They had been using that program, gleefully taking sines of random
numbers and using them to build planes, for a decade or two.

No gasoline because the computer is down?

Jim Barnes <decvax!wanginst!infinet!barnes@seismo.CSS.GOV>
Wed, 23 Jul 86 13:56:44 edt
Last Friday, on my way home, I stopped at the local gasoline station to
"fill 'er up".  However, they could not pump any gas because the "computer
was down".  It seems that the pumps at the station were the new kind (with
the digital displays for price per gallon, total, etc.) and were linked
through to some computer somewhere.  Who would have thought that a computer
failure could prevent us from being able to purchase gasoline?  But now that
I think of it, all those new point of sale terminals linked to a computer
could be in trouble if the computer fails.

It used to be that this kind of problem would occur only if there was an
electrical power outage, but now just having the computer down can cause the
same problem.

decvax!wanginst!infinet!barnes      Jim Barnes

HBO Hacker Captain Midnight Caught

23 Jul 1986 17:08-PDT
    JACKSONVILLE, Fla. (AP) - Investigators using a complicated process of
elimination have unmasked ''Captain Midnight,'' who admitted in court he
overrode HBO's satellite delivery system to transmit a message.
    John R. MacDougall, owner of a home satellite dish business in Ocala
that officials said was hurt by cable companies' decisions to scramble their
signals, agreed to plead guilty to illegal transmission of a satellite
signal in exchange for a $5,000 fine and one year probation.
    He could have faced a maximum $10,000 fine and a year imprisonment.
    MacDougall, who was released on a $5,000 bond, and his attorney,
John M. Green Jr., refused to comment as they left the federal court
building Tuesday after entering the plea before a U.S. magistrate.
    Sentencing is set for Aug. 26 and MacDougall can retract his plea if
the judge will not accept the arrangement.
    Early on April 27, MacDougall was the only one working at a satellite
transmission center called Central Florida Teleport with the kind of
equipment needed to disrupt the HBO signal, officials said.
    Although the video sneak attack was only a minor annoyance to HBO and
its viewers, the Federal Communications Commission launched a massive
investigation because of the potential problems a less selective video
hacker might cause.
    ''The potential for damage to critical satellite frequencies cannot be
underestimated,'' said Richard M. Smith, chief of the FCC's field operations
bureau. He noted that critical telephone calls, air traffic control,
military data and medical information are sent by satellite and that even an
accidental interruption of one of these messages could cause dire
    On April 27, HBO viewers saw a message replace the movie ''The
Falcon and the Snowman.'' The message said:
    ''Good Evening HBO
    ''From Captain Midnight
    ''$12.95 month
    ''No way!
    ''(Showtime Movie Channel beware.)''
    The wording was an apparent reference to HBO's decision to scramble
its satellite-delivered signal so it could not be watched by those
not paying for HBO, officials said.
    ''His company was sustaining substantial losses because of the
scrambing of HBO and threats of other scrambling,'' said Assistant
U.S. Attorney Lawrence Gentile III.
    MacDougall also interrupted HBO video signals on April 20, when he
transmitted a color bar pattern, officials said.
    On Jan. 15, HBO became the first cable TV network to scramble its
signal full time. Showtime and The Movie Channel scrambled their
programming full time on May 27.
    The scrambling makes pictures unwatchable without a descrambler and
slowed sales of satellite dishes.
    Of 580 satellite facilities with a transmitting dish large enough to
overpower HBO's signal, less than a dozen had sufficient power and
the right kind of electronic typewriter to write the protest message
Captain Midnight transmitted, investigators said.
    The investigation focused on Ocala after a tipster vacationing in
Florida reported to the FCC an overheard telephone call about Captain
Midnight. The tipster provided the caller's description and license
plate number.
    The caller who was overheard was not the suspect, but the FCC said
the information provided proved extremely beneficial.

   [The L.A. Times refined this a little, after noting that there were only
    580 appropriate candidate facilities:

        "By studying tapes of the illegal video signal, the FCC's field staff
      concluded that the message had been generated using a specific make
      and model of character-generator device to transmit symbols, such as
      letters and numbers, onto a television screen.
        "After visiting those plants, investigators had three prime suspects,
      including MacDougall.  When he was notified he was a suspect, MacDougall
      turned himself in."

   This seems like a nice bit of detective work, and certainly presents an
   interesting risk for would-be perpetrators — somewhat like radioactive 
   traces in dyes, watermarks in paper, imperfections in certain characters
   on a typewriter or printer, and voiceprints (all of which have been used
   successfully to identify or subset culprits).  On the other hand, the
   smart perpetrator, aware of such tell-tale signatures, might figure out a
   way to spoof someone else's tell-tale, similar to changing the answer-back 
   drum on a teletype or hacking your cellular telephone identifier (as noted
   in a previous RISKS by Geoff).  Will this case escalate the sophistication 
   of satellite attacks?  PGN]

Please report problems with the web pages to the maintainer