From: rdd@rascal.ics.utexas.edu (Robert Dorsett)
Newsgroups: sci.aeronautics
Subject: A320 throttles yet once again (re: coffee-cups)
Date: 24 Jul 1992 16:17:46 -0500
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Reply-To: rdd@rascal.ics.utexas.edu
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bcoleman@hayes.com (Bill Coleman) wrote:
> [post advocating summary execution of designers of A320 throttles]
>
> How would pilots react to a system where a "ghost-like" influence keeps
> moving levers on them? For example, the pilot chooses the reduce the
> throttle levers. The flight system, detecting a possible impending
> stall condition (or something), moves them back up. The pilot, seeing that
> the computer has moved the levels back up, again pulls them down. Soon
> we have a battle of wits in which the pilot stops flying the plane 
> because he is fighting with the computer. 

One solution is what Airbus did on the A310, and restrict "alpha-floor"
protections to the *autopilot* mode, only.  Turn off the A/P, and you don't
have to "fight" anything; your airplane is yours, pure and unprotected.


> As someone who designs user interface as part of his job, it isn't easy to
> say which is the better system: that which introduces indirect feedback vs
> physical feedback. The only way to descern the truth in these cases is to
> perform user testing. And even that may not single out a "best" solution.

In reality, "moving throttles" are used on most airliners in autothrottle
mode.  Note the difference between *autothrottle* and *throttle*; many 
complaints tend to deal with how the *autothrottle* behaves.


Some background, and info on the A320:

Fuel controllers:

On a conventional airplane, throttle travel commands a relative fuel
control setting in the engine fuel control unit.  Thus, full travel is
"100" per  cent, while  full-back is "0" per cent, or "idle", usually 
around 60% N1.  This is a minimum engine speed that can support stable 
combustion.

In early jet engines, the fuel control unit was a "hydromechanical"
controller-- lots of funky analog stuff.  In the early 1970's, this was
*augmented* with an *electronic* fuel controller, with a
hydromechanical controller retained as backup.  As the decade
progressed, the EFC's got more and more sophisticated.

In the late 1970's/early 1980's, *full* authority controllers were
introduced; these are commonly known as *full authority* digital engine
controllers.  What does this mean?  Simply that there is no
*hydromechanical* back-up controller.  There are usually two FADECs per
engine, however.  Is this arrangement bad?  Yes and no.  No: the FADEC 
design is much more straightforward in  design than hydromechanical devices. 
Yes: the FADEC software is subject to  many of the same sort of
software-reliability critiques as FBW systems, especially since the
backup FADEC *appears* to have the same hardware/software as the primary 
(this isn't discussed much, but does seem to be the case). 

Note that FADEC-equipped engines have been shown to be very reliable.


The Pilot:

What does all this mean, from the pilot's perspective?  In a
conventional control model, full-back is idle.  But full-forward really
"lays the gas" on the engine. There is an interval between maximum
*effective* thrust (take-off or go-around) and *maximum* thrust; the
difference can be 5-10%, and can be quite damaging to the engine, if
used on a long-term basis.  In general, in conventional airplanes, for 
each major segment of flight--takeoff, landing, climb--the flight 
engineer or first officer must re-calculate the maximum/go-around thrust 
limits (as EPR), maximum continuous thrust, etc.  Setting these thrust
ratings can also be somewhat awkward, a process of set-check, set-check.

In realization of the max-thrust problem, electronic fuel controllers
got  "smart," and tended to "re-map" the full-thrust command to maximum
*usable*  thrust.   The pilots would never have access to *full* power,
in this scheme.   In turbojets, the difference between max-avail and
max-possible could be  substantial; in the big fans, it was much
smaller, and, thus, less of an  issue.  In either case, the normal service
life of the engine was extended.  Engines also became more economical
to run, in conjunction with flight management systems.


Modelling:

Looking at how simulators model this is informative; modern airplanes
use something similar.  The throttle quadrant position is called a
"lever angle." This is represented from 0 to 100%.  This is mapped to a
reference EPR or N1 value at sea level.  This value, in turn, is mapped
back up to ambient atmos- pheric conditions, for a *command-EPR* value.
"Engine protections," in turn, determine whether this is actually a
safe value, and "clip" it to stay within the safe envelope.  This is
passed to the actual fuel controller, and the engine spools up or down
to the commanded setting.


Autothrottle:

Autothrottles were introduced along with EFC's; they were designed to 
assist the pilot in maintaining speed control.  Using them had two 
components: (1) the  autothrottle had to be turned on, as a separate 
component of the autoflight system, and (b) the pilot had to "dial in" a 
speed setting.  The autothrottle  would reduce or increase thrust to 
help achieve and maintain the desired airspeed.  

