Re: A320 pitch control (was: emergency procedure)

Date:         18 Apr 98 00:49:29 
From:         Michael Zaller <mikoza@pacbell.net>
Organization: Mikeysoft.
References:   1 2 3 4 5
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Julian Scarfe wrote:

> Pete Mellor wrote:
> >
> > 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.
>
> 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
>
> Alternate law: stick deflection controls lift, with maximum stick defelection
> corresponding to maximum permissible loading
>
> Normal law: stick deflection controls lift, with maximum stick defelection
> corresponding to maximum permissible AOA (or is the AOA protected in a
> different way?)
>
> Is that anything like correct?  How does the pilot select Alt or Direct law?

Here's a (slightly) more complete description:

Normal Law gives full flight envelope protection to the aircraft (except
below 100 ft radar altitude, otherwise you could never land...)  There
are five primary flight control computers, each with separate and
redundant systems.  The sidesticks are used to provide input to the
computers, which in turn command the actuators to move the flight
controls.  The sidesticks offer no feedback other than they are spring
loaded to neutral.  There is no mechanism to limit the deflections of
the sidestick; any limits are calculated by the primary flight control
computers (Elevator and AiLeron Computers, ELACs, and Spoiler and
Elevator Computers, SECs).  Also, the movements of the flight control
surfaces are adjusted based on airspeed.  In other words, the aileron
will deflect less at high speeds than at low speeds with the same amount
of sidestick input.

Alternate Law is in effect when there is some sort of degredation of the
flight control computers or their inputs (i.e., airspeed data).  There
are actually several different Alternate Laws, depending on which
systems have been degraded.  There is no difference in the sidestick
controls between Normal Law and Alternate Law; only certain flight
envelope protections are lost.

In Direct Law the electrical signals from the sidestick controllers act
directly upon the flight control actuators.  Test flight crews which I
know to have flown in this mode report that it flies much like an old
taildragger.  Keep in mind that the aircraft can be in Direct Law in one
axis (roll) and Alternate Law in another (pitch).

Also, don'f forget that the aircraft can be flown using rudder and
elevator trim as these are mechanical backups. However, I wouldn't
recommend it.

Michael Zaller
A319/A320 Fleet Engineering, United Airlines

P.S.  The views expressed are my own, and in no way reflect upon my employer.