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

From:         dtmedin@cca.rockwell.com (David T. Medin)
Organization: Rockwell Avionics - Collins, Cedar Rapids, IA
Date:         21 Jun 95 02:56:52 
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In article <airliners.1995.792@ohare.Chicago.COM>, Jean-Francois Mezei <MEZEI_JF@Eisner.DECUS.Org> writes:
> I read about the 777's engines stalling during takeoff during a test mission.
> Although this may have nothing to do with day to day operation of the 777, it
> lead me to the following question:

The P&W engine stalled several times, but on the 747 engine testbed,
specifically in a maneuver meant to stall the engine if it was
possible. The engine nacelle was redesigned. Static testing also
uncovered the nacelle problem, but later than the flight test.

The GE engine is the only one I know that stalled on the 777 airframe,
during a testing flight when those sort of problems are supposed to be
found and ironed out.

Keep in mind that the immediate engine management on the 777 is by a
DEC (full authority for the engine environment, anyway), so in that
regard a computer is monitoring and using feedback to control the
engine. A stall in an engine can be a very rapid event--hard to
predict and correct.

> We know that the computer plays is VERY important role in the 320 and 340.
> We know that Boeing decided to keep the pilots in control on the 777.
>
> Although the Airbus's FBW system makes actual flight decisions (eg: pilot
> trying to make a movement that is outside aircraft's capabilities) which can
> prevent stalling etc etc, I get the impression that Boeing's system is not as
> sophisticated and is really a glorified auto-pilot with a couple more warning
> buzzers. I get the impression that to Boeing, FBW means just that:
>
> make the interface between joystick and engine/rudders an electronic
  one.

Negative. The flight control systems have 3 (I think) modes that
provide incrementally increasing computer supervision of the
aircraft's envelope up to full limiting.

> Am I right in assuming that the Boeing system has less "smarts" built into
> it and that Boeing concentrated instead on providing mechanical feedback to
> those joysticks to replace the mechanical interfaces that existed between the
> pilot and co-pilot ?

The Boeing flight management system can be given full envelope
limiting authority in one of its modes. It still allows pilot override
in full computer authority mode by fighting the yoke (not joystick),
but you have to be a weightlifter to do it (I tried in the simulator
and failed, but I'm musclebound--bound to get 'em someday) or be under
the influence of adrenalin in an emergency.  There are other modes,
though, that give the pilot up to full, non-limited authority (raw
control laws).

I fly the small iron (aluminum?). I was surprised at the ferocity of
the stick shaker and the insistence on the airplane righting itself
when I exceeded 30 degrees of bank or tried to slip.

The link between the pilot's and copilot's yoke is direct mechanical,
on the 777 I believe. The control force feedback is simulated by a
single source for both.

I think the main exception when compared to Airbus is that Boeing
allows pilot override of the computers, either through fighting the
control inputs or switching flight control modes, and gives simulated
tactile feedback of conventional control feel. Airbus does neither.
That is the issue...

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