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 02

Saturday 2 June 1990


o Article on A320 in Aeronautique, April 1990
Pete Mellor
o Info on RISKS (comp.risks)

Article on A320 in Aeronautique, April 1990

Pete Mellor <>
Sat, 2 Jun 90 17:08:14 PDT
I was given this article by some colleagues who bought the magazine while
visiting France for a research project meeting. Having had a quick look at
it, I decided that it was *very* interesting. It contains chapter and verse
on a couple of hair-raising incidents on board the A320, and the author
obviously had access to the dossier of OEB's, from which he draws some
fascinating conclusions regarding the general state of readiness of the A320
on entry into service and the possible causes of the Habsheim accident. He
also includes an excellent summary of the legal wrangle surrounding the
investigation into Habsheim. So, because of:

a) the technical quality of the article,
b) the fact that it presents a French (and therefore not negatively biased?)
   view, and
c) the fact that it is not readily accessible to the majority of UK and US

I decided, at *enormous* cost in time and effort :-), to make a careful
translation of the whole article, and send it complete to RISKS and to
Aeronautics Digest. [...]

                       LES CRISES DE NERFS DE L'A320

Translation of article by Bertrand Bonneau: Aeronautique, April 1990, pp. 94-101
        [Translator's comments and additions are in square brackets.]

                       THE A320'S ATTACKS OF NERVES

- The first aircraft in the history of the world to be totally "managed" by
- computer; has the A320 been put into service before it is ready?
- The excessive number of incidents during its first year of use can only make
- one think so. How could the willingness to declare the pilots responsible for
- major accidents, even before the judges have returned their verdict, appear
- other than suspect? Even so, as everyone wished, the verdict whitewashed the
- aircraft.

At the start of 1988, the French authorities and Airbus Industrie congratulated
themselves on the certification of the A320 only one year after the first flight
of the prototype. In less than one year, the manufacturer had demonstrated the
reliability of this new generation aircraft to the authorities of four of the
States of the European Community.

However, controversy surrounding the aircraft would not be slow to surface at
the time of the inaugural flight of the Air France A320, on 28th March 1988
over Paris, with the Prime Minister of the time on board. This flight was
marked by a series of technical incidents, notably by the untimely setting off
of alarms. New controversies were to arise when an aircraft was destroyed in
the forest of Habsheim in Alsace (26th June 1988), and when an Indian Airlines
A320 crashed before reaching the runway in Bangalore last February. In both of
the last two cases, the aircraft was whitewashed as far as public opinion was
concerned before the slightest preliminary accident report was published...

Although what have come to be called the "Chirac flight" and the "Habsheim
affair" are the two facts most known to the public, the first year of operation
of the A320 has been marked by numerous incidents which have directly called
into question certain systems on the aeroplane. Often badly received by the
first crews qualified on this aircraft, and sometimes vigorously denied by the
technical directors of the launching companies, these incidents lead one to ask if the manufacturers and the certification authorities have not proceeded a
little too quickly.

*Twelve times more incidents than were foreseen.* In his statement on the first
year of operation of the A320 in the Air France fleet, a statement addressed to
the general department of civil aviation (Direction Generale de l'Aviation
Civile - DGAC) on the 11th July 1989, the technical sub-director of operations
management of the national company remarks that the first exercise has been
marked by "a greatly increased number of technical incidents altogether"
(page 12). Whereas the target set was one incident per thousand hours of flight,
the year 1988 ended with an incident rate of twelve per thousand hours of
flight. For comparison, this rate was 5/1 000 at the time of the first year of
operation of the Airbus A300.

The frequency of these incidents which have marked the A320 going into service
within Air France, Air Inter and British Airways has forced the manufacturer to
publish no fewer than 52 provisional flight notices (OEB, Operations Engineering
Bulletin) between April 1988 and April 1989. The launch of a new aircraft
requires on average four times fewer. OEB's are temporary notices sent out by
the manufacturer to the users. They form a list of anomalies or simply
functional features of the aircraft, which do not appear in the users' manual
for the equipment (FCOM, Flight Crew Operation Manual): they are only revealed
in the course of operation. In the case of Air France, these provisional
records are provided to the crews in the form of a volume of supplementary
technical information notices (Renseignements Complementaires Techniques
 - RCT's).

