Re: FLY-BY-WIRE (AIRBUS vs. BOEING)

From:         mezei_jf@eisner.decus.org (Jean-Francois Mezei)
Organization: DECUServe
Date:         30 Jun 95 03:47:16 
References:   1 2 3
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>>The 310 may have the same joystick as the 320 and 340
>
> It does not.  The A310 (and A300) both have a conventional yoke.
>

When I objected to people including A-310 crash stats in the FBW discussion, I
was told that the 310 had similar user interface and that it could therefore be
included, even if the computers behind it were not as sophisticated. I deducted
that since it had the same interface, the 310 also had that joystick.

Nevertheless, previous posts and now yours  have indicated that the 310 doesn't
share the same user interface as the 320.

> In FBW systems, electrons replace hydraulic fluid, wires replace
> pipes, and generators replace hydraulic pumps.  There need not be

OK, in the case of the 767 or even the 747-400 which are not "FBW", how does the
autopilot actually move control surfaces ? does it have servos in the cockpit
that push/pull those mechanical cables to control surfaces, or does the
autopilot have electrical wires that go directly to the hydraulic systems that
provide the force to move the surfaces ?

>>If the information given to the computer is wrong, then problems occur
>>(eg: crash of 320 in France where computer aborted a take-off even though the
>>plan was already in air because its altimeter "value" was still at 0).
>
> What crash is this?  The details do not even remotely match any of the
> A320 crashes, nor any other Airbus crash that I'm aware of.
>

I stand corrected. I reread an old article I had read in Science & Vie (France)
, April 1990 about the A 320. The plane which crashed at Habscheim had suffered
the altimeter problem hours before its crash but it crashed for other reasons.

The altimeter problem caused a discrepancy between the current altimeter
readings and what the computer thought the alitude was and was experienced on
at least the first 3 A320s delivered to Air France. (It had to do with how
Airbus calculated altitude based on either an airport point of reference and
atmospheric pressure). Furthermore, that problem occured downline from the
flight data recorders: as a result the later would indicate correct altitude
while the "decision" computers would have been fed incorrect information and
hence taken what would have seemed to investigators to be "random" decisions.

As a resust, there had been occurances during a descent where the altimeter
thought the plane was too low and would increase thrust and "regain" altitude.
(Roissy, 1988) This would definitely cause confusion in the cockpit as the
pilot was at the controls (no auto pilot) and the plane's computers kicked in
thinking there was an emergency. The article stated that the pilots were able
to quickly regain control. This suggests that there is a way to override the
computers.

The "thrust reverser" business, I probably got from another article which
probably speculated on the cause of a crash baed on the knledge of that
altimeter problem. So, I stand corrected on that. It has been a few years since
and all this info had become fuzzy, especially since I didn't have much of an
interest in airplanes at the time. Re-reading the SCIENCE & VIE article with
all the recent discussions in mind provided an interesting point of view.

The article's mention of the user interface is basically to point out that to
acheive the various control "commands", the "steering device" has various modes
used during various phases of a flight.

During its first year of operation at Air France, the 320 had an incident rate
of 12 incidents per 1000 hours of flights, which is 12 times higher than what
AF had expected. (in 1988) The plane was "debugged" by the airlines after it
was certified and after it was put in service. Part of the certification
problem was that these incidents didn't show up during the certification
process, and there were issues of the certification agencies not having "easy"
access to all the computer programs for proprietary-secrets reasons and having
to rely more on cause-effect testing.

The certification folks had no idea for instance that resetting the cabin
temperature controls by flight attendants might affect the engine thrust logic,
and probably never bothered to test that. But Air France found out and had to
instruct flight crews not to reset the temperature controls during flight to
prevent false alarms on the pilots side or erroneous thrust changes.

The article mentions that the majority of the problems occured because of the
alarms systems feeding the cockpit/computers erroneous information. (going from
Fire in bathrooms to improperly deployed landing gear or loss of pitch trim
capabilities). These incidents are the ones that caused a lot of confusion and
the "what is the plane doing" questions.

It took 9 months to fix the alarm computers and ship the enw version of the
software to the 320 operators.


> The 777's system has been described here several times before.  At the
> risk of boring everyone who has been paying attention, when a pilot

You have to remember that when my question had originally
been posted some time ago, ("posted" != distributed) this info had not yet been
made available, and by the time my post made it to my newsreader (and possibly
yours), the stuff had long been answered, making it look like I was asking a
question that had just been answered by other posts.

> A number of contributors to this group know quite a bit about the
> interface in the Airbus system, either from extensive study of it
> or in at least one case from flying the beasts.  Please do not insult
> them by claiming that an all-encompassing "we" are ignorant of the
> facts when the ignorance is primarily yours.

Sorry about the "we". However, you have to understand that from the reader's
point of view. There have been many posts stating that Aibus "chose not to
inform pilots of what the computers were doing" or that Airbus provided no
feedback to pilots. So far, I have seen only one post with actual information
stating that Airbus does emit audible alarms as well as indicating onn the CRTs
which pilot is in control etc.  Compared to the seemingly complete information
about Boeing's interface you have to admit that the information about Airbus is
not as complete.

Of course, if you are talking about crash statistics, this news groups has
complete stats on Airbus :-) :-) :-)