"Mortimer Schnerd, RN" mschnerdatcarolina.rr.com wrote in message
...


Somebody else has already explained the frailities of the vacuum system.
So why do we use it? Redundancy. If the electrical system takes a dump,
you've still got primary instruments along with the attitude indicator and
directional gyro... you've lost the lights, radios, and the turn
coordinator. But you're still able to keep the shiny side up without too
much sweat.

If the vacuum system craps out, you've lost the attitude indicator and the
directional gyro. But you've still got the primary instruments to keep
things upright.

The chances of both systems failing on the same flight are much less than
any one system... and even there failure is an uncommon event. But as
others have noted, you can add redundant instruments and even backup
sources.

Of course, the vacuum system is, IIRC, about 5-10 times more likely to "take
a dump".

One nice little gadget is an AH that has a battery backup completely
isolated from the standard electrical system.

Better yet is dual electrical systems that run off independant and isolated
electical buses.

Better still, is the above that runs of an AHRS, that has a MTBF of 30,000
hours as opposed to the vacuum system that typically has a MTBF of 28
minutes. :~)