Rationale behind vacuum instruments

IIRC, the FAA mandates a fully redundant electrical system (two
completely separate electric buses, each with its own alternator and
battery) before they'll allow you to run your primary gyros off
electric power. The added weight of such is often substantially more
than just a vacuum system alone.

Also, in almost every aircraft I've flown, Only the AG and DG run off
the vacuum system, while the Turn coordinator is electric. This way,
if either the vacuum or the electrical systems fail, you still have
enough of a partial panel to get you through a cloud layer, at the
very least.

Sometimes there is MX..

My aircraft has a "standby Vacuum system" using the manifold pressure
in the engine..

It is for emergency use only, and , of course, will NOT supply enough
vacuum if the altitude is too high or if the engine is operating at
too high a power setting..

But could sure be useful in a vacuum pump failure situation...

In some instances, thehigh flying pilot was able to reduce power
(producing vacuum) and decend safely on the manifold vacuum system
to an altitude that the system was able to produce enough vacuum for
continued operation.

Dave


On Mon, 09 Apr 2007 23:20:07 +0200, Mxsmanic
wrote:

kontiki writes:

It sounds like a free lunch doesn't it? :^O Well, tt works in a pinch
of course, but having to maintain a power setting that produces a
manifold pressure difference (between MP and outside static pressure)
just so that the gyros can spin sort of limits your flexibility.

Yes. I guess I got confused, thinking there would always be vacuum available
from the engine.

Dave writes:

My aircraft has a "standby Vacuum system" using the manifold pressure
in the engine..

It is for emergency use only, and , of course, will NOT supply enough
vacuum if the altitude is too high or if the engine is operating at
too high a power setting..

So you have the unenviable choice between crashing into a mountainside and
knowing you are about to do so, or staying safely above the mountain but
without any means of knowing it.

In some instances, thehigh flying pilot was able to reduce power
(producing vacuum) and decend safely on the manifold vacuum system
to an altitude that the system was able to produce enough vacuum for
continued operation.

Why don't vacuum pumps run out of vacuum at high altitudes? When the air gets
thin enough I should think it wouldn't be possible to maintain the necessary
pressure differential to keep things spinning.

--
Transpose mxsmanic and gmail to reach me by e-mail.

EridanMan writes:

IIRC, the FAA mandates a fully redundant electrical system (two
completely separate electric buses, each with its own alternator and
battery) before they'll allow you to run your primary gyros off
electric power. The added weight of such is often substantially more
than just a vacuum system alone.

If you're flying a twin that already has two engines and two alternators, what
would it add?

What's the backup for vacuum?

--
Transpose mxsmanic and gmail to reach me by e-mail.

If you're flying a twin that already has two engines and two alternators, what
would it add?

At most an extra battery. But most of us don't fly twins

What's the backup for vacuum?

If you want to be fancy about it, a venturi or a second vacuum pump
(equal in weight to an extra electrical system I suppose). If your
goal is to just get yourself on the ground in one piece, an Electric
Turn Coordinator, compass and ASI will get you through a cloud layer
and to an airport in short order if you have basic partial panel
skills (and recognize the problem early enough).

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. The GPS
tells me if I am turning (as does the Turn Cordinator, but the turn
coordinator is a bit more finicky, and turns are more difficult because
you can't really count on the whisky compass while turning, 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.

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

Robert,
You as a mere CFII are no match to Mxmanic the seer and no all...
Roy CFII, PGl

Robert M. Gary wrote:
On Apr 9, 8:17 am, Mxsmanic wrote:
Can someone tell me why vacuum power is popular for certain instruments? I
cannot see any special reliability of a vacuum pump as opposed to an electric
motor. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond what
was required by the engines themselves. However, a vacuum pump seems no more
reliable to me than an alternator. What are the reasons behind it all? I see
lots of descriptions of how the instruments work, but none that explain or
justify the choice of vacuum over electrical power. It also seems that vacuum
is subject to partial failures, whereas an electrical failure is usually much
more obvious.

--
Transpose mxsmanic and gmail to reach me by e-mail.

If I tell you are you going to tell me I'm wrong??

-robert, CFII

Mxsmanic wrote:
Can someone tell me why vacuum power is popular for certain instruments? I
cannot see any special reliability of a vacuum pump as opposed to an electric
motor. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond what
was required by the engines themselves. However, a vacuum pump seems no more
reliable to me than an alternator. What are the reasons behind it all? I see
lots of descriptions of how the instruments work, but none that explain or
justify the choice of vacuum over electrical power. It also seems that vacuum
is subject to partial failures, whereas an electrical failure is usually much
more obvious.

For once I agree with MXmoron. Vacuum instruments completely suck.

Gerald

"G. Sylvester" wrote in message
t...
Mxsmanic wrote:
Can someone tell me why vacuum power is popular for certain instruments?
I
cannot see any special reliability of a vacuum pump as opposed to an
electric
motor. The only reason I can think of is historical, in that many small
aircraft have traditionally not had on-board electrical systems beyond
what
was required by the engines themselves. However, a vacuum pump seems no
more
reliable to me than an alternator. What are the reasons behind it all?
I see
lots of descriptions of how the instruments work, but none that explain
or
justify the choice of vacuum over electrical power. It also seems that
vacuum
is subject to partial failures, whereas an electrical failure is usually
much
more obvious.

For once I agree with MXmoron. Vacuum instruments completely suck.


Then you would be wrong, just like MX usually is. Vaccum pumps suck!!!!