"Larry Dighera" wrote in message
At first glance, five levels of electrical power source redundancy
would seem safe in guarding against 'Complete electrical failure',
however there are other points of failure in electrically operated
systems. Each electrical circuit is fed through a circuit breaker.
How many circuit breakers are involved in the Airbus flight control
system? How many electrical buses?

Airbus understood from inception that their company would live or die by the
integrity of their fly-by-wire design. There are 7 computers responsible for
the primary flight control surfaces. Three are ELACs (elevator-aileron).
They perform the same functions with slight differences. Each is
manufactured by a different vendor with different computer code so that if
one assembly line has a batch of bad parts or bad code, the other two are
not affected. The same goes for the 2 SECs (spoiler-elevator) and the 2 FACs
(rudder plus flight protections). One ELAC, one SEC, and one FAC are powered
by a DC essential bus which switches automatically to any of the power
sources mentioned (AC or DC). The sister computers are powered by other
busses which are normally powered by a seperate main bus. A failure of any
single computer results in nothing more than a message to the crew. A
failure of any 2 like computers results in some crosswind limitations for
landing.

No, that is the phrase that describes the issue you addressed. I'm
more concerned about single points of failure.

Which was a concern of my sceptical way of viewing engineer's creations. I
couldn't find any single points of failure which would cause much concern.
However I have just begun to look.

If the circuit breaker (an electrical/mechanical device) feeding the
electrical bus supplying the flight control system were to
malfunction, regardless of power being available, the bus could be
de-energized, and no amount of available power would bring the flight
control system back into operation, unless you know of additional
redundancy designed in to the Airbus control system that you have not
yet discussed.

There is more than one bus feeding the flight control computers. The two
main busses have independent power supplies. The enormous amount of
redundancy explains why it takes 6 weeks of training to ultimately flip on
the autopilot at 100'AGL.

So you're intimating that only the ailerons and elevator are
electrically operated?

Electrically "controlled" and hydraulically "actuated", as are all the
flight control surfaces in normal operations. Each flight control surface
has multiple actuators receiving hydraulic power from seperate hydraulic
systems. For example, there are 3 spoilers on each wing used for roll
control (the forth is for speedbrake and the fifth is for ground spoiler).
Each spoiler has 2 actuators which are powered by different hydraulic
systems. A loss of any single hydraulic system won't affect control. A loss
of any 2 hydraulic systems will result in a loss of 1 of the 3 spoilers.

Are you saying there are three hydraulic actuators, one from each
hydraulic system, attached to each control surface?

There are at least 2 actuators, each from a different hydraulic system. All
control surfaces are hydraulically "actuated". In the event that all 3
hydraulic systems are lost, none of the control surfaces will move. The
pilots would have only engine thrust for control, just like the United DC-10
that crashed at Sioux City.

In the event of complete electrical loss, there is still hydraulic pressure,
and the pilots can manually control the hydraulic actuators for the rudder
and the horizontal stabilizer in addition to controlling engine thrust.

Hope it's all clear now :-)

D.