For the A320, the number of OEB's alone gives an account of the problems of
putting the aircraft into service. At the technical level, around twenty of the
fifty main computers of the first A320's coming off the production lines in
Toulouse have had to undergo modifications. For the A320 is the first aircraft
in the world to be completely computerised. Computers control the function of
all the systems of the aeroplane (motors, ailerons, but also the cabin
lighting, etc); it [sic] processes raw data, converts them, and
transmits them to the pilot. Now, the application of numerous modifications
defined by the manufacturer in order to correct defects in the systems or to
enhance them, has been the origin of new breakdowns. These new problems have
obliged the manufacturer to publish new OEB's before drawing up final

During service, companies have had to modify once or several times certain
procedures for operating their aircraft. Also, with the exception of Air Inter,
which reported only good results, the increased number of incidents was the
origin of poor availability and bad technical readiness of the first A320's
delivered. "Of 7 334 stop-overs [landing + take-off's (?)] carried
out up to April 1989," states the report of the technical sub-director of
Air-France, "one lists on technical grounds [i.e. something went wrong (?)]:
4 accelerations-stops on take-off, 36 about-turns on the ground,
10 about-turns in the air, 1 emergency descent procedure, the cabin altitude
being on the increase (without violent decompression), 1 engine stop in flight."
[If you think this lot is confusing, you should see the original French!
 I think an about-turn on the ground is an aborted take-off, and an about-turn
 in the air is a return to the departure port. I'm not sure what the difference
 is between an about-turn on the ground and an acceleration-stop. Presumably
 the latter means the engines raced or cut-out during approach to take-off.
 'Cabin *altitude* being on the increase' is a literal translation: I think it
 means the cabin atmosphere was below pressure, since they came *down*.
 Anyone with access to a dictionary of French avionic terms, or who knows the
 correct English avionic terms is welcome to correct me!]
It is advisable to add to these outcomes the grounding of aircraft due to
suspect behaviour, and 74 cancellations of flight before even starting up the

*Reliability in question*. For the aviation companies, the most serious problem
would seem to have been that of the reliability of the information given to the
crew by the various systems of the A320. The operating assessment by the
technical sub-director of Air France is edifying on this subject. One discovers
there, for example, that: "certain inconsistencies of piloting information
have led to certain confused and very distracting situations, where the
information presented to the pilots on the control screens, during flight, was
in contradiction to the physical reality of the equipment, not always
verifiable in flight", (report already cited, page 18).
[Presumably this means: "The instruments were lying, but the pilots couldn't
 get out and walk around to check this at 30 000 feet!" Nice to know that
 French technical officialese is as obscure as British or American! ;-}]

Without a doubt, Captain Claude Dalloz and First Officer Patrick Vacquand share
the views of the technical sub-director of Air France. On the 25th August 1988,
while taking off from Roissy on a flight to Amsterdam (flight AF 914), they had
the disagreeable surprise of seeing the message "Man pitch trim only" appear in
red on their control screens. In plain terms, this message informed the pilots
that the controls activating the pitch control mechanism were no longer in a
functional state. In this case, the only means of ensuring the longitudinal
stability of the aircraft is to manually move the trimmable horizontal
stabiliser by means of the pitch trim wheels.

Meanwhile, the copilot who was at the controls felt not the slightest difficulty
in controlling the aircraft. Then the crew witnessed a display of imaginary
alarms ("fire in the toilets", for example), and noticed new signalling
anomalies on the screens concerning the flight control systems, the position of
the landing gear, and also the situation of the automatic pilot.

It was therefore decided to return, but, during the approach, the gear at first
refused to come down normally. Given the uncertainty, three passes at low
altitude were made in front of the control tower to ascertain the real position
of the gear after having carried out safety manoeuvres. As the information
provided to the crew ("gear partially down") did not correspond to the
observations of the controllers at Roissy (gear down), the passenger cabin was
prepared for an eventual crash, which did not, very fortunately, occur. The
same incident recurred on another plane on 29th November 1988. It finally
required nine months of operation before a new, more reliable, version of the
Flight Warning Computer (FWC) called into question by these two cases was made
available to users.

*A temperamental altimeter*. A good many problems due to the design of
certain systems have revealed themselves since the start of operation. The
most spectacular, for the passengers, would have been the vagaries of the
integrated cabin communication system (CIDS), which modified explanations or
illuminating announcements in an eccentric fashion. More seriously, the crews
discovered that the temperature regulation of the passenger cabin could
interfere with the functioning of the engine power control computers (FADEC),
generating breakdowns and alarms. To avoid these interferences, crews were
asked not to "reinitialise" the cabin temperature regulation system while the
engines were running.

