Re: A320 emergency procedure

Date:         26 Apr 98 03:44:51 
From:         rdd@netcom.nospam.com
Organization: Netcom On-Line Services
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In article <airliners.1998.549@ohare.Chicago.COM> Michael Zaller <mikoza@pacbell.net> wrote:

> Lemme get this straight, you know someone who had some flight control
> cables fail and still prefer a mechanical airplane?  Anyway, the rudder
> and elevator trim controls on an A320 ARE mechanical.  It's not easy to
> land an airplane in this condition, but heck, they almost landed a DC10
> with much less...

The A320's "mechanical" revision mode is hydraulic.  The mode is designed
to provide a "get it on the ground" capability in the face of complete
*electrical* failure.  So cables go allllll the way to the back, whereupon
they work with hydraulic actuators in just the same way as any conventional
airliner.

In normal operation, the same hydraulic actuators (as well as those for
the elevators, spoilers, and ailerons) are controlled with electrical
signals.

The discussions of the real benefit of the "manual" reversion mode are
interesting.  In past discussions, responses have ranged from "it's
impossible" to "it's easy." Similarly, airline policies appear to range
from "we train for it" to "why bother?"

It appears to be much more difficult to handle in this mode than the
mechanical revision modes for, say, the 727 or 737, which, in the case of
complete hydraulic failure, will provide three-axis control by manipulating
control tabs on the control surfaces.  This aerodynamically moves the
surfaces, but results in very high control forces.

On the bright side, the A320 has triple-redundant hydraulics and five
levels of redundancy on the AC electrical side, so perhaps it's not
too much to worry about.  Failure concerns in the academic community tend
to center more around the theoretical reliability of the flight control
computers.

It's worth pointing out that complete hydraulic failure is considered
extremely improbable, and the lack of any true mechanical reversion is
consistent with the flight control system design for many other airliners,
ranging from the 747 to the 767.  Although for an "extremely improbable"
event, it's surprising how many airliners have experienced this emergency,
or gone down to one level of redundancy.

Regards,

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

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