Re: RISKS DIGEST 16.20

From:         rdd@netcom.com (Robert Dorsett)
Date:         07 Jul 94 00:13:06 
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"Robert Morrell Jr." <bmorrell@isnet.is.wfu.edu> writes:
>Subject: Airbus
>
>I recently had the opportunity to discuss at length the various RISKS Digest
>pieces on air safety and computer controls with a relative who is an
>experienced military and civilian industry pilot.
>
>He agreed with the thrust of the threads here, but added a specific and
>general comment about the A-320.
>
>Specifically he noted that the greatest problem with the aircraft is that it
>is unique in lacking a unified "off switch" for the autopilots. All other
>aircraft have one control that can be flipped or pressed that will turn off
>the computer pilot(s) and return control to the aircraft.  Apparently doing
>this in the A-320 is no small matter.

Is your friend actually an A320 pilot?  If so, I find his comments puzzling.

Like other modern aircraft, the A320 can operate in either "managed" flight
or "selected" flight.  In "selected" flight, the pilots can command various
flight parameters (airspeed, heading, altitude) simply by dialing in 
parameters into a (usually glareshield) interface.  In managed flight, 
they interact with a flight computer which makes turns, etc. in automatic
consultation with a pre-programmed flight plan.

"Selected" flight is performed via the equivalent of an autopilot control 
interface on the A320.  On this airplane, the management computer and guidance 
computers are integrated, and called a "Flight Management and Guidance 
System." (FMGS).  Under either managed or selected flight, these command 
the computers that comprise the electronic flight control system (EFCS) to 
perform the desired tasks.

BUT ONLY if the pilot specifically presses one or both of the autopilot
switches on the glareshield.  If the switches are not engaged, the airplane
will NOT be controlled by the FMGS (in either selected or managed mode), and
the airplane's EFCS will be controlled directly by the sidesticks, in one
of the myriad flight control laws.

Disengagement?  It's as simple as depressing a red button on either of the
pilots' sidesticks.  Then they're in control.  Or manually "clicking off" the
autopilot engage switches (there are only two, they're illuminated, and they're
side-by-side) on the glareshield.


>Generally, though he and other pilots like the A-320, it is known for having a
>"mind of its own" literally. Most pilots, according to my relative, have
>stories of the plane suddenly "up and deciding to begin an approach, go around
>or enter a traffic pattern" It seems amusing usually, but then my relative had
>never had it happen low to the ground....

As far as I can tell, these are just stories.  In reality, the flight control
system, for all the theoretical bickering, is probably quite safe.  The
flight management system is pretty conventional, and well-understood.  It
may have some idiosyncrasies, but they're not unique to this type of air-
plane (if they exist on the A320, they will occur on the 757, 767, virtually
any modern airplane which has an FMS).  

Not a single one of the "FMS take-over" stories has been proven.  Many of
the stories originate among pilots and lay people who have not used FMS-
driven cockpits, and confuse the FMS featureset with the fly-by-wire
controversy.   Many have assumed the stature of urban legends.

What IS clear, however, is that the A320 user interface suffers in other
respects, such as not clearly providing mode differentiation, feedback, etc.
This is a whole other can of worms, however, and doesn't really pertain to
a lack of ability to "click it off."



--              
Robert Dorsett                                                       
rdd@netcom.com