However, the most worrying phenomenon for the crews has been the untimely
alterations to the setting of the altimeters during flight. Having reached
a certain altitude, the pilots set their altimeters in a standard way,
calculated in relation to the theoretical atmospheric pressure at sea level
(1 013 hPa), in order that all aircraft using the airspace should have the same
reference for altitude (QNH base). Relative to this base, the altimeter
indicates a pressure altitude, which is a "QNE" altitude. While the aircraft
is descending, at a predetermined height the crew must set their altimeters
in relation to the altitude of the destination airport (QFE base). Apart from
some very rare landing strips situated below sea-level, airports are above this
[sea-] level. Since pressure diminishes with altitude, the value of QFE is
generally less than 1 013 hPa. The sudden alteration of the altimeter setting
by the flight programming computer (FCU, Flight Control Unit) sometimes occurs
in uncomfortable conditions. So, in July 1988, during an approach to Roissy,
the untimely alteration of the altimetric setting, which conveyed itself as
a reversal of the altimeter reading, provoked an automatic delivery of fuel in
order to compensate for the false deviation in altitude generated by the
defaulting computer and detected automatically by the safety systems of the
aircraft. This delivery of fuel occurred while the aircraft was being flown
manually on its descent. The rapid intervention of the pilot could not avoid
the aircraft going into overdrive for several seconds.

Untimely alterations of altimetric settings showed up on at least the first
three planes delivered to Air France, among them the aircraft which crashed at
Habsheim. The commission of enquiry has revealed in its final report that such
an incident had taken place on the plane several hours before its crash,
concluding immediately that this anomaly due to a design error had played no
part at all in the accident. Moreover, the flight report (CRM, compte-rendu
materiel) of a crew, concerning a third aircraft of Air France, made mention of
vagaries of the altimeter.

It is therefore surprising that the report of the technical sub-director of
Air France limits this type of incident to a single A320 of his fleet (the
aircraft registered F-GFKB), when it has also occurred on at least two other
planes (registered F-GFKA and F-GFKC). But the most amazing thing remains that
this functional anomaly should cease without anyone being able to identify its

*Recording of parameters*. In an indirect manner, these two types of incidents
have revealed another potential source of problems in the level of the
recording of parameters by the "black box recorder" (DFDR, Digital Flight Data
Recorder). In effect, each piece of information given to the pilot is handled
by a cascade of computers. Now, this "black box" records the majority of its
information on the intermediate computers and not at the start
or end of the processing chain. When examining this data, therefore, there
is nothing that allows one to know precisely what the pilots had for
information, since there is no recording at the output of the symbol generator
[DMC] for their screens.

The problems posed by the flight data recording system can be illustrated by
referring to the two incidents mentioned. If the Paris/Amsterdam flight
recalled above had ended in a crash, the "black box recorder", which captures
a large part of its information from the flight warning computer (FWC), would
have revealed that the crew no longer had pitch control available. In fact,
all the flight controls were functioning, but the flight warning computer,
which is one of the principal sources of information of the "black box
recorder", had failed (diagram, p.98).

Equally, if the untimely alterations of the altimeter readings had ended in
a crash, the "black box recorder" would have revealed no malfunction of the
altimeter assembly, since the recording of pressure altitudes (QNE), which was
correct, is effected by equipment located upstream of the failing computer.
This computer (FCU) incorrectly processed the information which had been sent
to it, and an erroneous indication of altitude was sent to the control screens
(diagram above, p. 99).

*Modification Campaigns*. Before the A320's went into service, the launch
companies' instructors - who cannot be accused of bias since
they were all volunteers - complained of having had no contact with the
test pilots of Airbus Industrie. The report of the technical subdirector of
Air France, for its part, confirms this worry by revealing that it had at last
been possible to establish a "frank relationship" (page 17) after six
months. The adaptation of failing systems has been progressively integrated
in the course of several modification campaigns begun at the start and
middle of 1989 as problems were found and listed. It was necessary
to wait until the end of last year to obtain the definitive version of certain
pieces of equipment, that is to say, eighteen months after the certification
and entry into commercial service of the A320.

At the end of last year, the dossier of supplementary technical notices (RCT's)
distributed to A320 crews already comprised eleven pages, whereas the RCT's of
other aircraft in the Air France fleet rarely got beyond three pages.

