Rationale behind vacuum instruments

On 04/09/07 17:48, Ron Natalie wrote:
Mxsmanic wrote:

Which types of pumps are used in most modern small aircraft?

Dry engine-driven pumps followed by wet engine-driven pumps.
Electric pumps and direct electric driven come last.

There's also the manifold-powered suction backup unit (Precise Flight).



How many small aircraft have redundant pumps and gyros?

There are a lot with the backup systems either manifold or
electric. As for dual gyros, air-driven gyros rarely fail.


Are any small aircraft using RLGs?

I don't know of any certificated ones suitable for light aircraft.

How hard is it to spot a pump failure? I've gotten the impression from what
I've read here and elsewhere that vacuum pumps may fail gradually and
insidiously, whereas (presumably) an electric motor fails in a much more
obvious way.

Nope you have the wrong impression. The dry pump fails instantly. A
flag, light, or well place gauge will tell you instantly. The issue
is that if it fails and you don't notice the lack of vacuum, it takes
a few minutes as the gyro slowly spins down and becomes unstable to
notice.
Does a failure involve the gyro coming to a stop, or can it just slow down and
thereby cause problems?
Precisely.

But how does that help you if the AI has failed? The GPS wouldn't tell you
the attitude of your aircraft.


The altimeter/vsi tells me if I am climbing or descending.

and the airspeed indicator... The altimeter tells you where you
are; the VSI and ASI tell you where you are going.

The GPS
tells me if I am turning (as does the Turn Cordinator, but the turn
coordinator is a bit more finicky,

Finicky? It's very accurate, if that's what you mean. It's provides an
easy way to determine if your wings are level (assuming coordinated
flight) among other things. I think it's a wonderful back-up for an inop
DG.

and turns are more difficult because
you can't really count on the whisky compass while turning,

Really? I can. I can turn to a heading using the wet compass. You just
have to know how to use it.

you do
timed turns, but with the GPS you just watch it's simulation of
the HSI).

Instrument pilots do a decent amount of practice with the gyros
simulated failed.

Well, instrument pilots are required to show proficiency. Some practice
more than that though...


--
Mark Hansen, PP-ASEL, Instrument Airplane, USUA Ultralight Pilot
Cal Aggie Flying Farmers
Sacramento, CA