The RISKS Digest
Volume 7 Issue 12

Thursday, 30th June 1988

Forum on Risks to the Public in Computers and Related Systems

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

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o Airbus 320
Steve Philipson
o Background on the A-320 incident
Willis Ware
o Fly-By-Wire
John O. Rutemiller
o Airbus 320
H.Ludwig Hausen
o $40 million Pentagon computer system failure
Rodney Hoffman
o Re: Another "silent fault tolerance" example: DWIM
Tim Budd via Mark Brader
o Info on RISKS (comp.risks)

Airbus 320 (RISKS-7.11)

Steve Philipson <>
Thu, 30 Jun 88 13:18:20 PDT
Here are some comments on the A-320 crash, official reports, and 
unofficial speculation.

   It is truly amazing that official sources attributed the accident
to pilot error virtually before the investigation began.  A decision 
seems to have been made to protect Airbus and blame the flight crew
no matter what the facts.  It was grossly irresponsible to exonerate
the aircraft before the flight data recorders were examined, or experts 
had time to analyse the video tapes and other data.  

   There are several questions that many people have not asked that are
critical to an understanding of this accident.  If the aircraft was
indeed only at 30 feet of altitude, why was it that low?  Did the pilot
intend to descend to that height?  What was the minimum altitude authorized
for the low pass?  What was the actual airspeed, the pilot-commanded/desired
airspeed, and the minimum authorized airspeed?  

   Observers claimed that they heard the engines spool-up too late.  The 
conclusion is that the pilots did not allow for the spool-up delay.  Such 
a delay is NOT specific to the A-320 but is rather a characteristic of all 
turbo-jet engines.  Why would an experienced crew have delayed adding power, 
or even throttle back to low idle, when a high-speed idle would have made 
engine response much quicker.  When did the crew command a power-increase?  
Was it several seconds BEFORE witnesses heard the engines START to spool-up?

   Some maintain that the "computer was turned off".  This is an interesting
concept for an aircraft that flies completely by electronic controls.  There
are no direct manual controls of the primary flight surfaces or engine
controls.  Some auto-matic features may have been de-selected. It is of 
great interest which these were and how the selected systems operated.

> The planes automatic escape chutes, which opened as soon as the plane crashed,

   That's an interesting report.  As far as I know, there is no automatic
door opening / escape slide deployment mechanism.  Slides inflate auto-
matically once doors are opened manually.  The likely reason that so few
lives were lost is that the aircraft contacted the trees in wings level,
controlled flight, with the nose up.  The lower fuselage structure and wings
absorbed much of the impact, and as trees were destroyed they relatively
gradually slowed the aircraft.  

> I believe that manslaughter proceedings may be brought against the pilot.

   What will be the outcome if we find that an aircraft system was at 
fault? Do we bring charges against Airbus management, engineers, or 
software programmers?  

   It is interesting that British Airways is satisfied that the aircraft 
was not at fault.  How could they have made that decision before any data on
the accident was released?  Every day that their airplanes sit on the ground
costs a lot of money in lost revenue, and is negative publicity for their
shinny new airplanes.  The airlines thus have a proprietary interest in
keeping them flying.

   Officials blame a flight crew that was chosen for its experience and
abilities.  This crew was extensively trained and passed certification
exams on systems and flight of this aircraft.  That such a crew could be
involved in an accident like this indicates that the aircraft is not
immune to accidents, even with it's advanced technology.

   The A-320 may have been designed to be more safe than older technology 
craft, but this does not mean that it is.  Only operational experience 
will establish the actual risk.  A major concern of systems analysts and
pilots is that automated systems may actually increase risk as the pilot 
is not "in the loop", at the least not to the extent he is in other 
aircraft.  Operational experience has shown us that automation introduces
new problems as it addresses some old ones.  As of yet, we have no data
to show that the highly automated A-320 will be as safe, less safe, or 
more safe than older aircraft.  We may learn a lot about the aircraft
from an analysis of this accident.  We will certainly not know why the
accident occurred or if anyone is to blame until the investigation is

                        Steve Philipson

Background on the A-320 incident

Thu, 30 Jun 88 10:14:10 PDT
Given the extensive commenting on the A-320, perhaps these observations
will be useful.  They're based on my personal knowledge of avionics as it
is implemented and practiced in USAF aircraft and in the 757/767.

Incidentally Klaus Brunstein quotes the investment in the A-320 as $10
Billion.  I wonder whose "billion" he's using; if it's the English
billion, that's an investment of $ 10*13 — lotsa bucks.