Contrary to the fears expressed many times in the course of these last years,
not only by certain pilots' unions, but also by the American certification
authorities (FAA, Federal Aviation Authority), the electrical flight controls
and the electronic engine control system, which constitute the two great
technological innovations of the A320, would never be the direct cause
of any significant incident, notably in stormy conditions. During test
just as in service, the A320 was struck by lightning several times without the
least influence on the flight controls.

The majority of the teething troubles and design faults of the A320
therefore concern more classical systems. The report of the technical
sub-director of Air France is once again definitive: "Pressurisation,
management of cabin communications (CIDS), pneumatic generation, auxiliary
power units (APU)... have been for a long time of an unacceptable reliability.
Everything is still not under control to this day (NDLR: 11th July 1989)."
(Report already cited, page 17).

*Industrial secret*. It could therefore be thought that the certificator has
turned his attention above all to the innovative elements (flight controls,
FADEC, etc.) of the A320. However, this explanation, although not completely
without foundation, does not take into account the fact that the systems called
classical are also subject to major innovations, since they practically all
require computer automation.

Without invoking the young demons of computing, the embedding of numerous
pieces of software on board aircraft of the new generation (A320, but also
McDonnell-Douglas MD 11, Boeing 747-400, among others) can pose problems for
the official agencies. Up until then, the certificators were confronted by
much more simple systems (cabling, for example) and by perfectly mastered
technologies (electricity, for example). With the A320, the certificator
found himself before a gigantic interactive data processing system, made up
of "boxes" which consisted of inputs and outputs. No-one having foreseen
such a rapid installation of computers on board service aircraft, it was not
possible to find, in the international regulations, standards directly
applicable to this domain.

Furthermore, the certificator came up against protection of embedded software
by industrial secret. The official agencies were finally forced sometimes to
give their agreement to a piece of equipment, on a simple demonstration of the
required result, without being able to know precisely the organisation of the
system which allowed it to be reached. In such a context, only a more thorough
programme of tests would have permitted the major design faults of certain
systems to be revealed with certainty and would have avoided certain launch
companies having to proceed with the modification of nearly half of the main
computers of their first A320's.

The protection of software by industrial secret constitutes a source of
problems also for the users' maintenance services, who must leave it to the
manufacturer to understand the reasons for its failure. For the time being,
the best equipped companies are provided with certain software test sets, but
eventually the users will have to be able to test their systems directly on
the battery of test sets of the manufacturer through data transmission networks.

*The dead-ends of certification*. It is interesting to note that use has
revealed several loopholes in certification. A provisional information bulletin
dating from the month of May 1988 (OEB no. 06/2) reveals for example that the
single information source for the pilot's and copilot's altimeters on the A320,
is not compatible with certification standards [i.e. it makes a single point
failure possible?].

Another provisional information bulletin sent out in August 1988 (OEB no. 33/1)
indicates that the safety lighting system of the floor of the passenger cabin
does not conform to certification standards. This system would not
automatically illuminate when one of the emergency evacuation devices of the
aircraft was activated, in the case of the loss of the normal electricity
supply. Now, this lighting system provides an illuminated pathway in the
central aisle of the cabin, which must allow passengers to find the safety
exits or doors during an evacuation in the dark or in smoke. This design
defect was underlined by the final report of the commission of enquiry into
the Habsheim accident.

"To be the launch client of a new aircraft is sometimes a painful task,"
one of the directors of Air France confided last year, before adding that "the
A320 would attain the level of reliability of the fleet (of Air France) by the
start of 1990." This would be practically two years after the certification of
the aircraft. A last example: it has been necessary to wait until the month of
July 1989 for it to be noticed, in the course of a test flight, that the
landing gear could, in certain cases, not retract fully in case of a shut-down
of engine no. 1 during take-off (OEB no. 62/1). (1)

The totality of these elements, then, could allow one to believe that speed and
haste had been confused.

                                  Bertrand Bonneau
     (1) The OEB's are intended to be temporary. As a consequence, the
     anomalies with which they are concerned when they are sent out, have
     normally been corrected.

