Re: A320 sidestick description + references (Re: Airbus safety)

From:         palmer@icat.larc.nasa.gov (Michael T. Palmer)
Organization: NASA Langley Research Center, Hampton, VA  USA
Date:         14 Dec 92 14:11:43 PST
References:   1 2 3 4 5 6
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Robert Dorsett <rdd@rascal.ics.utexas.edu> writes:

>In <airliners.1992.147@ohare.Chicago.COM> palmer@icat.larc.nasa.gov (Michael T. Palmer) writes:

>> rdd@cactus.org (Robert Dorsett) writes:
>> >On the A320, there is no interconnect between the
>> >sidesticks: the captain can command a full-left in an emergency evasive
>> >maneuver, the F/O full-right, and the net result will be an algebraically
>> >added "zero."

>> I believe this is incorrect, though I don't have the documentation here
>> right now.  My understanding is that whenever one of the sticks reaches
>> a critical percentage of deflection (say, 75%), it becomes automatically
>> the selected input device.  At this point, the other control stick is
>> ignored.  So it's a race.  Whoever slams their stick to the stops first
>> wins, and the only way for the other crewmember to override is to physically
>> attack the winner.  Neat, huh?

>I've looked into this closely.  Unless there have been significant, recent
>changes, it doesn't work this way (other designs do, though).  Here's an 
>excerpt from the impending A320 paper that Pete Mellor and I are writing
>("The A320 Electronic Flight Control System," title subject to change), which 
>might help clear things up.


Okee dokee.  You're right; I must have been remembering the specs for a
different system.  I can now recall the stories about the thumb switch for
overriding the other stick (my officemate and some of our test pilots
participated in a week of A320 training down in Florida last year).  Their
reaction to it sparked quite a lengthy debate about the various sidestick
implementations.

Personally, the lack of feedback about what the other crewmember is doing
is just astounding.  Does nobody remember their flight training?!?  How
do you think your instructors (or YOU, if you teach) were able to know what
you were doing even before the aircraft responded to your inputs?  Tactile
feedback can be a powerful and rich source of information.

And when, pray tell, would an algebraic sum of the control inputs be the
desired method of responding to the flight crew's actions?  Did the
designers think that the pilots would agree that the captain would only
move his stick left/right and the first officer only fore/aft?  "I'll be
the base of the triangle, you be the height, and we'll let the EFCS do the
hypoteneuse!"  (For those with a math/statics background:  "I'll be the
i, you be the j, and we'll let the EFCS do the resultant vector!").

My point is that if the crewmembers are trying to do something different,
the system should make that MORE not LESS visible.  Otherwise, when the
aircraft does not respond as they expect it to, each crewmember will simply
increase the magnitude of his control input without really understanding
what the h--- is going on.  I guess this is what Reason would identify as
a "latent system error."  The pilots will eventually make an error; yep,
the designers made sure of that.

-- 
Michael T. Palmer, M/S 152, NASA Langley Research Center, Hampton, VA 23681
Voice: 804-864-2044,   FAX: 804-864-7793,   Email: m.t.palmer@larc.nasa.gov
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