The USAF F-16 tactical fighter is a fly-by-wire and implements flight
control with a quad-redundant analog (YES - analog) computer in the belly
of the plane.  It is a microcircuit analog implementation to be sure, but
nonetheless avoids the software problem in flight control.  There are no
cables from the cockpit to the control surfaces except for trim tabs.  If
the quad-redundant machines are lost, the pilot can try to land the plane
by manipulating trim tabs.  Needless to say, one of the earliest changes
to the aircraft was to put protective armor around the flight controllers.

There is a separate 65K (as I recall) computer that integrates aircraft
functions, delivers signals to the flight control analog system, receives
signals from it, talks to the software-controlled heads-up-display, receives
digital inputs from the inertial navigator and from manual pilot inputs,
manages the radios, receives signals from and sends control signals to the
software-controlled radar, transmits pilot inputs to the digital
weapons-control subsystem and receives status information from it, receives
and logs all fault indications from the entire aircraft and runs diagnostic
tests on itself and on other subsystems, AND manages cockpit instrumentation
and display.  The sytem also monitors for some danger conditions, e.g.,
wheels up but ground clearance approaching zero.  Everything communicates
with everything else on a 1 Megabit/sec digital redundant MUX bus.

As I recall, pilot intentions (originating by a non-moving pressure-
senstive side stick) are communicated directly to the flight control
system which then relays them to the digital systems.  Thus the pilot
could fly the airplane IF the master digital system failed, although he
might have no instrumentation (at one time there was a debate about
leaving a few "round instruments" in the cockpit as fallback).

USAF has experimented with and test flown a digital fly-by-wire system,
but I don't know whether it's been implemented in more recent aircraft, or
even in the upgraded F-16.  Odds are that newer fighters (e.g., F-18s and
F-20s) are all-digital.  Almost surely the ATF will be all-digital.

Even though the analog system actually flys the F-16, it gets inputs and
control from the digital systems.  In particular, the acclerometers of the
inertial nav system report dynamic acceleration and if it's too high, the
intent of the pilot is overriden by the software controls to avoid tearing
the wings off the plane and blacking out the pilot.  In fact pilots have
complained about this because they would be willing to risk aircraft
damage and/or blackout to escape a pursuer or perform some maneuver.

In US commercial aircraft that have "glass cockpits" (as the CRT displays
are called), flight control has continued to be traditional direct cable
linkage for the most part although hydraulic boost is almost always
needed.  There are aircraft in which the stick motions control only a
hydraulic system which is the only linkage to the surfaces.

There usually is an all-digital system called by some such name as "Flight
Director" that does or helps with navigation (especially on inertial-nav
equipped aircraft), controls the aircraft trajectory in conjunction with
an autopilot or other nav inputs, might support fuel management, might
handle the aircraft through Cat III (blind-landing) procedures, controls
the descent from flight altitude to minimize fuel consumption, handles the
throttles to conserve fuel consumption, etc.

Without a lot of technical details, one cannot know how the A-320
designers implemented their software and distributed the functionality
across one or more computers.  Odds are that the flight control is in a
separate and redundant set of machines for safety.  If the designers were
astute, the redundant machines are powered from separate power sources and
through individual circuit breakers. [There is a recorded instance in a
commercial aircraft of the pilot losing an important instrument because it
was powered through a circuit breaker that also happened to control some
inconsequential other thing — such as lighting.]

So any comment about "shutting off the computer" (in the A-320) must refer
to the flight management system, not to flight control.  Airlines are
forever interested in optimizing cost of operation, and who knows what
flight-profile or flight-maneuvers may have been incorporated into the
A-320 systems?  Who knows what combination of danger situations have been
programmed in — and the flight crew blundered into?  Who knows whether
the aerodynamically possible and economically desired flight envelope
has been built into the software — and the flight crew accidentally

One can easily imagine a software requirement that says something like:

   IF   engines throttled back AND wheels down AND altitude less
    than ..... AND rate of descent equal to .... AND speed less
    than .....

   THEN aircraft is landing and adjust angle of attack to .....;
    also check for proper fuel-flow conditions for landing, check
    flap position, ......

In spite of what has been said, I personally am not yet ready to conclude
that a software anomaly lurks in the A-320.  Personal opinion of course,
but it sounds suspicious to me.