[End of main article. Text in boxes accompanying illustrations follows:]


[Box on p. 95, below photograph of cockpit:]

*Up to the customer to complete the tests.* The standards and certification
procedures of civil aircraft are not adapted to the A320, an aircraft which,
for the first time in the history of civil aviation, is massively equipped
with data processing systems. For example, the software in the flight warning
computer [FWC] included a fault which a good computer scientist could have
repaired without a doubt. But this software is protected by industrial secret,
and as luck would have it the fault did not show itself at the time of the
certification campaign. Result: an aircraft has been sold with a certain
number of latent defects, which the first customers have discovered bit by bit.


[Box on p. 97, accompanying photographs of instrument panels in cockpits of
 (1) A320, (2) A310, (3) A300:]


The A320 (1) is the first aircraft whose cockpit panel is entirely equipped
with cathode tube screens [CRT's]. Only three traditional instruments are still
found there, in case of failure of the former [i.e. CRT's]. The new screens
display more synthetic and more complete information to the crew. So, the whole
navigation of the flight is directly visible to one of them, and the image
evolves in real time along with the movement of the aircraft, whereas on
traditional aircraft, this tracking is effected by the pilot on a piece of
paper on which he reports the successive positions given by the on-board
equipment (radiobeacon receiver, radiocompass, inertial platform [IRS?], etc.).
But these screens can also, unfortunately, deliver erroneous information if
one of the systems that supplies them is failing; and the irony is that often
this information cannot be verified by the pilots in flight (see diagrams,
p.98). An aircraft of the preceding generation, the A310 (2), was already
equipped with some screens, whereas the A300 (3), which was developed at the
start of the 70's, is only equipped with classical electromechanical


[Box on pp. 98-99, illustrating two incidents described in the main
text, labelled case A and case B in the boxed text to allow cross-reference
between that and the two accompanying diagrams, which show by numbered labels
the placing, and communication between, the following:]

 1. PFD. Piloting screen. It is this which displayed "manual pitch trim only"
    in case A in the text, and the erroneous QFE altitude in case B.

 2. ECAM. Screen which gives information about the aircraft systems (motors,
    lighting, etc.).

 3. PA. Automatic Pilot.

 4. Side-stick.

 5. FCU. Flight Control Unit.

 6. DMC. Symbol generator for screen displays [Display Management Computer].

 7. SEC-ELAC-FAC. Computer [sic] for flight controls (ailerons, pitch control
    surface, flaps, spoilers, etc.).

 8. ADIRU. Air Data Inertial Reference Unit

 9. SDAC. System Data Acquisition Concentrator, which translates into data
    processing language the data received from systems upstream of it (sensors,
    controls, etc.).

10. FDIU. [Flight Data Interface Unit] Computer for the flight data
    recording system, which manages the "black box recorder" [DFDR].

11. Hydraulic servo-mechanism for pitch control surface.

12. Trimmable Horizontal Stabiliser [THS] and pitch control surface.

13. DFDR. "Black box recorder" [Digital Flight Data Recorder].

14. Switch for display of QFE pressure [on FCU]


*A. Alarmist computers.* This simplified diagram [p. 98] of the A320 systems
(which takes no account of the actual location of the computers) shows how the
crew of flight AF 914 of 25th August 1988 found themselves confronted by
nonsensical information generated by the flight warning computer (FWC). This
sent the erroneous message "manual pitch trim only" to the piloting screen
(PFD) and to the "black box recorder" (DFDR), a message informing of a loss of
control of the pitch control surfaces (red arrows). [Sorry. Colour diagrams are
difficult over e-mail ;-)] Put simply, the pilot can no longer control the
climb or descent of his aircraft with the stick (but only by means of a manual
back-up control). In fact (green arrows), this control [i.e. the electronic one]
was functioning perfectly.

*B. Imaginary altitude.* The second diagram [p. 99] shows how the pilots had
on their screens an untimely alteration to their altimetric setting, generated
by the flight control unit (FCU), whereas the altitude data in the air data
inertial reference unit (ADIRU) was correct. The FCU prompted an inversion
between the pressure altitude (QNE) and the altitude of the destination landing
strip (QFE). As the QFE was giving an altitude below the QNE altitude (which
would allow one to believe that the aircraft was flying dangerously lower than
it was in reality), the safety systems of the aircraft demanded an automatic
delivery of fuel to regain height.


[Box on pp. 100-101:]


On the 26th June 1988, the air show organised by the little flying club of
Habsheim, in the Haut-Rhin, turned to drama when an Air France A320 crashed
with 130 passengers in the forest which bordered the landing field, in the
course of its display flight. In a few minutes, the aircraft was almost
completely burned. Toll: 3 dead, 34 injured, the other 93 occupants unhurt.

