Re: A320 emergency procedure

Date:         30 Mar 98 04:31:27 
From:         Pete Mellor <pm@csr.city.ac.uk>
References:   1 2 3
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On Tue Mar 24 19:51:04 1998, Karl Swarz wrote:
> Is the yaw damper still effective in Direct Law?  Without
> it, and with the other controls largely crippled, I'd expect you'd
> end up fighting an uphill battle with Dutch Roll and phugoid
> oscillations, ...

As I understood it, the original query related to the use of
mechanical backup after complete loss of FCS. Yaw damping is
one of the functions of the ELAC (Elevator and Aileron Computer)
of which there are two in the FCS. While at least one ELAC still
functions, it will compute rudder commands to achieve yaw damping
and turn coordination, which are transmitted to the rudder actuators
by the FAC (Flight Augmentation Computer), of which there are also
two, and which achieve the ELAC's yaw orders as well as being
responsible for rudder trim and rudder travel limit.

If both FACs are down, only manual control of rudder is available.

If both ELACs are down, the FCS reverts to alternate law, which
implies pitch alternate law (reduced pitch protections), roll direct
law (direct stick-to-surface-position relationship) and yaw alternate
law (which provides only yaw damping, presumably computed by FACs).

The next step in the degradation path is direct law, which implies
direct stick-to-surface-position relationship for both pitch and
roll, and only manual control of rudder for yaw.

After that, it's mechanical backup. (Note that the trim-wheels on
the central pedestal that are used for pitch control move the
trimmable horizontal stabilizer (THS), not the elevators.)

Peter Mellor, Centre for Software Reliability, City University, Northampton
Square, London EC1V 0HB, UK. Tel: +44 (171) 477-8422, Fax: +44 (171) 477-8585
E-mail: p.mellor@csr.city.ac.uk, Web: http://www@csr.city.ac.uk/
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