Re: Airbus trim system?

Date:         19 Jul 98 22:07:25 
Organization: Deja News - The Leader in Internet Discussion
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In article <airliners.1998.983@ohare.Chicago.COM>,
  Scott Macmillan <> wrote:
> Hello folks,  I was watching one of those 'in the cockpit' videos
> the other day that featured the Airbus A320.  During the video I could
> see the trim wheel moving back and forward of its own accord.

> Anyway, what I would like to know is how do the computers know how the
> pilot wants the plane to fly?

You were on the right track Scott. The computer and the autopilot through the
FMC or its equivalent does all the work BUT follows the pilots commands. In
auto cruise the pitch command mode is 'altitude hold', some altitude has been
selected by the pilots on the APFDMCP and the autopilot maintains that
selection. The autothrottle maintains the selected speed for cruise, either
via an FMC or a speed selection in some speed select mode again on the
APFDMCP. Equilibrium flight, alls well. Instability due to wind shear. Nose
rises, little movement with the autothrottle, as speed decreases so does drag
and the aeroplane, depending on its speed stability, re-accelerates to the
equilibrium speed position again with thrust equalling drag. Same for minor
slowdowns. Different problem if the speed is lost due to temp change, this is
permanent and requires thrust lever movement to re-establish the cruise
equilibrium state again but with a different thrust setting and thrust lever
angle. For an auto climb, after clearance, a new altitude is selected on the
APFDMCP and the autothrottle system applies climb or max climb thrust as the
autopilot pitches the nose up and commences climb. The climb speed, if climb
is commenced t optimum weight for climb to new altitude, will be close to the
same as previous and this will also be near/the same for the cruise speed at
the new level. Manually the pilot applies climb thrust and commences a climb
not unlike that which you do in your 'bug smasher'. He trims the stabiliser
into the attitudehe requires and maintains his climb speed. If the auto
throttle is engaged then as he pitches to climb, it will, in one mode, apply
power to maintain the selected speed until it has max climb thrust on. This
is sloppy technique and in another mode it would be commanded just to apply
climb thrust and the pilot has control in pitch over his speed and/or a
particular rate of climb. He won't for long however be able to achieve both
and  usually he settles for the desired speed.

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