Terence Wilson wrote:
Hello,

I was wondering how the primary vacuum is powered in a single engined
aircraft. If it was powered directly by the engine, a engine failure
would result in the immediate loss of attitude indicator and heading
indicator. So I guess most aircraft use an electrical pump that can be
driven by the alternator or battery, is this the case?


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.



--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com