"Tarver Engineering" wrote in message ...
"Gord Beaman" wrote in message
...
(sid) wrote:


I should have framed the question this way:
How far would either aircraft fly if there is trouble in the E&E bay
that compromises the electrical system and you are down to DC
power...And then you lose even that?

Isn't that like saying "what would happen if the bloody wings
were to fall off"?...pretty silly statement imo.

I can't understand the obsession with DC power either. Airplanes mostly use
AC power for controls. I have yet to see a synchro that runs on DC.

Further assurance of maintaining proper attitude can come from
dedicated power sources. In the last five minutes of the 1998 tragedy
of Swissair Flight 111, with a raging electrically stoked fire
spreading from the attic space in the cockpit, the two pilots had no
attitude reference at all—not even a turn needle or turn coordinator
to fall back on. They saw inky blackness on the outside and black
primary flight displays [PDFs] inside the cockpit. The standby ADI,
even if it had been working, was located near the bottom center of the
instrument panel. It was poorly lit and even more difficult to see
while wearing smoke masks.

The Swissair jet's standby was lost because it had been hooked to a
vulnerable hot battery bus. When it failed, the powerless gyro started
to spin down. Therein lies a vital object lesson in redundancy:
Standby instruments need their own batteries, so that a loss of
generated power doesn't take down the sole fallback attitude
reference.