Re: A320 pitch control (was: emergency procedure)

Date:         18 Apr 98 00:49:30 
From:         rdd@nospam.com
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
References:   1 2 3
Followups:    1
Next article
View raw article
  or MIME structure

In article <airliners.1998.584@ohare.Chicago.COM> Julian Scarfe <jas1@scigen.co.uk> writes:
>Could you clarify the systems for pitch control on the A320 and family for
>me.  My understanding so far is:
>
>Direct law: direct relation between elevator deflection and stick position,
>with maximum stick defelection corresponding to maximum elev deflection

Correct, except there's filtering for center of gravity position (i.e.,
elevator travel is limited by the CG position).  There are, otherwise, no
flight envelope protections.

>Alternate law: stick deflection controls lift, with maximum stick defelection
>corresponding to maximum permissible loading

Sort of.  In pitch, it performs similarly to Normal Law (below).  The main
differences are in how pitch and low/high speed stability protections are
performed.  Alternate law can also exist in a mode without high- or low-
speed stability protections (just load factor protections).

>Normal law: stick deflection controls lift, with maximum stick defelection
>corresponding to maximum permissible AOA (or is the AOA protected in a
>different way?)

Look at it a different way.  When you move the stick, you're commanding
a load factor.  The flight control system reacts to provide that factor.
So suppose you pull back.  You're increasing load, increasing pitch.  You
let go.  Stick centers.  You're now commanding 1G in the last known attitude.
Airplane holds the pitch angle and maintains 1G.

Normal law also includes load protections, pitch protections, high AOA
protections, and high speed protections.  It also includes a load alleviation
function, which reduces wing loading in turbulence by automatically
modulating selected spoiler panels.

>Is that anything like correct?  How does the pilot select Alt or Direct law?

The pilot reaches up and turns off the various flight control computers,
with successive degrees of degradation.  There is no "on/off" switch for
the various laws.

--
Robert Dorsett                         Moderator, sci.aeronautics.simulation
rdd@netcom.com                         aero-simulation@cactus.org

           "Bother," said Pooh when his engine quit on take-off.