In most conventional autothrottle designs, the throttles moved.  Since
these  are a *command* indicator of relative engine power (i.e., if you
have the throttles full-forward and have a 13% N1, you're in deep
trouble), this would be useful in detecting discrepancies between
command-speed and what direction the throttles move.  Autothrottles are
*not* perfect, and they do break.  An experienced pilot can
notice significant discrepancies, in a given configuration and selected
airspeed, between what the throttles SHOULD be set at, and what they
ARE set at.



Now, for the A320.  There are three major components to consider:

1.  The A320 system is called "FADEC."  This is somewhat of a misnomer:
it's like referring to any car as a "V-6", regardless of whether the
engine has conventional or electronic fuel injection, cruise control, manual
choke/mixture, etc.  In the above, "FADEC" refers to the *engine* controller; 
Airbus refers to it as a *system*, from the engine through to the cockpit.

2.  The throttles can be used in a conventional manner, over the short
term.  I.e., they can be moved to command an engine N1 in a "manual" mode.

3.  However, they have fixed "detents," which the throttle can be slid into.
These are:
	* Full Reverse
	* Reverse Idle
	* Flight Idle
	* Maximum Climb
	* Maximum Continuous Thrust (MCT)/Flex Take-off
	* Takeoff/Go-Around (TOGA)
	
The detents have two meanings.  First, they are simple, automatic
thrust *mode* settings.  Want takeoff thrust?  Just advance the
thrust levers all  the way forward.  The airplane knows as much about
its environment as the F/O,  after all, and it's trivial for it to
calculate the "command" settings from a  look-up table and
automatically command the engine to the desired setting.   Similarly,
the other "notches" can be set, to quickly obtain the desired rating.
This is fairly nice, sort of like automatic transmission in a car.

The notches also define what goes on when the *autothrottle* is engaged.
They define *limits* of thrust.  For example, suppose the throttle is
in the maximum-climb notch.  In this case, the engines CANNOT produce more
thrust than that setting (unless the alpha-floor protection-mode engages).
Similarly, if the throttles are one degree out of flight idle, then the
maximum limit is *maximum climb*.  It is intended as a "gentle" protection
system, to help guard against excessive speed-busts.


The problems pilots have with this arrangement are:

1.  Autothrottles make mistakes.  It becomes more difficult to track them
down if the throttles don't move.

2.  The flight management system has full-thrust/zero-thrust climb modes.
If these are inadvertently selected, a moving autothrottle helps show this.
For example, even if the *gobs* of information on the flight displays were
ignored in the Strasbourg crash, the pilots might have questioned why the 
throttle levers--big black things, sticking in the air--had moved all the way 
back.

3.  The A320 throttle design has less angular movement, in total, than a
conventional throttle.  Errors in "hand-setting" thrust thus increase
several times, and make hand-flying more difficult.

4.  In a normal airplane, to maintain a "rated" climb, for instance,
one must frequently re-adjust throttle position to maintain the
command-reference.   In the conventional scheme, remember, throttle
angle is more of a *fuel  control* value than a "what you want to do"
value.  In the A320, all one has  to do is place the throttle in a slot
and it will MAINTAIN that command. On a conventional airplane, engine
performance would "drift" as ambient conditions change, with a constant
throttle setting.  The A320 "detent" features makes engine-control,
over long-term climbs or descents, quite a  bit different than on a
conventional airplane, much easier (but put the pilot that much more out
of the loop).

5.  Early reports showed a "negative" discrepancy between airspeed and
throttle direction (i.e., higher airspeed resulting in a lower thrust
setting).  A moving throttle would have helped locate this.

6.  Non-moving throttles, even though the system can command full-throttle
in certain situations, may lead the pilots to view the throttle architecture
as conventional, rather than mode/command-oriented, which can lead to 
mistakes.


The main defenses of the system are:

1.  A few pounds are saved, due to the lack of the actuators.

2.  A potentially troublesome maintenance item is also eliminated.  When
autothrottles break, they usually break "in the cockpit."

3.  The "command" structure doesn't really accomodate a moving-
autothrottle concept.  If moving autothrottles were adopted, only
two major flight modes, flight idle and max-thrust, could be supported.
In computer terms, what Airbus did was "design the interface around 
the features."

4.  For normal flight operations, the "detents" greatly reduce pilot 
workload, allowing the pilot to "navigate" within an often complex ATC 
environment, and not have to devote much brain-power to flying the airplane.






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