A320 sidestick description + references (Re: Airbus safety)

From:         Robert Dorsett <rdd@rascal.ics.utexas.edu>
Date:         13 Dec 92 12:14:17 PST
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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.

I've inserted a couple of comments in brackets; these clarify passages,
based on respondent comments.


4.1.  Sidesticks; [AIRB88, 1.09.20, 3-5; ZIE86, COR88]

The main flight control interface for the EFCS is one of two
"sidestick" controllers.  Conventional airplanes have two "control
columns," mounted between each pilot's legs.  The A320 does away with
these, and instead has sidesticks mounted on the side-walls of the
cockpit (incidentally resulting in outstanding pilot visibility of
flight instrumentation).

Conventional control devices reflect control forces to some degree. 
This "feel" is usually either supplied aerodynamically by the airplane,
or, as in most airliners, via an artificial feed-back system.  The
sidesticks on the A320 do not have artificial feel.  On the A320,
springs are used to discourage abrupt control movements.  Flight
control specialists seem to regard this as a valid "artificial"
feedback mechanism, but the point must be emphasized that the pilot is
only reacting to the qualities of the spring: no tactile feedback
relates to what the airplane may be doing (unlike a conventional
control system); thus, secondary cues, such as the design of the flight
displays, take on more importance.  


Following are some of the force-characteristics of the sidesticks on
the A320 [adapted from COR86]:

                 Pitch            In          Out        
Max.  load       10 daN           3 daN       2 daN        
Threshold        0.5 daN          0.4 daN     0.4 daN    
Deflection       +-16 deg.        20 deg.     20 deg.    
Orientation      20 deg fwd.      12 deg in   12 deg in.    

Proper pilot arm position is important for proper use of the sidestick. 
[actually, not that important: an early concern that proved unjustified] 
Thus, Airbus has included a fully-adjustable seat-arm, which features
an LCD readout for arm angle.  A pilot entering an aircraft need only
remember his optimal settings, and set them up.  The arm-rest rest
position may be changed within an interval of [+20,-15] degrees. 
Supporting arm position may be adapted in the interval of [+15, -12]

The sidesticks are not mechanically interconnected: inputs to one can't
be felt on the other.  Their inputs are algebraically added, with a
maximum limit corresponding to the maximum deflection of one sidestick.
No weight is given to the captain.  Thus, if the captain pulls full
left, and his first officer pulls full right, the net effect is zero. 
The last pilot to click on an override thumb-button (also used to
disconnect the autopilot) obtains control: a small indicator light in
front of the other pilot signals this fact.  The potential exists for
the pilots to "fight" over control of the sidesticks.  Rather than the
"strongest" pilot winning, the one with the fastest thumb will win.  If
the override button is held down for more than 30 seconds, it will
"deactivate" the other sidestick.  If the deactivated sidestick's
override button is pushed, it will re-activate.

This scheme has been the subject of much criticism from pilots: it is
widely felt that one pilot should feel what the other pilot is up to,
through the stick.  Inter-pilot communication in an emergency may also
be enhanced through better tactile feedback [PIK88, HEL86, SUM87].

Note that there is no trim control on the sidestick, since this is
normally handled by the Normal and Alternate control laws (see below). 
In Direct law, pitch [trim!] must be set through controls on the center 
control pedestal.  

The only other button, besides the override switch, is a push-to-talk
trigger, for the radios.

The sidesticks are not merely a different interface, otherwise causing
the same functional effects as a conventional airplane.  The pilot's
role in flight management is fundamentally changed, depending upon the
mode the flight control computers are in.  For example, in Normal Law,
which is what the airplane is normally flown in (see below, 5.1), in
a turn, a pilot must normally pull back on the stick, to compensate for
lost lift.  On the A320, this is not necessary: the pilot just moves
the stick in the direction he wishes to turn, and the airplane will
turn, automatically supplying the necessary elevator to maintain
altitude [HOP87].  The sidestick is more of a "flight path command"
interface, rather than a conventional "flight control surface
deflection" interface.  If, that is, the appropriate computer support
is there: if not, the same sidesticks are used with one of the other
"redundant" control laws, which are much more conventional in design. 
This raises an interesting issue of whether a pilot, who, with a
properly-functioning system, will fly in "Normal" law almost all the
time, will be "current" enough to satisfactorily fly the airplane in a
significantly degraded mode, more akin to conventional control laws. 
This issue is addressed in training, not the interface.

AIRB88    Airbus Industrie/Aeroformation, Flight Crew Operating
Manual, 1988.  

COR88    S. G. Corps, "Airbus A320 side stick and flyPbyPwirePPan
update," Society of Automotive Engineers Paper 861801. [Very GOOD

HEL86    Peter H.  Heldt, "Airline requirements on a fly-by-wire
aircraft--a pilot's view," Society of Automotive Engineers paper 861804.

HOP87     Harry Hopkins, "Simulating the A320," Flight International,
12 September 1987. [good article, weak on this issue]

PIK88    J.  R.  Pike, "A320 in service--initial report," British Air
Line Pilots Association, July 31, 1988. [extensive comments]

SUM87    L.G.  Summers, et.  al., "Fly-by-wire sidestick controller
evaluation," a paper presented at the SAE Aerospace Technology
Conference and Exposition, Oct.  5-8, 1987. [ this is a decent overview
of the MANY options available ]

ZIE86    Bernard Ziegler, "Front seat on the future," Aerospace
America, April 1986. [nicely-illustrated pap]


How accurate is all this?  Besides the cited sources, I've run into
several other comments on the "algebraic" nature of the sidesticks.
I haven't tried a neutral deflection in a simulator, but a recent email
comment indicated that pilots, while trying to avoid an aircraft on the
ground, commanded opposite inputs, thus leaving the flight path 
unchanged.  If true, this would also tend to support the "algebraic,"
additive nature of inputs.

In addition, of the A320 pilots who have reviewed the paper thus far,
none have contested this point.  One pilot did raise the issue that
later transition work isn't as difficult as might be gathered from the
last paragraph.  So far, this is a minority viewpoint.

Robert Dorsett
Internet: rdd@rascal.ics.utexas.edu
UUCP: ...cs.utexas.edu!rascal.ics.utexas.edu!rdd