Windmilling Prop & Vacuum Pump

Ok, a curious question just popped in my mind. Say that one was flying
IFR in a piston single, maybe a 172 or 182. While enroute, either in
actual or above a cloud layer, the engine fails. Will the windmilling
prop keep the vacuum pump going enough to make the AI and DG usuable
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

Matt Young wrote:
Ok, a curious question just popped in my mind. Say that one was flying
IFR in a piston single, maybe a 172 or 182. While enroute, either in
actual or above a cloud layer, the engine fails. Will the windmilling
prop keep the vacuum pump going enough to make the AI and DG usuable
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

No and no.

Matt Young wrote:
Ok, a curious question just popped in my mind. Say that one was flying
IFR in a piston single, maybe a 172 or 182. While enroute, either in
actual or above a cloud layer, the engine fails. Will the windmilling
prop keep the vacuum pump going enough to make the AI and DG usuable
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

Whilst doing a engine runup at about 2000 rpm my vacuum indicates
'normal', and even during taxying at around 1000 rpm the AH is stable.

So., if the engine fails *but the prop still keeps turning* at a
reasonable rate (eg above 1000rpm) then I'd expect that there
should still be sufficient 'vacuum' to operate the gyro instruments.
With *no* vacuum, (eg after engine shutdown) my gyros starts
drifting after about a minute.

Matt Young wrote:
Ok, a curious question just popped in my mind. Say that one was flying
IFR in a piston single, maybe a 172 or 182. While enroute, either in
actual or above a cloud layer, the engine fails. Will the windmilling
prop keep the vacuum pump going enough to make the AI and DG usuable

Depends on your pump and what RPM's you can expect the prop to windmill
at. My pump will produce 4" at idle and if you just shut off the fuel
to the engine, the prop will windmill in the glide at well above idle
(actually, you can't really tell by the RPM's that the engine isn't
running).

Of course, if your engine siezes, you're not going to get any windmilling.

during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

It's amazing how FAST the gyros spin down (and how long it takes them
to spin up). I had a pump crump on takeoff roll and before I was at
pattern altitude the AI had started to lean over (fortunately in VMC).

In article et,
Matt Young wrote:
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

I question the wisdom of relying on an instrument that you already know to
have failed. You would need to know just how long you can stare at it
in a very stressful situation before you can no longer believe what it
tells you. I would think that immediately covering the instrument would
be your best course of action in this scenario.

No Such User wrote:
In article et,
Matt Young wrote:

during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?


I question the wisdom of relying on an instrument that you already know to
have failed.

What makes you think it has failed? If the vacuum guage still shows a vacuum,
then the gyro's got to spin (unless you are unlucky enough to have a second
failure of the vacuum system at the same time that the engine crumped).

What I've never understood is why there isn't more obvious indication that
there is no power (vacuum or electric as required) going to the instrument.
Jeez, the VOR which isn't as essential to instrument flight has a better
indication that it's not on.

Dave Butler wrote:
Matt Young wrote:

Ok, a curious question just popped in my mind. Say that one was
flying IFR in a piston single, maybe a 172 or 182. While enroute,
either in actual or above a cloud layer, the engine fails. Will the
windmilling prop keep the vacuum pump going enough to make the AI and
DG usuable during descent through the clouds, or will the gyros keep
spinning fast enough long enough to make the vacuum pump irrelevant?


No and no.

Personal experience, credible sources, or just a guess?


All the best,


David

David Megginson wrote:
Dave Butler wrote:

Matt Young wrote:

Ok, a curious question just popped in my mind. Say that one was
flying IFR in a piston single, maybe a 172 or 182. While enroute,
either in actual or above a cloud layer, the engine fails. Will the
windmilling prop keep the vacuum pump going enough to make the AI and
DG usuable during descent through the clouds, or will the gyros keep
spinning fast enough long enough to make the vacuum pump irrelevant?



No and no.


Personal experience, credible sources, or just a guess?

1A. Will it keep the vacuum pump going? No, just a guess. You might have a
better shot at it with a controllable prop. That assumes, of course, that
whatever caused the engine to stop making noise didn't prevent it from spinning
'round.

1B. Will the instruments keep spinning enough? No, just a guess, reinforced by
my experience testing the Precise Flight backup vacuum system when it was
installed in the Archer I used to own. To calibrate the altitude limits on the
placard, you have to fly with varying levels of vacuum and note the instrument
performance. The vacuum doesn't have to be much below specified minimum (is it 4
inHg?) before the gauges get really squirrely. Noting how long it takes the
gyros to spin down while the plane is stationary on the ramp after you shut down
is not a useful measure.

2. It's easy to test. Try it for yourself if you really want to know.

3. I wouldn't rely on a usenet answer if it were 'yes'.

4. Even if I tested it and it worked, I wouldn't rely on it working when I need it.

Short answer: it was a guess.

Dave Butler wrote:

3. I wouldn't rely on a usenet answer if it were 'yes'.

Very wise.

4. Even if I tested it and it worked, I wouldn't rely on it working when
I need it.

Short answer: it was a guess.

Fair enough. My gyros behave fine on an instrument approach down to 1500
rpm, so I'd be pretty comfortable trusting them as long as I could keep my
Warrior's speed up high enough to spin the prop that fast; unfortunately,
with an engine out, that might mean descending a lot faster than I really
need to. As long as my TC were usable, I would find it hard to justify
sacrificing a lot of glide range just to keep the vacuum pump working.


All the best,


David

Dave Butler wrote in message ...
Matt Young wrote:
Ok, a curious question just popped in my mind. Say that one was flying
IFR in a piston single, maybe a 172 or 182. While enroute, either in
actual or above a cloud layer, the engine fails. Will the windmilling
prop keep the vacuum pump going enough to make the AI and DG usuable
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

No and no.

No and maybe.

Especially with a constant speed prop, you will wish to pull the
prop pitch to low rpm to get the longest glide. Your pump will be doing
about nothing -- tho the wet ones will make some vacuum even as you crank.

The gyros will typically run for ~ 5 min, but they lose the erection
function caused by the vacuum loss.

But heck, unless you are REALLY high, they will run at least sort
of until you contact the earth. It's not that many minutes.

Bill Hale