There is one other observation about glass cockpits that my friends in the
business have told me.  Round instruments not only indicate some parameter
but they also convey rate-of-change information.  Glass instruments
evidently have been implemented to convey only the parameter value, not
the derivative of the parameter.  If true, one can imagine that in some
circumstances the flight crew is denied helpful and possibly critical
information about events.

                    Willis Ware


"John O. Rutemiller" <Rutemiller@DOCKMASTER.ARPA>
Thu, 30 Jun 88 09:12 EDT
When considering the safety of a fly-by-wire system compared to a "normal"
system, you must consider whether the overall safety of the system is improved.
Granted there are failure modes in fly-by-wire that would not otherwise
exist, but there are also a lot of things you can do with fly-by-wire that
you would not be able to do with a "normal" system.  If the overall safety
of the system is improved, then it is a better system.

   [But don't forget that if the aircraft is designed to be aerodynamically
   unstable (i.e., without the computer) — as are some high-performance planes
   — overall safety can be nonexistent under certain conditions.  At the
   IEEE COMPASS '88 this week, John Cullyer noted that in a fully
   fly-by-wire plane such as the planned Eurofighter, the pilot will
   have at most two seconds in which to decide whether to eject, after
   which it may generally be too late.  (The Experimental Aircraft Project
   is developing a plane that will be -12% (un)stable.  John described the
   VIPER effort, which is being subjected to extensive formal proofs, and
   which is being designed for the EAP.)  PGN]

Airbus 320 (Re: RISKS-7.10)

"H.Ludwig Hausen +49-2241142426" <HAUSEN%DBNGMD21.BITNET@CUNYVM.CUNY.EDU>
Thu, 30 Jun 88 09:54
 There was a video on German TV Newsshows (ARD, ZDF, SAT1)
indicating that the Airbus plane
1. was 10 meters above the ground
2. in a position making the pilot unable to see the trees at the
   end of the runway (which was too short for landing an Airbus)
3. engines got power far to late to get back into secure hights.

Seeing the video one will get the impression that the whole thing
(going to a small, badly equipped airfield and doing demonstrations)
was a very risky PR manoeuver.  There might be some serious
problems with this Airbus plane (see B. Littlewoods comments) but
I guess this time it was the pilot's fault.  H.L. Hausen

$40 million Pentagon computer system failure

Rodney Hoffman <>
30 Jun 88 07:45:04 PDT (Thursday)
From the Los Angeles Times, June 29, 1988:


The Pentagon's latest effort to unscramble its tangled foreign military sales
accounts has been a $40-million failure, the House Government Operations
Committee said Tuesday.  It said a costly new computer system for straightening
out the botched program was two years behind schedule, had thousands of
unresolved problems and ultimately could cost $75 million without performing

As a result, the committee said, the Defense Department cannot say why there is
an unreconciled $1-billion difference between cash on hand in a special trust
fund and total payments by foreign countries that purchase U.S. military
equipment through the foreign military sales program.  "The [foreign military
sales] trust fund system is in shambles," Committee Chairman Jack Brooks
(D-Tex.) said.... 

The new accounting system, developed by the SAGE firm of Rockville, Md., for the
military, was supposed to be ready in October, 1986.  Now the completion date
has been postponed until next October, or two years behind schedule.

Re: Another "silent fault tolerance" example: DWIM

Mark Brader <>
Mon, 27 Jun 88 16:11:44 EDT
Path: sq!utfyzx!utgpu!utzoo!attcan!uunet!husc6!bloom-beacon!!rutgers!orstcs!mist!budd
From: (Tim Budd)
Newsgroups: comp.lang.misc
Subject: Re: What makes a language "easy" to program in?
Date: 17 Jun 88 20:31:42 GMT
Organization: Oregon State Universtiy - CS - Corvallis, Oregon

No, I'm not sure you want a ``do what I want'' command.

The following story is true, I've just forgotten some of the less important
details.  There was a Lisp system once that had something called DWIM (do
what I mean).  If you typed an expression, if it didn't make sense, it would
try various techniques to see if something close to it made sense, and do
that.  Now a friend of mine was using this system and kept having amazingly
slow programs.  It turned out he was saying things like (CAR ...) when the
system wanted (car ...).  It would not recognize CAR, go through some
analyzing, discover that a probable meaning was car, then do it.  Problem
was there was no feedback - no indication he was doing anything wrong, it
produced the right answer, just slowly.  So there are dangers in ``do what I
want'' systems even when (and this is a big if) they can exactly figure out
what it is that you want.

[Forwarded to Risks by Mark Brader] 
     [That was Warren Teitelman's INTERLISP environment.  PGN]

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