Nearly 18 months after the accident, the Commission of enquiry delivered its
report. Contrary to what it had been possible to affirm, this document (called
the "Bechet report" after the name of the president of that commission) does
not establish any responsibility, but limits itself to stating the facts and
suggesting some measures. After all, only judges are entitled to decide blame
and responsibility. Now, this decision has not taken place. The investigating
magistrate has even requested recently the reopening of the inquiry for
supplementary information.

No-one knows, then, what the Mulhouse magistrate thinks, but the context in
which the enquiry into this accident was begun could be marked by certain
irregularities. Indeed, on the evening of the drama, the director general of
Civil Aviation was filmed by a television crew as he took charge of the
transport of the two "black boxes" (CVR and DFDR). Now these two recorders
are the essential elements for the enquiry. The presence of the director
general of civil Aviation at the scene of the accident and the particularly
active role that he played that evening seem hardly compatible with the
ministerial directive of the 3rd January 1953 relating to the coordination
of the judicial inquiry and the technical investigation and with
directive no. 300 IGAC/SA of the 3rd June 1957 concerning the steps to be
taken in case of irregularity, incident or accident in aviation. The General
Directorate of Civil Aviation having had the responsibility of certifying the
aircraft and having authorised the holding of the meeting, it is legitimate
to ask oneself if its director is not simultaneously judge and party to the
case. Moreover, the authority designated by the regulations as being competent
in the matter of enquiries is not the DGAC but the General Inspectorate of
Civil Aviation (IGAC), placed under the direct authority of the Minister of

A second factor, which follows from the first, could leave one to suspect
that the concern of the only technical enquiry had overridden that of
the judicial enquiry. First, it was necessary to wait two days for an
investigating judge to be appointed, whereas that is generally done in half a
day for major accidents; and this is one of them [i.e. major], with, moreover,
a considerable amount at stake. Furthermore, the two black boxes were left for
nine days without any judicial control, since the placing under seal was only
done on the 5th July (let us recall that the accident took place on 26th June).
In the meantime, parts of the recording of the conversations held in the cockpit
during the flight were published in the press, in defiance of the secrecy
required by the directive [i.e. no. 300 IGAC/SA of 3rd June 1957(?)].

Reading of the Bechet report (page 41) reveals that the assessment of the
damaged aircraft engines was entrusted to their own manufacturer (CFMI), on the
SNECMA premises at Melun-Villaroche. Without casting doubt on the quality of
the assessment achieved by the manufacturer on the premises of one of his
partners with the participation of the Commission of Enquiry, it seems
astonishing that the manufacturer should have had control over a procedure
which concerned him so directly. As one knows, in the case of an enquiry
relating to an accident, an assessment is always likely to have judicial

That is all the more surprising since the engines had been directly implicated
by the statements of the crew immediately after the accident. One can therefore
ask oneself why the assessment of the two CFM56-5A1's was not entrusted to the
experts of the Propeller Test Centre of Saclay, which comes under the Flight
Test Centre. Indeed, this centre does not have any judicial, industrial or
commercial links with the equipment in question.

Even if the conclusions of the commission of enquiry, based on that assessment
and on the recording of the "black box recorder" [DFDR], categorically
rule out the two engines, that will not cut short some of the objections which
some of the lawyers would have been able to try to set out before the judges
of Mulhouse. Such would not have been the case if that investigation had been
entrusted to an organisation which was not also an interested party.

If the defects of acceleration of the CFM56-5A1 engines of the Air France A320,
noticed sometimes in certain cases of low altitude flight, did not exhibit
themselves at the time of the accident, why, then, was a provisional information
bulletin (OEB 19/1) sent out in May 1988, modified in the following August
(OEB 19/2)? Moreover, the adjustment of the stator blades (counterbalancing [?]
of the jacks which modify their pitch [?]) of these engines, which has a direct
link with their efficiency at low speed and at low altitude, was also modified
a short time after the accident. There again, why?



1. The opinions expressed in this article are not necessarily the opinions
   of City University, of the Centre for Software Reliability, or of the

2. Misprints in the original are the responsibility of the publisher.

3. Factual errors in the original are the responsibility of the author.

4. Errors in translation are the responsibility of my O-level French mistress.

5. I am not responsible for ANYTHING! ;-}

Peter Mellor

Please report problems with the web pages to the maintainer